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87 \title{Automated Composition of Refactorings}
88 \subtitle{Implementing and evaluating a search-based Extract and Move Method
90 \author{Erlend Kristiansen}
93 \newglossaryentry{profiling}
96 description={is to run a computer program through a profiler/with a profiler
99 \newglossaryentry{profiler}
102 description={A profiler is a program for analyzing performance within an
103 application. It is used to analyze memory consumption, processing time and
104 frequency of procedure calls and such}
106 \newglossaryentry{xUnit}
108 name={xUnit framework},
109 description={An xUnit framework is a framework for writing unit tests for a
110 computer program. It follows the patterns known from the JUnit framework for
111 Java\citing{fowlerXunit}
113 plural={xUnit frameworks}
115 \newglossaryentry{softwareObfuscation}
117 name={software obfuscation},
118 description={makes source code harder to read and analyze, while preserving
121 \newglossaryentry{extractClass}
123 name=\refa{Extract Class},
124 description={The \refa{Extract Class} refactoring works by creating a class,
125 for then to move members from another class to that class and access them from
126 the old class via a reference to the new class}
128 \newglossaryentry{designPattern}
130 name={design pattern},
131 description={A design pattern is a named abstraction that is meant to solve a
132 general design problem. It describes the key aspects of a common problem and
133 identifies its participators and how they collaborate},
134 plural={design patterns}
136 \newglossaryentry{enclosingClass}
138 name={enclosing class},
139 description={An enclosing class is the class that surrounds any specific piece
140 of code that is written in the inner scope of this class},
142 \newglossaryentry{mementoPattern}
144 name={memento pattern},
145 description={The memento pattern is a software design pattern that is used to
146 capture an object's internal state so that it can be restored to this state
147 later\citing{designPatterns}},
149 %\newglossaryentry{extractMethod}
151 % name=\refa{Extract Method},
152 % description={The \refa{Extract Method} refactoring is used to extract a
153 %fragment of code from its context and into a new method. A call to the new
154 %method is inlined where the fragment was before. It is used to break code into
155 %logical units, with names that explain their purpose}
157 %\newglossaryentry{moveMethod}
159 % name=\refa{Move Method},
160 % description={The \refa{Move Method} refactoring is used to move a method from
161 % one class to another. This is useful if the method is using more features of
162 % another class than of the class which it is currently defined. Then all calls
163 % to this method must be updated, or the method must be copied, with the old
164 %method delegating to the new method}
167 \bibliography{bibliography/master-thesis-erlenkr-bibliography}
168 \DefineBibliographyStrings{english}{%
169 bibliography = {References},
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194 \pgf@yc=\pgf@yb \advance\pgf@yc by-10pt
195 % construct main path
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197 \pgfpathlineto{\pgfpoint{\pgf@xa}{\pgf@yb}}
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199 \pgfpathlineto{\pgfpoint{\pgf@xb}{\pgf@yc}}
200 \pgfpathlineto{\pgfpoint{\pgf@xb}{\pgf@ya}}
203 \pgfpathmoveto{\pgfpoint{\pgf@xc}{\pgf@yb}}
204 \pgfpathlineto{\pgfpoint{\pgf@xc}{\pgf@yc}}
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245 Can be done by removing ``draft'' from documentclass.}}
246 \todoin{Write abstract}
254 %\setcounter{page}{13}
256 \chapter{Introduction}
258 \section{Motivation and structure}
260 For large software projects, complex program source code is an issue. It impacts
261 the cost of maintenance in a negative way. It often stalls the implementation of
262 new functionality and other program changes. The code may be difficult to
263 understand, the changes may introduce new bugs that are hard to find and its
264 complexity can simply keep people from doing code changes in fear of breaking
265 some dependent piece of code. All these problems are related, and often lead to
266 a vicious circle that slowly degrades the overall quality of a project.
268 More specifically, and in an object-oriented context, a class may depend on a
269 number of other classes. Sometimes these intimate relationships are appropriate,
270 and sometimes they are not. Inappropriate \emph{coupling} between classes can
271 make it difficult to know whether or not a change that is aimed at fixing a
272 specific problem also alters the behavior of another part of a program.
274 One of the tools that are used to fight complexity and coupling in program
275 source code is \emph{refactoring}. The intention for this master's thesis is
276 therefore to create an automated composite refactoring that reduces coupling
277 between classes. The refactoring shall be able to operate automatically in all
278 phases of a refactoring, from performing analysis to executing changes. It is
279 also a requirement that it should be able to process large quantities of source
280 code in a reasonable amount of time.
282 The current chapter proceeds in \mysimpleref{sec:refactoring} by describing what
283 refactoring is. Then the project is presented in \mysimpleref{sec:project},
284 before the chapter is concluded with a brief discussion of related work in
285 \mysimpleref{sec:relatedWork}.
287 \Mysimpleref{ch:extractAndMoveMethod} shows the workings of our refactoring
288 together with a simple example illustrating this.
290 \todoin{Structure. Write later\ldots}
293 \section{What is refactoring?}\label{sec:refactoring}
295 This question is best answered by first defining the concept of a
296 \emph{refactoring}, what it is to \emph{refactor}, and then discuss what aspects
297 of programming make people want to refactor their code.
299 \subsection{Defining refactoring}
300 Martin Fowler, in his classic book on refactoring\citing{refactoring}, defines a
301 refactoring like this:
304 \emph{Refactoring} (noun): a change made to the internal
305 structure\footnote{The structure observable by the programmer.} of software to
306 make it easier to understand and cheaper to modify without changing its
307 observable behavior.~\cite[p.~53]{refactoring}
310 \noindent This definition assigns additional meaning to the word
311 \emph{refactoring}, beyond the composition of the prefix \emph{re-}, usually
312 meaning something like ``again'' or ``anew'', and the word \emph{factoring},
313 which can mean to isolate the \emph{factors} of something. Here a \emph{factor}
314 would be close to the mathematical definition of something that divides a
315 quantity, without leaving a remainder. Fowler is mixing the \emph{motivation}
316 behind refactoring into his definition. Instead it could be more refined, formed
317 to only consider the \emph{mechanical} and \emph{behavioral} aspects of
318 refactoring. That is to factor the program again, putting it together in a
319 different way than before, while preserving the behavior of the program. An
320 alternative definition could then be:
322 \definition{A \emph{refactoring} is a transformation
323 done to a program without altering its external behavior.}
325 From this we can conclude that a refactoring primarily changes how the
326 \emph{code} of a program is perceived by the \emph{programmer}, and not the
327 \emph{behavior} experienced by any user of the program. Although the logical
328 meaning is preserved, such changes could potentially alter the program's
329 behavior when it comes to performance gain or -penalties. So any logic depending
330 on the performance of a program could make the program behave differently after
333 In the extreme case one could argue that \gloss{softwareObfuscation} is
334 refactoring. It is often used to protect proprietary software. It restrains
335 uninvited viewers, so they have a hard time analyzing code that they are not
336 supposed to know how works. This could be a problem when using a language that
337 is possible to decompile, such as Java.
339 Obfuscation could be done composing many, more or less randomly chosen,
340 refactorings. Then the question arises whether it can be called a
341 \emph{composite refactoring} or not \see{compositeRefactorings}? The answer is
342 not obvious. First, there is no way to describe the mechanics of software
343 obfuscation, because there are infinitely many ways to do that. Second,
344 obfuscation can be thought of as \emph{one operation}: Either the code is
345 obfuscated, or it is not. Third, it makes no sense to call software obfuscation
346 \emph{a refactoring}, since it holds different meaning to different people.
348 This last point is important, since one of the motivations behind defining
349 different refactorings, is to establish a \emph{vocabulary} for software
350 professionals to use when reasoning about and discussing programs, similar to
351 the motivation behind \glosspl{designPattern}\citing{designPatterns}.
353 So for describing \emph{software obfuscation}, it might be more appropriate to
354 define what you do when performing it rather than precisely defining its
355 mechanics in terms of other refactorings.
358 \subsection{The etymology of 'refactoring'}
359 It is a little difficult to pinpoint the exact origin of the word
360 ``refactoring'', as it seems to have evolved as part of a colloquial
361 terminology, more than a scientific term. There is no authoritative source for a
362 formal definition of it.
364 According to Martin Fowler\citing{etymology-refactoring}, there may also be more
365 than one origin of the word. The most well-known source, when it comes to the
366 origin of \emph{refactoring}, is the
367 Smalltalk\footnote{\label{footNote}Programming language} community and their
368 infamous \name{Refactoring
369 Browser}\footnote{\url{http://st-www.cs.illinois.edu/users/brant/Refactory/RefactoringBrowser.html}}
370 described in the article \tit{A Refactoring Tool for
371 Smalltalk}\citing{refactoringBrowser1997}, published in 1997.
372 Allegedly\citing{etymology-refactoring}, the metaphor of factoring programs was
373 also present in the Forth\textsuperscript{\ref{footNote}} community, and the
374 word ``refactoring'' is mentioned in a book by Leo Brodie, called \tit{Thinking
375 Forth}\citing{brodie2004}, first published in 1984\footnote{\tit{Thinking Forth}
376 was first published in 1984 by the \name{Forth Interest Group}. Then it was
377 reprinted in 1994 with minor typographical corrections, before it was
378 transcribed into an electronic edition typeset in \LaTeX\ and published under a
379 Creative Commons license in
380 2004. The edition cited here is the 2004 edition, but the content should
381 essentially be as in 1984.}. The exact word is only printed one
382 place~\cite[p.~232]{brodie2004}, but the term \emph{factoring} is prominent in
383 the book, which also contains a whole chapter dedicated to (re)factoring, and
384 how to keep the (Forth) code clean and maintainable.
387 \ldots good factoring technique is perhaps the most important skill for a
388 Forth programmer.~\cite[p.~172]{brodie2004}
391 \noindent Brodie also express what \emph{factoring} means to him:
394 Factoring means organizing code into useful fragments. To make a fragment
395 useful, you often must separate reusable parts from non-reusable parts. The
396 reusable parts become new definitions. The non-reusable parts become arguments
397 or parameters to the definitions.~\cite[p.~172]{brodie2004}
400 Fowler claims that the usage of the word \emph{refactoring} did not pass between
401 the \name{Forth} and \name{Smalltalk} communities, but that it emerged
402 independently in each of the communities.
404 \subsection{Reasons for refactoring}
405 There are many reasons why people want to refactor their programs. They can for
406 instance do it to remove duplication, break up long methods or to introduce
407 design patterns into their software systems. The shared trait for all these is
408 that peoples' intentions are to make their programs \emph{better}, in some
409 sense. But what aspects of their programs are becoming improved?
411 As just mentioned, people often refactor to get rid of duplication. They are
412 moving identical or similar code into methods, and are pushing methods up or
413 down in their class hierarchies. They are making template methods for
414 overlapping algorithms/functionality, and so on. It is all about gathering what
415 belongs together and putting it all in one place. The resulting code is then
416 easier to maintain. When removing the implicit coupling\footnote{When
417 duplicating code, the duplicate pieces of code might not be coupled, apart
418 from representing the same functionality. So if this functionality is going to
419 change, it might need to change in more than one place, thus creating an
420 implicit coupling between multiple pieces of code.} between code snippets, the
421 location of a bug is limited to only one place, and new functionality need only
422 to be added to this one place, instead of a number of places people might not
425 A problem you often encounter when programming, is that a program contains a lot
426 of long and hard-to-grasp methods. It can then help to break the methods into
427 smaller ones, using the \ExtractMethod refactoring\citing{refactoring}. Then
428 you may discover something about a program that you were not aware of before;
429 revealing bugs you did not know about or could not find due to the complex
430 structure of your program. Making the methods smaller and giving good names to
431 the new ones clarifies the algorithms and enhances the \emph{understandability}
432 of the program \see{magic_number_seven}. This makes refactoring an excellent
433 method for exploring unknown program code, or code that you had forgotten that
436 Most primitive refactorings are simple, and usually involves moving code
437 around\citing{kerievsky2005}. The motivation behind them may first be revealed
438 when they are combined into larger --- higher level --- refactorings, called
439 \emph{composite refactorings} \see{compositeRefactorings}. Often the goal of
440 such a series of refactorings is a design pattern. Thus the design can
441 \emph{evolve} throughout the lifetime of a program, as opposed to designing
442 up-front. It is all about being structured and taking small steps to improve a
445 Many software design pattern are aimed at lowering the coupling between
446 different classes and different layers of logic. One of the most famous is
447 perhaps the \pattern{Model-View-Controller}\citing{designPatterns} pattern. It
448 is aimed at lowering the coupling between the user interface, the business logic
449 and the data representation of a program. This also has the added benefit that
450 the business logic could much easier be the target of automated tests, thus
451 increasing the productivity in the software development process.
453 Another effect of refactoring is that with the increased separation of concerns
454 coming out of many refactorings, the \emph{performance} can be improved. When
455 profiling programs, the problematic parts are narrowed down to smaller parts of
456 the code, which are easier to tune, and optimization can be performed only where
457 needed and in a more effective way\citing{refactoring}.
459 Last, but not least, and this should probably be the best reason to refactor, is
460 to refactor to \emph{facilitate a program change}. If one has managed to keep
461 one's code clean and tidy, and the code is not bloated with design patterns that
462 are not ever going to be needed, then some refactoring might be needed to
463 introduce a design pattern that is appropriate for the change that is going to
466 Refactoring program code --- with a goal in mind --- can give the code itself
467 more value. That is in the form of robustness to bugs, understandability and
468 maintainability. Having robust code is an obvious advantage, but
469 understandability and maintainability are both very important aspects of
470 software development. By incorporating refactoring in the development process,
471 bugs are found faster, new functionality is added more easily and code is easier
472 to understand by the next person exposed to it, which might as well be the
473 person who wrote it. The consequence of this, is that refactoring can increase
474 the average productivity of the development process, and thus also add to the
475 monetary value of a business in the long run. The perspective on productivity
476 and money should also be able to open the eyes of the many nearsighted managers
477 that seldom see beyond the next milestone.
479 \subsection{The magical number seven}\label{magic_number_seven}
480 The article \tit{The magical number seven, plus or minus two: some limits on our
481 capacity for processing information}\citing{miller1956} by George A. Miller,
482 was published in the journal \name{Psychological Review} in 1956. It presents
483 evidence that support that the capacity of the number of objects a human being
484 can hold in its working memory is roughly seven, plus or minus two objects. This
485 number varies a bit depending on the nature and complexity of the objects, but
486 is according to Miller ``\ldots never changing so much as to be
489 Miller's article culminates in the section called \emph{Recoding}, a term he
490 borrows from communication theory. The central result in this section is that by
491 recoding information, the capacity of the amount of information that a human can
492 process at a time is increased. By \emph{recoding}, Miller means to group
493 objects together in chunks, and give each chunk a new name that it can be
497 \ldots recoding is an extremely powerful weapon for increasing the amount of
498 information that we can deal with.~\cite[p.~95]{miller1956}
501 By organizing objects into patterns of ever growing depth, one can memorize and
502 process a much larger amount of data than if it were to be represented as its
503 basic pieces. This grouping and renaming is analogous to how many refactorings
504 work, by grouping pieces of code and give them a new name. Examples are the
505 fundamental \ExtractMethod and \refa{Extract Class}
506 refactorings\citing{refactoring}.
508 An example from the article addresses the problem of memorizing a sequence of
509 binary digits. The example presented here is a slightly modified version of the
510 one presented in the original article\citing{miller1956}, but it preserves the
511 essence of it. Let us say we have the following sequence of
512 16 binary digits: ``1010001001110011''. Most of us will have a hard time
513 memorizing this sequence by only reading it once or twice. Imagine if we instead
514 translate it to this sequence: ``A273''. If you have a background from computer
515 science, it will be obvious that the latter sequence is the first sequence
516 recoded to be represented by digits in base 16. Most people should be able to
517 memorize this last sequence by only looking at it once.
519 Another result from the Miller article is that when the amount of information a
520 human must interpret increases, it is crucial that the translation from one code
521 to another must be almost automatic for the subject to be able to remember the
522 translation, before \heshe is presented with new information to recode. Thus
523 learning and understanding how to best organize certain kinds of data is
524 essential to efficiently handle that kind of data in the future. This is much
525 like when humans learn to read. First they must learn how to recognize letters.
526 Then they can learn distinct words, and later read sequences of words that form
527 whole sentences. Eventually, most of them will be able to read whole books and
528 briefly retell the important parts of its content. This suggests that the use of
529 design patterns is a good idea when reasoning about computer programs. With
530 extensive use of design patterns when creating complex program structures, one
531 does not always have to read whole classes of code to comprehend how they
532 function, it may be sufficient to only see the name of a class to almost fully
533 understand its responsibilities.
536 Our language is tremendously useful for repackaging material into a few chunks
537 rich in information.~\cite[p.~95]{miller1956}
540 Without further evidence, these results at least indicate that refactoring
541 source code into smaller units with higher cohesion and, when needed,
542 introducing appropriate design patterns, should aid in the cause of creating
543 computer programs that are easier to maintain and have code that is easier (and
546 \subsection{Notable contributions to the refactoring literature}
549 \item[1992] William F. Opdyke submits his doctoral dissertation called
550 \tit{Refactoring Object-Oriented Frameworks}\citing{opdyke1992}. This work
551 defines a set of refactorings that are behavior-preserving given that their
552 preconditions are met. The dissertation is focused on the automation of
554 \item[1999] Martin Fowler et al.: \tit{Refactoring: Improving the Design of
555 Existing Code}\citing{refactoring}. This is maybe the most influential text
556 on refactoring. It bares similarities with Opdykes thesis\citing{opdyke1992}
557 in the way that it provides a catalog of refactorings. But Fowler's book is
558 more about the craft of refactoring, as he focuses on establishing a
559 vocabulary for refactoring, together with the mechanics of different
560 refactorings and when to perform them. His methodology is also founded on
561 the principles of test-driven development.
562 \item[2005] Joshua Kerievsky: \tit{Refactoring to
563 Patterns}\citing{kerievsky2005}. This book is heavily influenced by Fowler's
564 \tit{Refactoring}\citing{refactoring} and the ``Gang of Four'' \tit{Design
565 Patterns}\citing{designPatterns}. It is building on the refactoring
566 catalogue from Fowler's book, but is trying to bridge the gap between
567 \emph{refactoring} and \emph{design patterns} by providing a series of
568 higher-level composite refactorings, that makes code evolve toward or away
569 from certain design patterns. The book is trying to build up the reader's
570 intuition around \emph{why} one would want to use a particular design
571 pattern, and not just \emph{how}. The book is encouraging evolutionary
572 design \see{relationToDesignPatterns}.
575 \subsection{Tool support (for Java)}\label{toolSupport}
576 This section will briefly compare the refactoring support of the three IDEs
577 \name{Eclipse}\footnote{\url{http://www.eclipse.org/}}, \name{IntelliJ
578 IDEA}\footnote{The IDE under comparison is the \name{Community Edition},
579 \url{http://www.jetbrains.com/idea/}} and
580 \name{NetBeans}\footnote{\url{https://netbeans.org/}}. These are the most
581 popular Java IDEs\citing{javaReport2011}.
583 All three IDEs provide support for the most useful refactorings, like the
584 different extract, move and rename refactorings. In fact, Java-targeted IDEs are
585 known for their good refactoring support, so this did not appear as a big
588 The IDEs seem to have excellent support for the \ExtractMethod refactoring, so
589 at least they have all passed the first ``refactoring
590 rubicon''\citing{fowlerRubicon2001,secondRubicon2012}.
592 Regarding the \MoveMethod refactoring, the \name{Eclipse} and \name{IntelliJ}
593 IDEs do the job in very similar manners. In most situations they both do a
594 satisfying job by producing the expected outcome. But they do nothing to check
595 that the result does not break the semantics of the program
596 \see{sec:correctness}.
597 The \name{NetBeans} IDE implements this refactoring in a somewhat
598 unsophisticated way. For starters, the refactoring's default destination for the
599 move, is the same class as the method already resides in, although it refuses to
600 perform the refactoring if chosen. But the worst part is, that if moving the
601 method \method{f} of the class \type{C} to the class \type{X}, it will break the
602 code. The result is shown in \myref{lst:moveMethod_NetBeans}.
606 \begin{minted}[samepage]{java}
619 \begin{minted}[samepage]{java}
629 \caption{Moving method \method{f} from \type{C} to \type{X}.}
630 \label{lst:moveMethod_NetBeans}
633 \name{NetBeans} will try to create code that call the methods \method{m} and \method{n}
634 of \type{X} by accessing them through \var{c.x}, where \var{c} is a parameter of
635 type \type{C} that is added the method \method{f} when it is moved. (This is
636 seldom the desired outcome of this refactoring, but ironically, this ``feature''
637 keeps \name{NetBeans} from breaking the code in the example from
638 \myref{sec:correctness}.) If \var{c.x} for some reason is inaccessible to
639 \type{X}, as in this case, the refactoring breaks the code, and it will not
640 compile. \name{NetBeans} presents a preview of the refactoring outcome, but the
641 preview does not catch it if the IDE is about break the program.
643 The IDEs under investigation seem to have fairly good support for primitive
644 refactorings, but what about more complex ones, such as
645 \gloss{extractClass}\citing{refactoring}? \name{IntelliJ} handles this in a
646 fairly good manner, although, in the case of private methods, it leaves unused
647 methods behind. These are methods that delegate to a field with the type of the
648 new class, but are not used anywhere. \name{Eclipse} has added its own quirk to
649 the \refa{Extract Class} refactoring, and only allows for \emph{fields} to be
650 moved to a new class, \emph{not methods}. This makes it effectively only
651 extracting a data structure, and calling it \refa{Extract Class} is a little
652 misleading. One would often be better off with textual extract and paste than
653 using the \refa{Extract Class} refactoring in \name{Eclipse}. When it comes to
654 \name{NetBeans}, it does not even show an attempt on providing this refactoring.
656 \subsection{The relation to design patterns}\label{relationToDesignPatterns}
658 Refactoring and design patterns have at least one thing in common, they are both
659 promoted by advocates of \emph{clean code}\citing{cleanCode} as fundamental
660 tools on the road to more maintainable and extendable source code.
663 Design patterns help you determine how to reorganize a design, and they can
664 reduce the amount of refactoring you need to do
665 later.~\cite[p.~353]{designPatterns}
668 Although sometimes associated with
669 over-engineering\citing{kerievsky2005,refactoring}, design patterns are in
670 general assumed to be good for maintainability of source code. That may be
671 because many of them are designed to support the \emph{open/closed principle} of
672 object-oriented programming. The principle was first formulated by Bertrand
673 Meyer, the creator of the Eiffel programming language, like this: ``Modules
674 should be both open and closed.''\citing{meyer1988} It has been popularized,
675 with this as a common version:
678 Software entities (classes, modules, functions, etc.) should be open for
679 extension, but closed for modification.\footnote{See
680 \url{http://c2.com/cgi/wiki?OpenClosedPrinciple} or
681 \url{https://en.wikipedia.org/wiki/Open/closed_principle}}
684 Maintainability is often thought of as the ability to be able to introduce new
685 functionality without having to change too much of the old code. When
686 refactoring, the motivation is often to facilitate adding new functionality. It
687 is about factoring the old code in a way that makes the new functionality being
688 able to benefit from the functionality already residing in a software system,
689 without having to copy old code into new. Then, next time someone shall add new
690 functionality, it is less likely that the old code has to change. Assuming that
691 a design pattern is the best way to get rid of duplication and assist in
692 implementing new functionality, it is reasonable to conclude that a design
693 pattern often is the target of a series of refactorings. Having a repertoire of
694 design patterns can also help in knowing when and how to refactor a program to
695 make it reflect certain desired characteristics.
698 There is a natural relation between patterns and refactorings. Patterns are
699 where you want to be; refactorings are ways to get there from somewhere
700 else.~\cite[p.~107]{refactoring}
703 This quote is wise in many contexts, but it is not always appropriate to say
704 ``Patterns are where you want to be\ldots''. \emph{Sometimes}, patterns are
705 where you want to be, but only because it will benefit your design. It is not
706 true that one should always try to incorporate as many design patterns as
707 possible into a program. It is not like they have intrinsic value. They only add
708 value to a system when they support its design. Otherwise, the use of design
709 patterns may only lead to a program that is more complex than necessary.
712 The overuse of patterns tends to result from being patterns happy. We are
713 \emph{patterns happy} when we become so enamored of patterns that we simply
714 must use them in our code.~\cite[p.~24]{kerievsky2005}
717 This can easily happen when relying largely on up-front design. Then it is
718 natural, in the very beginning, to try to build in all the flexibility that one
719 believes will be necessary throughout the lifetime of a software system.
720 According to Joshua Kerievsky ``That sounds reasonable --- if you happen to be
721 psychic.''~\cite[p.~1]{kerievsky2005} He is advocating what he believes is a
722 better approach: To let software continually evolve. To start with a simple
723 design that meets today's needs, and tackle future needs by refactoring to
724 satisfy them. He believes that this is a more economic approach than investing
725 time and money into a design that inevitably is going to change. By relying on
726 continuously refactoring a system, its design can be made simpler without
727 sacrificing flexibility. To be able to fully rely on this approach, it is of
728 utter importance to have a reliable suit of tests to lean on \see{testing}. This
729 makes the design process more natural and less characterized by difficult
730 decisions that has to be made before proceeding in the process, and that is
731 going to define a project for all of its unforeseeable future.
733 \subsection{The impact on software quality}
735 \subsubsection{What is software quality?}
736 The term \emph{software quality} has many meanings. It all depends on the
737 context we put it in. If we look at it with the eyes of a software developer, it
738 usually means that the software is easily maintainable and testable, or in other
739 words, that it is \emph{well designed}. This often correlates with the
740 management scale, where \emph{keeping the schedule} and \emph{customer
741 satisfaction} is at the center. From the customers point of view, in addition to
742 good usability, \emph{performance} and \emph{lack of bugs} is always
743 appreciated, measurements that are also shared by the software developer. (In
744 addition, such things as good documentation could be measured, but this is out
745 of the scope of this document.)
747 \subsubsection{The impact on performance}
749 Refactoring certainly will make software go more slowly\footnote{With today's
750 compiler optimization techniques and performance tuning of e.g. the Java
751 virtual machine, the penalties of object creation and method calls are
752 debatable.}, but it also makes the software more amenable to performance
753 tuning.~\cite[p.~69]{refactoring}
756 \noindent There is a common belief that refactoring compromises performance, due
757 to increased degree of indirection and that polymorphism is slower than
760 In a survey, Demeyer\citing{demeyer2002} disproves this view in the case of
761 polymorphism. He did an experiment on, what he calls, ``Transform Self Type
762 Checks'' where you introduce a new polymorphic method and a new class hierarchy
763 to get rid of a class' type checking of a ``type attribute``. He uses this kind
764 of transformation to represent other ways of replacing conditionals with
765 polymorphism as well. The experiment is performed on the C++ programming
766 language and with three different compilers and platforms. Demeyer concludes
767 that, with compiler optimization turned on, polymorphism beats middle to large
768 sized if-statements and does as well as case-statements. (In accordance with
769 his hypothesis, due to similarities between the way C++ handles polymorphism and
773 The interesting thing about performance is that if you analyze most programs,
774 you find that they waste most of their time in a small fraction of the
775 code.~\cite[p.~70]{refactoring}
778 \noindent So, although an increased amount of method calls could potentially
779 slow down programs, one should avoid premature optimization and sacrificing good
780 design, leaving the performance tuning until after \gloss{profiling} the
781 software and having isolated the actual problem areas.
783 \subsection{Composite refactorings}\label{compositeRefactorings}
784 Generally, when thinking about refactoring, at the mechanical level, there are
785 essentially two kinds of refactorings. There are the \emph{primitive}
786 refactorings, and the \emph{composite} refactorings.
788 \definition{A \emph{primitive refactoring} is a refactoring that cannot be
789 expressed in terms of other refactorings.}
791 \noindent Examples are the \refa{Pull Up Field} and \refa{Pull Up
792 Method} refactorings\citing{refactoring}, that move members up in their class
795 \definition{A \emph{composite refactoring} is a refactoring that can be
796 expressed in terms of two or more other refactorings.}
798 \noindent An example of a composite refactoring is the \refa{Extract
799 Superclass} refactoring\citing{refactoring}. In its simplest form, it is composed
800 of the previously described primitive refactorings, in addition to the
801 \refa{Pull Up Constructor Body} refactoring\citing{refactoring}. It works
802 by creating an abstract superclass that the target class(es) inherits from, then
803 by applying \refa{Pull Up Field}, \refa{Pull Up Method} and
804 \refa{Pull Up Constructor Body} on the members that are to be members of
805 the new superclass. If there are multiple classes in play, their interfaces may
806 need to be united with the help of some rename refactorings, before extracting
807 the superclass. For an overview of the \refa{Extract Superclass}
808 refactoring, see \myref{fig:extractSuperclass}.
812 \includegraphics[angle=270,width=\linewidth]{extractSuperclassItalic.pdf}
813 \caption{The Extract Superclass refactoring, with united interfaces.}
814 \label{fig:extractSuperclass}
817 \subsection{Manual vs. automated refactorings}
818 Refactoring is something every programmer does, even if \heshe does not known
819 the term \emph{refactoring}. Every refinement of source code that does not alter
820 the program's behavior is a refactoring. For small refactorings, such as
821 \ExtractMethod, executing it manually is a manageable task, but is still prone
822 to errors. Getting it right the first time is not easy, considering the method
823 signature and all the other aspects of the refactoring that has to be in place.
825 Consider the renaming of classes, methods and fields. For complex programs these
826 refactorings are almost impossible to get right. Attacking them with textual
827 search and replace, or even regular expressions, will fall short on these tasks.
828 Then it is crucial to have proper tool support that can perform them
829 automatically. Tools that can parse source code and thus have semantic knowledge
830 about which occurrences of which names belong to what construct in the program.
831 For even trying to perform one of these complex tasks manually, one would have
832 to be very confident on the existing test suite \see{testing}.
834 \subsection{Correctness of refactorings}\label{sec:correctness}
835 For automated refactorings to be truly useful, they must show a high degree of
836 behavior preservation. This last sentence might seem obvious, but there are
837 examples of refactorings in existing tools that break programs. In an ideal
838 world, every automated refactoring would be ``complete'', in the sense that it
839 would never break a program. In an ideal world, every program would also be free
840 from bugs. In modern IDEs the implemented automated refactorings are working for
841 \emph{most} cases, which is enough for making them useful.
843 I will now present an example of a \emph{corner case} where a program breaks
844 when a refactoring is applied. The example shows an \ExtractMethod refactoring
845 followed by a \MoveMethod refactoring that breaks a program in both the
846 \name{Eclipse} and \name{IntelliJ} IDEs\footnote{The \name{NetBeans} IDE handles this
847 particular situation without altering the program's behavior, mainly because
848 its \refa{Move Method} refactoring implementation is a bit flawed in other ways
849 \see{toolSupport}.}. The target and the destination for the composed
850 refactoring are shown in \myref{lst:correctnessExtractAndMove}. Note that the
851 method \method{m(C c)} of class \type{X} assigns to the field \var{x} of the
852 argument \var{c} that has type \type{C}.
856 \begin{minted}[linenos,frame=topline,label={Refactoring
857 target},framesep=\mintedframesep]{java}
859 public X x = new X();
871 \begin{minted}[frame=topline,label={Method
872 destination},framesep=\mintedframesep]{java}
876 // If m is called from
877 // c, then c.x no longer
884 \caption{The target and the destination for the composition of the Extract
885 Method and \refa{Move Method} refactorings.}
886 \label{lst:correctnessExtractAndMove}
890 The refactoring sequence works by extracting line 6 through 8 from the original
891 class \type{C} into a method \method{f} with the statements from those lines as
892 its method body (but with the comment left out, since it will no longer hold any
893 meaning). The method is then moved to the class \type{X}. The result is shown
894 in \myref{lst:correctnessExtractAndMoveResult}.
896 Before the refactoring, the methods \method{m} and \method{n} of class \type{X}
897 are called on different object instances (see line 6 and 8 of the original class
898 \type{C} in \cref{lst:correctnessExtractAndMove}). After the refactoring, they
899 are called on the same object, and the statement on line
900 3 of class \type{X} (in \cref{lst:correctnessExtractAndMoveResult}) no longer
901 has the desired effect in our example. The method \method{f} of class \type{C}
902 is now calling the method \method{f} of class \type{X} (see line 5 of class
903 \type{C} in \cref{lst:correctnessExtractAndMoveResult}), and the program now
904 behaves different than before.
908 \begin{minted}[linenos]{java}
910 public X x = new X();
920 \begin{minted}[linenos]{java}
935 \caption{The result of the composed refactoring.}
936 \label{lst:correctnessExtractAndMoveResult}
939 The bug introduced in the previous example is of such a nature\footnote{Caused
940 by aliasing. See \url{https://en.wikipedia.org/wiki/Aliasing_(computing)}}
941 that it is very difficult to spot if the refactored code is not covered by
942 tests. It does not generate compilation errors, and will thus only result in
943 a runtime error or corrupted data, which might be hard to detect.
945 \subsection{Refactoring and the importance of testing}\label{testing}
947 If you want to refactor, the essential precondition is having solid
948 tests.\citing{refactoring}
951 When refactoring, there are roughly three classes of errors that can be made.
952 The first class of errors is the one that makes the code unable to compile.
953 These \emph{compile-time} errors are of the nicer kind. They flash up at the
954 moment they are made (at least when using an IDE), and are usually easy to fix.
955 The second class is the \emph{runtime} errors. Although these errors take a bit
956 longer to surface, they usually manifest after some time in an illegal argument
957 exception, null pointer exception or similar during the program execution.
958 These kinds of errors are a bit harder to handle, but at least they will show,
959 eventually. Then there are the \emph{behavior-changing} errors. These errors are
960 of the worst kind. They do not show up during compilation and they do not turn
961 on a blinking red light during runtime either. The program can seem to work
962 perfectly fine with them in play, but the business logic can be damaged in ways
963 that will only show up over time.
965 For discovering runtime errors and behavior changes when refactoring, it is
966 essential to have good test coverage. Testing in this context means writing
967 automated tests. Manual testing may have its uses, but when refactoring, it is
968 automated unit testing that dominate. For discovering behavior changes it is
969 especially important to have tests that cover potential problems, since these
970 kinds of errors do not reveal themselves.
972 Unit testing is not a way to \emph{prove} that a program is correct, but it is a
973 way to make you confident that it \emph{probably} works as desired. In the
974 context of test-driven development (commonly known as TDD), the tests are even a
975 way to define how the program is \emph{supposed} to work. It is then, by
976 definition, working if the tests are passing.
978 If the test coverage for a code base is perfect, then it should, theoretically,
979 be risk-free to perform refactorings on it. This is why automated tests and
980 refactoring is such a great match.
982 \subsubsection{Testing the code from correctness section}
983 The worst thing that can happen when refactoring is to introduce changes to the
984 behavior of a program, as in the example on \myref{sec:correctness}. This
985 example may be trivial, but the essence is clear. The only problem with the
986 example is that it is not clear how to create automated tests for it, without
987 changing it in intrusive ways.
989 Unit tests, as they are known from the different \glosspl{xUnit} around, are
990 only suitable to test the \emph{result} of isolated operations. They can not
991 easily (if at all) observe the \emph{history} of a program.
993 This problem is still open.
997 Assuming a sequential (non-concurrent) program:
1000 tracematch (C c, X x) {
1002 call(* X.m(C)) && args(c) && cflow(within(C));
1004 call(* X.n()) && target(x) && cflow(within(C));
1006 set(C.x) && target(c) && !cflow(m);
1010 { assert x == c.x; }
1014 %\begin{minted}{java}
1015 %tracematch (X x1, X x2) {
1017 % call(* X.m(C)) && target(x1);
1019 % call(* X.n()) && target(x2);
1021 % set(C.x) && !cflow(m) && !cflow(n);
1025 % { assert x1 != x2; }
1031 \section{The project}\label{sec:project}
1032 In this section we look at the work that shall be done for this project, its
1033 building stones and some of the methodologies used.
1035 \subsection{Project description}
1036 The aim of this master's project will be to explore the relationship between the
1037 \ExtractMethod and the \MoveMethod refactorings. This will be done by composing
1038 the two into a composite refactoring. The refactoring will be called the
1039 \ExtractAndMoveMethod refactoring.
1041 The two primitive \ExtractMethod and \MoveMethod refactorings must already be
1042 implemented in a tool, so the \ExtractAndMoveMethod refactoring is going to be
1043 built on top of those.
1045 The composition of the \ExtractMethod and \MoveMethod refactorings springs
1046 naturally out of the need to move procedures closer to the data they manipulate.
1047 This composed refactoring is not well described in the literature, but it is
1048 implemented in at least one tool called
1049 \name{CodeRush}\footnote{\url{https://help.devexpress.com/\#CodeRush/CustomDocument3519}},
1050 which is an extension for \name{MS Visual
1051 Studio}\footnote{\url{http://www.visualstudio.com/}}. In CodeRush it is called
1052 \refa{Extract Method to
1053 Type}\footnote{\url{https://help.devexpress.com/\#CodeRush/CustomDocument6710}},
1054 but I choose to call it \ExtractAndMoveMethod, since I feel this better
1055 communicates which primitive refactorings it is composed of.
1057 The project will consist of implementing the \ExtractAndMoveMethod refactoring,
1058 as well as executing it over a larger code base, as a case study. To be able to
1059 execute the refactoring automatically, I have to make it analyze code to
1060 determine the best selections to extract into new methods.
1062 \subsection{The premises}
1063 Before we can start manipulating source code and write a tool for doing so, we
1064 need to decide on a programming language for the code we are going to
1065 manipulate. Also, since we do not want to start from scratch by implementing
1066 primitive refactorings ourselves, we need to choose an existing tool that
1067 provides the needed refactorings. In addition to be able to perform changes, we
1068 need a framework for analyzing source code for the language we select.
1070 \subsubsection{Choosing the target language}
1071 Choosing which programming language the code that shall be manipulated shall be
1072 written in, is not a very difficult task. We choose to limit the possible
1073 languages to the object-oriented programming languages, since most of the
1074 terminology and literature regarding refactoring comes from the world of
1075 object-oriented programming. In addition, the language must have existing tool
1076 support for refactoring.
1078 The \name{Java} programming language\footnote{\url{https://www.java.com/}} is
1079 the dominating language when it comes to example code in the literature of
1080 refactoring, and is thus a natural choice. Java is perhaps, currently the most
1081 influential programming language in the world, with its \name{Java Virtual
1082 Machine} that runs on all of the most popular architectures and also supports
1083 dozens of other programming languages\footnote{They compile to Java bytecode.},
1084 with \name{Scala}, \name{Clojure} and \name{Groovy} as the most prominent ones.
1085 Java is currently the language that every other programming language is compared
1086 against. It is also the primary programming language for the author of this
1089 \subsubsection{Choosing the tools}
1090 When choosing a tool for manipulating Java, there are certain criteria that
1091 have to be met. First of all, the tool should have some existing refactoring
1092 support that this thesis can build upon. Secondly it should provide some kind of
1093 framework for parsing and analyzing Java source code. Third, it should itself be
1094 open source. This is both because of the need to be able to browse the code for
1095 the existing refactorings that is contained in the tool, and also because open
1096 source projects hold value in them selves. Another important aspect to consider
1097 is that open source projects of a certain size, usually has large communities of
1098 people connected to them, that are committed to answering questions regarding the
1099 use and misuse of the products, that to a large degree is made by the community
1102 There is a certain class of tools that meet these criteria, namely the class of
1103 \emph{IDEs}\footnote{\emph{Integrated Development Environment}}. These are
1104 programs that are meant to support the whole production cycle of a computer
1105 program, and the most popular IDEs that support Java, generally have quite good
1106 refactoring support.
1108 The main contenders for this thesis is the \name{Eclipse IDE}, with the
1109 \name{Java development tools} (JDT), the \name{IntelliJ IDEA Community Edition}
1110 and the \name{NetBeans IDE} \see{toolSupport}. \name{Eclipse} and
1111 \name{NetBeans} are both free, open source and community driven, while the
1112 \name{IntelliJ IDEA} has an open sourced community edition that is free of
1113 charge, but also offer an \name{Ultimate Edition} with an extended set of
1114 features, at additional cost. All three IDEs supports adding plugins to extend
1115 their functionality and tools that can be used to parse and analyze Java source
1116 code. But one of the IDEs stand out as a favorite, and that is the \name{Eclipse
1117 IDE}. This is the most popular\citing{javaReport2011} among them and seems to be
1118 de facto standard IDE for Java development regardless of platform.
1121 \subsection{The primitive refactorings}
1122 The refactorings presented here are the primitive refactorings used in this
1123 project. They are the abstract building blocks used by the \ExtractAndMoveMethod
1126 \paragraph{The Extract Method refactoring}
1127 The \refa{Extract Method} refactoring is used to extract a fragment of code
1128 from its context and into a new method. A call to the new method is inlined
1129 where the fragment was before. It is used to break code into logical units, with
1130 names that explain their purpose.
1132 An example of an \ExtractMethod refactoring is shown in
1133 \myref{lst:extractMethodRefactoring}. It shows a method containing calls to the
1134 methods \method{foo} and \method{bar} of a type \type{X}. These statements are
1135 then extracted into the new method \method{fooBar}.
1138 \begin{multicols}{2}
1139 \begin{minted}[samepage,frame=topline,label={Before},framesep=\mintedframesep]{java}
1150 \begin{minted}[samepage,frame=topline,label={After},framesep=\mintedframesep]{java}
1162 \caption{An example of an \ExtractMethod refactoring.}
1163 \label{lst:extractMethodRefactoring}
1166 \paragraph{The Move Method refactoring}
1167 The \refa{Move Method} refactoring is used to move a method from one class to
1168 another. This can be appropriate if the method is using more features of another
1169 class than of the class which it is currently defined.
1171 \Myref{lst:moveMethodRefactoring} shows an example of this refactoring. Here a
1172 method \method{fooBar} is moved from the class \type{C} to the class \type{X}.
1175 \begin{multicols}{2}
1176 \begin{minted}[samepage,frame=topline,label={Before},framesep=\mintedframesep]{java}
1195 \begin{minted}[samepage,frame=topline,label={After},framesep=\mintedframesep]{java}
1212 \caption{An example of a \MoveMethod refactoring.}
1213 \label{lst:moveMethodRefactoring}
1216 \subsection{The Extract and Move Method refactoring}
1217 The \ExtractAndMoveMethod refactoring is a composite refactoring composed of the
1218 primitive \ExtractMethod and \MoveMethod refactorings. The effect of this
1219 refactoring on source code is the same as when extracting a method and moving it
1220 to another class. Conceptually, this is done without an intermediate step. In
1221 practice, as we shall see later, an intermediate step may be necessary.
1223 An example of this composite refactoring is shown in
1224 \myref{lst:extractAndMoveMethodRefactoring}. The example joins the examples from
1225 \cref{lst:extractMethodRefactoring} and \cref{lst:moveMethodRefactoring}. This
1226 means that the selection consisting of the consecutive calls to the methods
1227 \method{foo} and \method{bar}, is extracted into a new method \method{fooBar}
1228 located in the class \type{X}.
1231 \begin{multicols}{2}
1232 \begin{minted}[samepage,frame=topline,label={Before},framesep=\mintedframesep]{java}
1248 \begin{minted}[samepage,frame=topline,label={After},framesep=\mintedframesep]{java}
1265 \caption{An example of the \ExtractAndMoveMethod refactoring.}
1266 \label{lst:extractAndMoveMethodRefactoring}
1269 \subsection{The Coupling Between Object Classes metric}\label{sec:CBO}
1270 The best known metric for measuring coupling between classes in object-oriented
1271 software is called \metr{Coupling Between Object Classes}, usually abbreviated
1272 as CBO. The metric is defined in the article \tit{A Metrics Suite for Object
1273 Oriented Design}\citing{metricsSuite1994} by Chidamber and Kemerer, published in
1276 \definition{\emph{CBO} for a class is a count of the number of other classes to
1277 which it is coupled.}
1279 An object is coupled to another object if one of them acts on the other by using
1280 methods or instance variables of the other object. This relation goes both ways,
1281 so both outgoing and incoming uses are counted. Each coupling relationship is
1282 only considered once when measuring CBO for a class.
1284 \paragraph{How can the Extract and Move Method refactoring improve CBO?}
1285 \Myref{lst:CBOExample} shows how CBO changes for a class when it is refactored
1286 with the \ExtractAndMoveMethod refactoring. In the example we consider only the
1287 CBO value of class \type{C}.
1290 \begin{multicols}{2}
1291 \begin{minted}[linenos,samepage,frame=topline,label={Before},framesep=\mintedframesep]{java}
1322 \begin{minted}[linenos,samepage,frame=topline,label={After},framesep=\mintedframesep]{java}
1354 \caption{An example of improving CBO. Class \type{C} has a CBO value of 4
1355 before refactoring it, and 3 after.}
1356 \label{lst:CBOExample}
1359 Before refactoring the class \type{C} with the \ExtractAndMoveMethod
1360 refactoring, it has a CBO value of 4. The class uses members of the classes
1361 \type{A} and \type{B}, which accounts for 2 of the coupling relationships of
1362 class \type{C}. In addition to this, it uses its variable \var{x} with type
1363 \type{X} and also the methods \method{foo} and \method{bar} declared in class
1364 \type{Y}, giving it a total CBO value of 4.
1366 The after-part of the example code in \mysimpleref{lst:CBOExample} shows the
1367 result of extracting the lines
1368 5 and 6 of class \type{C} into a new method \method{fooBar}, with a subsequent
1369 move of it to class \type{X}.
1371 With respect to the CBO metric, the refactoring action accomplishes something
1372 important: It eliminates the uses of class \type{Y} from class \type{C}. This
1373 means that the class \type{C} is no longer coupled to \type{Y}, only the classes
1374 \type{A}, \type{B} and \type{X}. The CBO value of class \type{C} is therefore 3
1375 after refactoring, while no other class have received any increase in CBO.
1377 The example shown here is an ideal situation. Coupling is reduced for one class
1378 without any increase of coupling for another class. There is also another point
1379 that is important. It is the fact that to reduce the CBO value for a class, we
1380 need to remove \emph{all} its uses of another class. This is done for the class
1381 \type{C} in \myref{lst:CBOExample}, where all uses of class \type{Y} is removed
1382 by the \ExtractAndMoveMethod refactoring.
1383 \todoin{Highlight code}
1386 \subsection{Research questions}\label{sec:researchQuestions}
1387 The main question that I seek an answer to in this thesis is:
1390 Is it possible to automate the analysis and execution of the
1391 \ExtractAndMoveMethod refactoring, and do so for all of the code of a larger
1395 \noindent The secondary questions will then be:
1397 \paragraph{Can we do this efficiently?} Can we automate the analysis and
1398 execution of the refactoring so it can be run in a reasonable amount of time?
1400 \paragraph{Can we perform changes safely?} Can we take actions to prevent the
1401 refactoring from breaking the code? By breaking the code we mean to either do
1402 changes that do not compile, or make changes that alter the behavior of the
1405 \paragraph{Can we improve the quality of source code?} Assuming that the
1406 refactoring is safe: Is it feasible to assure that the code we refactor actually
1407 gets better in terms of coupling?
1409 \paragraph{How can the automation of the refactoring be helpful?} Assuming the
1410 refactoring does in fact improve the quality of source code and is safe to use:
1411 What is the usefulness of the refactoring in a software development setting? In
1412 what parts of the development process can the refactoring play a role?
1414 \subsection{Methodology}
1415 This section will present some of the methods used during the work of this
1418 \subsubsection{Evolutionary design}
1419 In the programming work for this project, I have tried using a design strategy
1420 called evolutionary design, also known as continuous or incremental
1421 design\citing{wiki_continuous_2014}. It is a software design strategy advocated
1422 by the Extreme Programming community. The essence of the strategy is that you
1423 should let the design of your program evolve naturally as your requirements
1424 change. This is seen in contrast with up-front design, where design decisions
1425 are made early in the process.
1427 The motivation behind evolutionary design is to keep the design of software as
1428 simple as possible. This means not introducing unneeded functionality into a
1429 program. You should defer introducing flexibility into your software, until it
1430 is needed to be able to add functionality in a clean way.
1432 Holding up design decisions, implies that the time will eventually come when
1433 decisions have to be made. The flexibility of the design then relies on the
1434 programmer's abilities to perform the necessary refactoring, and \his confidence
1435 in those abilities. From my experience working on this project, I can say that
1436 this confidence is greatly enhanced by having automated tests to rely on
1439 The choice of going for evolutionary design developed naturally. As Fowler
1440 points out in his article \tit{Is Design Dead?}, evolutionary design much
1441 resembles the ``code and fix'' development strategy\citing{fowler_design_2004}.
1442 A strategy that most of us have practiced in school. This was also the case when
1443 I first started this work. I had to learn the inner workings of Eclipse and its
1444 refactoring-related plugins. That meant a lot of fumbling around with code I did
1445 not know, in a trial and error fashion. Eventually I started writing tests for
1446 my code, and my design began to evolve.
1448 \subsubsection{Test-driven development}\label{tdd}
1449 As mentioned before, the project started out as a classic code and fix
1450 development process. My focus was aimed at getting something to work, rather
1451 than doing so according to best practice. This resulted in a project that got
1452 out of its starting blocks, but it was not accompanied by any tests. Hence it
1453 was soon difficult to make any code changes with the confidence that the program
1454 was still correct afterwards (assuming it was so before changing it). I always
1455 knew that I had to introduce some tests at one point, but this experience
1456 accelerated the process of leading me onto the path of testing.
1458 I then wrote tests for the core functionality of the plugin, and thus gained
1459 more confidence in the correctness of my code. I could now perform quite drastic
1460 changes without ``wetting my pants``. After this, nearly all of the semantic
1461 changes done to the business logic of the project, or the addition of new
1462 functionality, were made in a test-driven manner. This means that before
1463 performing any changes, I would define the desired functionality through a set
1464 of tests. I would then run the tests to check that they were run and that they
1465 did not pass. Then I would do any code changes necessary to make the tests
1466 pass. The definition of how the program is supposed to operate is then captured
1467 by the tests. However, this does not prove the correctness of the analysis
1468 leading to the test definitions.
1470 \subsection{Case studies}
1471 The case study methodology is used to show how the \ExtractAndMoveMethod
1472 refactoring performs on real code, not just toy examples. The case studies are
1473 used to analyze our project so we can conclude on its completeness and
1476 \subsection{Dogfooding}
1477 Dogfooding is a methodology where you use your own tools to do your job, also
1478 referred to as ``eating your own dog food''\citing{harrisonDogfooding2006}. It
1479 is used in this project to see if we can refactor our own refactoring code and
1480 still use it to refactor other code.
1482 \section{Related work}\label{sec:relatedWork}
1483 Here we present some work related to automated composition of refactorings.
1485 \subsection{Refactoring safety}
1486 This section presents a couple of approaches to improving the safety of
1487 performing refactorings. In these approaches, the problems that are addressed
1488 are not compilation problems, but behavior-altering problems that are not easily
1489 discovered during static analysis of source code. An example of such a problem
1490 is presented in \myref{sec:correctness}.
1492 \subsubsection{Project ``Safer Refactorings''}
1493 \tit{Safer Refactorings}\citing{stolzSaferRefactorings} is a proposal for a
1494 master's thesis. The proposer is my supervisor, Volker Stolz from the University
1497 The proposed solution for making refactorings safer, is to insert assertions
1498 into source code when refactoring it. For the example in
1499 \myref{lst:correctnessExtractAndMoveResult}, which is the result of a
1500 refactoring, it is suggested that we insert an assert statement between lines 9
1501 and 10. In this example, the assert statement
1502 would be \mint{java}|assert c.x == this;| which would discover the aliasing
1503 problems of this example.
1505 \subsubsection{``Making Program Refactoring Safer''}
1506 This is the name of an article\citing{soaresSafer2010} about providing a way to
1507 improve safety during refactoring. Soares et al. approaches the problem of
1508 preserving behavior during refactoring by analyzing a transformation and then
1509 generate a test suite for it, using static analysis. These tests are then run
1510 for both the before- and after-code, and is compared to assure that they are
1513 \subsection{Search-based refactoring}
1514 \tit{Search-Based Refactoring: an
1515 empirical study}\citing{okeeffeSearchBased2008} is a paper by Mark O'Keeffe and
1516 Mel Ó Cinnéide published in 2008. The authors present an empirical study of
1517 different algorithmic approaches to search-based refactoring.
1519 The common approach for all these algorithms is to generate a set of changes to
1520 a program for then to use a ``fitness function'' to evaluate if they improve its
1521 design or not. The fitness function consists of a weighted sum of different
1522 object-oriented metrics.
1524 Among other things, the authors conclude that even with small input programs,
1525 their solution representation is memory-intensive, at least for some algorithms.
1526 The programs they refactor on have in average 4,000 lines of code, spread over
1527 57 classes. I.e. considerably smaller than one of the programs that will be
1528 subject to refactoring in this project.
1531 \subsection{The compositional paradigm of refactoring}
1532 This paradigm builds upon the observation of Vakilian et
1533 al.\citing{vakilian2012}, that of the many automated refactorings existing in
1534 modern IDEs, the simplest ones are dominating the usage statistics. The report
1535 mainly focuses on \name{Eclipse} as the tool under investigation.
1537 The paradigm is described almost as the opposite of automated composition of
1538 refactorings \see{compositeRefactorings}. It works by providing the programmer
1539 with easily accessible primitive refactorings. These refactorings shall be
1540 accessed via keyboard shortcuts or quick-assist menus\footnote{Think
1541 quick-assist with Ctrl+1 in \name{Eclipse}} and be promptly executed, opposed to in the
1542 currently dominating wizard-based refactoring paradigm. They are meant to
1543 stimulate composing smaller refactorings into more complex changes, rather than
1544 doing a large upfront configuration of a wizard-based refactoring, before
1545 previewing and executing it. The compositional paradigm of refactoring is
1546 supposed to give control back to the programmer, by supporting \himher with an
1547 option of performing small rapid changes instead of large changes with a lesser
1548 degree of control. The report authors hope this will lead to fewer unsuccessful
1549 refactorings. It also could lower the bar for understanding the steps of a
1550 larger composite refactoring and thus also help in figuring out what goes wrong
1551 if one should choose to op in on a wizard-based refactoring.
1553 Vakilian and his associates have performed a survey of the effectiveness of the
1554 compositional paradigm versus the wizard-based one. They claim to have found
1555 evidence of that the \emph{compositional paradigm} outperforms the
1556 \emph{wizard-based}. It does so by reducing automation, which seems
1557 counterintuitive. Therefore they ask the question ``What is an appropriate level
1558 of automation?'', and thus questions what they feel is a rush toward more
1559 automation in the software engineering community.
1563 \chapter{The search-based Extract and Move Method
1564 refactoring}\label{ch:extractAndMoveMethod}
1565 In this chapter I will delve into the workings of the search-based
1566 \ExtractAndMoveMethod refactoring. We will see the choices it must make along
1567 the way and why it chooses a text selection as a candidate for refactoring or
1570 After defining some concepts, I will introduce an example that will be used
1571 throughout the chapter to illustrate how the refactoring works in some simple
1574 \section{The inputs to the refactoring}
1575 For executing an \ExtractAndMoveMethod refactoring, there are two simple
1576 requirements. The first thing the refactoring needs is a text selection, telling
1577 it what to extract. Its second requirement is a target for the subsequent move
1580 The extracted method must be called instead of the selection that makes up its
1581 body. Also, the method call has to be performed via a variable, since the method
1582 is not static. Therefore, the move target must be a variable in the scope of the
1583 extracted selection. The actual new location for the extracted method will be
1584 the class representing the type of the move target variable. But, since the
1585 method also must be called through a variable, it makes sense to define the move
1586 target to be either a local variable or a field in the scope of the text
1589 \section{Defining a text selection}
1590 A text selection, in our context, is very similar to what you think of when
1591 selecting a bit of text in your editor or other text processing tool with your
1592 mouse or keyboard. It is an abstract construct that is meant to capture which
1593 specific portion of text we are about to deal with.
1595 To be able to clearly reason about a text selection done to a portion of text in
1596 a computer file, which consists of pure text, we put up the following
1599 \definition{A \emph{text selection} in a text file is defined by two
1600 non-negative integers, in addition to a reference to the file itself. The first
1601 integer is an offset into the file, while the second reference is the length of
1602 the text selection.}
1604 This means that the selected text consist of a number of characters equal to the
1605 length of the selection, where the first character is found at the specified
1608 \section{Where we look for text selections}
1610 \subsection{Text selections are found in methods}
1611 The text selections we are interested in are those that surround program
1612 statements. Therefore, the place we look for selections that can form candidates
1613 for an execution of the \ExtractAndMoveMethod refactoring, is within the body of
1616 \paragraph{On ignoring static methods}
1617 In this project we are not analyzing static methods for candidates to the
1618 \ExtractAndMoveMethod refactoring. The reason for this is that in the cases
1619 where we want to perform the refactoring for a selection within a static method,
1620 the first step is to extract the selection into a new method. Hence this method
1621 also becomes static, since it must be possible to call it from a static context.
1622 It would then be difficult to move the method to another class, make it
1623 non-static and calling it through a variable. To avoid these obstacles, we
1624 simply ignore static methods.
1626 \begin{listing}[htb]
1627 \def\charwidth{5.8pt}
1628 \def\indent{2*\charwidth}
1629 \def\lineheight{\baselineskip}
1630 \def\mintedtop{2*\lineheight+5.8pt}
1632 \begin{tikzpicture}[overlay, yscale=-1, xshift=3.8pt+\charwidth*31]
1633 \tikzstyle{overlaybox}=[fill=lightgray,opacity=0.2]
1635 \draw[overlaybox] (\indent,\mintedtop+\lineheight*4) rectangle
1636 +(23*\charwidth,17*\lineheight);
1639 \draw[overlaybox] (2*\indent,\mintedtop+5*\lineheight) rectangle
1640 +(15*\charwidth,3*\lineheight);
1641 \draw[overlaybox] (2*\indent,\mintedtop+15*\lineheight) rectangle
1642 +(15*\charwidth,3*\lineheight);
1643 \draw[overlaybox] (2*\indent,\mintedtop+19*\lineheight) rectangle
1644 +(15*\charwidth,\lineheight);
1646 \begin{multicols}{2}
1647 \begin{minted}[linenos,frame=topline,label=Clean,framesep=\mintedframesep]{java}
1649 A a; B b; boolean bool;
1651 void method(int val) {
1675 \begin{minted}[frame=topline,label={With statement
1676 sequences},framesep=\mintedframesep]{java}
1678 A a; B b; boolean bool;
1680 void method(int val) {
1703 \caption{Classes \type{A} and \type{B} are both public. The methods
1704 \method{foo} and \method{bar} are public members of class \type{A}.}
1705 \label{lst:grandExample}
1708 \subsection{The possible text selections of a method body}
1709 The number of possible text selections that can be made from the text in a
1710 method body, are equal to all the sub-sequences of characters within it. For our
1711 purposes, analyzing program source code, we must define what it means for a text
1712 selection to be valid.
1714 \definition{A \emph{valid text selection} is a text selection that contains all
1715 of one or more consecutive program statements.}
1717 For a sequence of statements, the text selections that can be made from it, are
1718 equal to all its sub-sequences. \Myref{lst:textSelectionsExample} show an
1719 example of all the text selections that can be made from the code in
1720 \myref{lst:grandExample}, lines 16-18. For convenience and the clarity of this
1721 example, the text selections are represented as tuples with the start and end
1722 line of all selections: $\{(16), (17), (18), (16,17), (16,18), (17,18)\}$.
1724 \begin{listing}[htb]
1725 \def\charwidth{5.7pt}
1726 \def\indent{4*\charwidth}
1727 \def\lineheight{\baselineskip}
1728 \def\mintedtop{\lineheight-1pt}
1730 \begin{tikzpicture}[overlay, yscale=-1]
1731 \tikzstyle{overlaybox}=[fill=lightgray,opacity=0.2]
1734 \draw[overlaybox] (2*\charwidth,\mintedtop) rectangle
1735 +(16*\charwidth,\lineheight);
1738 \draw[overlaybox] (2*\charwidth,\mintedtop+\lineheight) rectangle
1739 +(16*\charwidth,\lineheight);
1742 \draw[overlaybox] (2*\charwidth,\mintedtop+2*\lineheight) rectangle
1743 +(16*\charwidth,\lineheight);
1745 \draw[overlaybox] (\indent-3*\charwidth,\mintedtop) rectangle
1746 +(18*\charwidth,2*\lineheight);
1748 \draw[overlaybox] (3*\charwidth,\mintedtop+\lineheight) rectangle
1749 +(14*\charwidth,2*\lineheight);
1752 \draw[overlaybox] (\indent,\mintedtop) rectangle
1753 +(12*\charwidth,3*\lineheight);
1755 % indent should be 5 spaces
1756 \begin{minted}[linenos,firstnumber=16]{java}
1761 \caption{Example of how the text selections generator would generate text
1762 selections based on a lists of statements. Each highlighted rectangle
1763 represents a text selection.}
1764 \label{lst:textSelectionsExample}
1767 Each nesting level of a method body can have many such sequences of statements.
1768 The outermost nesting level has one such sequence, and each branch contains
1769 its own sequence of statements. \Myref{lst:grandExample} has a version of some
1770 code where all such sequences of statements are highlighted for a method body.
1772 To complete our example of possible text selections, I will now list all
1773 possible text selections for the method in \myref{lst:grandExample}, by nesting
1774 level. There are 23 of them in total.
1777 \item[Level 1 (10 selections)] \hfill \\
1778 $\{(5,9), (11), (12), (14,21), (5,11), (5,12), (5,21), (11,12),
1779 (11,21), \\(12,21)\}$
1781 \item[Level 2 (13 selections)] \hfill \\
1782 $\{(6), (7), (8), (6,7), (6,8), (7,8), (16), (17), (18), (16,17), (16,18), \\
1786 \subsubsection{The complexity}\label{sec:complexity}
1787 The complexity of how many text selections that need to be analyzed for a body
1788 of in total $n$ statements, is bounded by $O(n^2)$. A body of statements is here
1789 all the statements in all nesting levels of a sequence of statements. A method
1790 body (or a block) is a body of statements. To prove that the complexity is
1791 bounded by $O(n^2)$, I present a couple of theorems and prove them.
1794 The number of text selections that need to be analyzed for each list of
1795 statements of length $n$, is exactly
1798 \sum_{i=1}^{n} i = \frac{n(n+1)}{2}
1799 \label{eq:complexityStatementList}
1801 \label{thm:numberOfTextSelection}
1805 For $n=1$ this is trivial: $\frac{1(1+1)}{2} = \frac{2}{2} = 1$. One statement
1806 equals one selection.
1808 For $n=2$, you get one text selection for the first statement, one selection
1809 for the second statement, and one selection for the two of them combined.
1810 This equals three selections. $\frac{2(2+1)}{2} = \frac{6}{2} = 3$.
1812 For $n=3$, you get 3 selections for the two first statements, as in the case
1813 where $n=2$. In addition you get one selection for the third statement itself,
1814 and two more statements for the combinations of it with the two previous
1815 statements. This equals six selections. $\frac{3(3+1)}{2} = \frac{12}{2} = 6$.
1817 Assume that for $n=k$ there exists $\frac{k(k+1)}{2}$ text selections. Then we
1818 want to add selections for another statement, following the previous $k$
1819 statements. So, for $n=k+1$, we get one additional selection for the statement
1820 itself. Then we get one selection for each pair of the new selection and the
1821 previous $k$ statements. So the total number of selections will be the number
1822 of already generated selections, plus $k$ for every pair, plus one for the
1823 statement itself: $\frac{k(k+1)}{2} + k +
1824 1 = \frac{k(k+1)+2k+2}{2} = \frac{k(k+1)+2(k+1)}{2} = \frac{(k+1)(k+2)}{2} =
1825 \frac{(k+1)((k+1)+1)}{2} = \sum_{i=1}^{k+1} i$
1828 %\definition{A \emph{body of statements} is a sequence of statements where every
1829 %statement may have sub-statements.}
1832 The number of text selections for a body of statements is maximized if all the
1833 statements are at the same level.
1834 \label{thm:textSelectionsMaximized}
1838 Assume we have a body of, in total, $k$ statements. Then, the sum of the
1839 lengths of all the lists of statements in the body, is also $k$. Let
1840 $\{l,\ldots,m,(k-l-\ldots-m)\}$ be the lengths of the lists of statements in
1841 the body, with $l+\ldots+m<k \Rightarrow \forall i \in \{l,\ldots,m\} : i < k$.
1843 Then, the number of text selections that are generated for the $k$ statements
1849 \frac{l(l+1)}{2} + \ldots + \frac{m(m+1)}{2} +
1850 \frac{(k-l-\ldots-m)((k-l-\ldots-m)+ 1)}{2} = \\
1851 \frac{l^2+l}{2} + \ldots + \frac{m^2+m}{2} + \frac{k^2 - 2kl - \ldots - 2km +
1852 l^2 + \ldots + m^2 + k - l - \ldots - m}{2} = \\
1853 \frac{2l^2 - 2kl + \ldots + 2m^2 - 2km + k^2 + k}{2}
1857 \noindent It then remains to show that this inequality holds:
1860 \frac{2l^2 - 2kl + \ldots + 2m^2 - 2km + k^2 + k}{2} < \frac{k(k+1)}{2} =
1864 \noindent By multiplication by $2$ on both sides, and by removing the equal
1868 2l^2 - 2kl + \ldots + 2m^2 - 2km < 0
1871 Since $\forall i \in \{l,\ldots,m\} : i < k$, we have that $\forall i \in
1872 \{l,\ldots,m\} : 2ki > 2i^2$, so all the pairs of parts on the form $2i^2-2ki$
1873 are negative. In sum, the inequality holds.
1877 Therefore, the complexity for the number of selections that need to be analyzed
1878 for a body of $n$ statements is $O\bigl(\frac{n(n+1)}{2}\bigr) = O(n^2)$.
1880 \section{Disqualifying a selection}
1881 Certain text selections would lead to broken code if used as input to the
1882 \ExtractAndMoveMethod refactoring. To avoid this, we have to check all text
1883 selections for such conditions before they are further analyzed. This section
1884 is therefore going to present some properties that make a selection unsuitable
1885 for our refactoring. When analyzing all these properties, it is assumed that the
1886 source code does not contain any compilation errors.
1888 \subsection{A call to a protected or package-private method}
1889 If a text selection contains a call to a protected or package-private method, it
1890 would not be safe to move it to another class. The reason for this, is that we
1891 cannot know if the called method is being overridden by some subclass of the
1892 \gloss{enclosingClass}, or not.
1894 Imagine that the protected method \method{foo} is declared in class \m{A},
1895 and overridden in class \m{B}. The method \method{foo} is called from within a
1896 selection done to a method in \m{A}. We want to extract and move this selection
1897 to another class. The method \method{foo} is not public, so the \MoveMethod
1898 refactoring must make it public, making the extracted method able to call it
1899 from the extracted method's new location. The problem is, that the now public
1900 method \method{foo} is overridden in a subclass, where it has a protected
1901 status. This makes the compiler complain that the subclass \m{B} is trying to
1902 reduce the visibility of a method declared in its superclass \m{A}. This is not
1903 allowed in Java, and for good reasons. It would make it possible to make a
1904 subclass that could not be a substitute for its superclass.
1906 The problem this check helps to avoid, is a little subtle. The problem does not
1907 arise in the class where the change is done, but in a class derived from it.
1908 This shows that classes acting as superclasses are especially fragile to
1909 introducing errors in the context of automated refactoring.
1911 This is also shown in bug\ldots \todoin{File Eclipse bug report}
1914 \subsection{A double class instance creation}
1915 The following is a problem caused solely by the underlying \MoveMethod
1916 refactoring. The problem occurs if two classes are instantiated such that the
1917 first constructor invocation is an argument to a second, and that the first
1918 constructor invocation takes an argument that is built up using a field. As an
1919 example, say that \var{name} is a field of the enclosing class, and we have the
1920 expression \code{new A(new B(name))}. If this expression is located in a
1921 selection that is moved to another class, \var{name} will be left untouched,
1922 instead of being prefixed with a variable of the same type as it is declared in.
1923 If \var{name} is the destination for the move, it is not replaced by
1924 \code{this}, or removed if it is a prefix to a member access
1925 (\code{name.member}), but it is still left by itself.
1927 Situations like this would lead to code that will not compile. Therefore, we
1928 have to avoid them by not allowing selections to contain such double class
1929 instance creations that also contain references to fields.
1931 \todoin{File Eclipse bug report}
1934 \subsection{Instantiation of non-static inner class}
1935 When a non-static inner class is instantiated, this must happen in the scope of
1936 its declaring class. This is because it must have access to the members of the
1937 declaring class. If the inner class is public, it is possible to instantiate it
1938 through an instance of its declaring class, but this is not handled by the
1939 underlying \MoveMethod refactoring.
1941 Performing a move on a method that instantiates a non-static inner class, will
1942 break the code if the instantiation is not handled properly. For this reason,
1943 selections that contain instantiations of non-static inner classes are deemed
1944 unsuitable for the \ExtractAndMoveMethod refactoring.
1946 \subsection{References to enclosing instances of the enclosing class}
1947 To ``reference an enclosing instance of the enclosing class'' is to reference
1948 another instance than the one for the immediately enclosing class. Imagine there
1949 is a (non-static) class \m{C} that is declared in the inner scope of another
1950 class. That class can again be nested inside a third class, and so on. Hence,
1951 the nested class \m{C} can have access to many enclosing instances of its
1952 innermost enclosing class. A selection in a method declared in class \m{C} is
1953 disqualified if it contains a statement that contains a reference to one or more
1954 instances of these enclosing classes of \m{C}.
1956 The problem with this, is that these references may not be valid if they are
1957 moved to another class. Theoretically, some situations could easily be solved by
1958 passing, to the moved method, a reference to the instance where the problematic
1959 referenced member is declared. This should work in the case where this member is
1960 publicly accessible. This is not done in the underlying \MoveMethod refactoring,
1961 so it cannot be allowed in the \ExtractAndMoveMethod refactoring either.
1963 \subsection{Inconsistent return statements}
1964 To verify that a text selection is consistent with respect to return statements,
1965 we must check that if a selection contains a return statement, then every
1966 possible execution path within the selection ends in either a return or a throw
1967 statement. This property is important regarding the \ExtractMethod refactoring.
1968 If it holds, it means that a method could be extracted from the selection, and a
1969 call to it could be substituted for the selection. If the method has a non-void
1970 return type, then a call to it would also be a valid return point for the
1971 calling method. If its return value is of the void type, then the \ExtractMethod
1972 refactoring will append an empty return statement to the back of the method
1973 call. Therefore, the analysis does not discriminate on either kind of return
1974 statements, with or without a return value.
1976 A \emph{throw} statement is accepted anywhere a return statement is required.
1977 This is because a throw statement causes an immediate exit from the current
1978 block, together with all outer blocks in its control flow that does not catch
1979 the thrown exception.
1981 We separate between explicit and implicit return statements. An \emph{explicit}
1982 return statement is formed by using the \code{return} keyword, while an
1983 \emph{implicit} return statement is a statement that is not formed using
1984 \code{return}, but must be the last statement of a method that can have any side
1985 effects. This can happen in methods with a void return type. An example is a
1986 statement that is inside one or more blocks. The last statement of a method
1987 could for instance be a synchronized statement, but the last statement that is
1988 executed in the method, and that can have any side effects, may be located
1989 inside the body of the synchronized statement.
1991 We can start the check for this property by looking at the last statement of a
1992 selection to see if it is a return statement (explicit or implicit) or a throw
1993 statement. If this is the case, then the property holds, assuming the selected
1994 code do not contain any compilation errors. All execution paths within the
1995 selection should end in either this, or another, return or throw statement.
1997 If the last statement of the selection is not a \emph{return} or \emph{throw},
1998 the execution of it must eventually end in one of these types of statements for
1999 the selection to be legal. This means that all branches of the last statement of
2000 every branch must end in a return or throw. Given this recursive definition,
2001 there are only five types of statements that are guaranteed to end in a return
2002 or throw if their child branches do. All other statements would have to be
2003 considered illegal. The first three: Block-statements, labeled statements and
2004 do-statements are all kinds of fall-through statements that always get their
2005 body executed. Do-statements would not make much sense if written such that they
2006 always end after the first round of execution of their body, but that is not our
2007 concern. The remaining two statements that can end in a return or throw are
2008 if-statements and try-statements.
2010 For an if-statement, the rule is that if its then-part does not contain any
2011 return or throw statements, this is considered illegal. If the then-part does
2012 contain a return or throw, the else-part is checked. If its else-part is
2013 non-existent, or it does not contain any return or throw statements, the
2014 statement is considered illegal. If an if-statement is not considered illegal,
2015 the bodies of its two parts must be checked.
2017 Try-statements are handled much the same way as if-statements. The body of a
2018 try-statement must contain a return or throw. The same applies to its catch
2019 clauses and finally body. \todoin{finally body?}
2021 \subsection{Ambiguous return values}
2022 The problem with ambiguous return values arises when a selection is chosen to be
2023 extracted into a new method, but if refactored it needs to return more than one
2024 value from that method.
2026 This problem occurs in two situations. The first situation arises when there is
2027 more than one local variable that is both assigned to within a selection and
2028 also referenced after the selection. The other situation occurs when there is
2029 only one such assignment, but the selection also contain return statements.
2031 Therefore we must examine the selection for assignments to local variables that
2032 are referenced after the text selection. Then we must verify that not more than
2033 one such reference is done, or zero if any return statements are found.
2035 \subsection{Illegal statements}
2036 An illegal statement may be a statement that is of a type that is never allowed,
2037 or it may be a statement of a type that is only allowed if certain conditions
2040 Any use of the \var{super} keyword is prohibited, since its meaning is altered
2041 when moving a method to another class.
2043 For a \emph{break} statement, there are two situations to consider: A break
2044 statement with or without a label. If the break statement has a label, it is
2045 checked that whole of the labeled statement is inside the selection. If the
2046 break statement does not have a label attached to it, it is checked that its
2047 innermost enclosing loop or switch statement also is inside the selection.
2049 The situation for a \emph{continue} statement is the same as for a break
2050 statement, except that it is not allowed inside switch statements.
2052 Regarding \emph{assignments}, two types of assignments are allowed: Assignments
2053 to non-final variables and assignments to array access. All other assignments
2054 are regarded illegal.
2056 \paragraph{Incompleteness.} The list of illegal statements is not complete, and
2057 a lot of situations that can lead to compilation errors or behavior changes are
2058 not considered. It is not feasible to consider all such situations within the
2059 limits of this master's project, and maybe not outside of them either. The
2060 feasibility of this problem could be explored further by others.
2062 \section{Disqualifying selections from the
2063 example}\label{sec:disqualifyingExample}
2064 Among the selections we found for the code in \myref{lst:grandExample}, not many
2065 of them must be disqualified on the basis of containing something illegal. The
2066 only statement causing trouble is the break statement in line 18. None of the
2067 selections on nesting level 2 can contain this break statement, since the
2068 innermost switch statement is not inside any of these selections.
2070 This means that the text selections $(18)$, $(16,18)$ and $(17,18)$ can be
2071 excluded from further consideration, and we are left with the following
2075 \item[Level 1 (10 selections)] \hfill \\
2076 $\{(5,9), (11), (12), (14,21), (5,11), (5,12), (5,21), (11,12),
2077 (11,21), \\(12,21)\}$
2079 \item[Level 2 (10 selections)] \hfill \\
2080 $\{(6), (7), (8), (6,7), (6,8), (7,8), (16), (17), (16,17), (20)\}$
2083 \section{Finding a move target}
2084 In the analysis needed to perform the \ExtractAndMoveMethod refactoring
2085 automatically, the selection we choose is found among all the selections that
2086 have a possible move target. Therefore, the best possible move target must be
2087 found for all the candidate selections, so that we are able to sort out the
2088 selection that is best suited for the refactoring.
2090 To find the best move target for a specific text selection, we first need to
2091 find all the possible targets. Since the target must be a local variable or a
2092 field, we are basically looking for names within the selection; names that
2093 represents references to variables.
2095 The names we are looking for, we call prefixes. This is because we are not
2096 interested in names that occur in the middle of a dot-separated sequence of
2097 names. We are only interested in names constituting prefixes of other names, and
2098 possibly themselves. The reason for this, is that two lexically equal names need
2099 not be referencing the same variable, if they themselves are not referenced via
2100 the same prefix. Consider the two method calls \code{a.x.foo()} and
2101 \code{b.x.foo()}. Here, the two references to \code{x}, in the middle of the
2102 qualified names both preceding \code{foo()}, are not referencing the same
2103 variable. Even though the variables may share the type, and the method
2104 \method{foo} thus is the same for both, we would not know through which of the
2105 variables \var{a} or \var{b} we should call the extracted method.
2107 The possible move targets are then the prefixes that are not among a subset of
2108 the prefixes that are not valid move targets \see{s:unfixes}. Also, prefixes
2109 that are just simple names, and have only one occurrence, are left out. This is
2110 because they are not going to have any positive effect on coupling between
2111 classes, and are only going to increase the complexity of the code.
2113 For finding the best move target among these safe prefixes, a simple heuristic
2114 is used. It is as simple as choosing the prefix that is most frequently
2115 referenced within the selection.
2117 \section{Unfixes}\label{s:unfixes}
2118 We will call the prefixes that are not valid as move targets for unfixes.
2120 A name that is assigned to within a selection can be an unfix. The reason for
2121 this is that the result would be an assignment to the \type{this} keyword, which
2122 is not valid in Java \see{eclipse_bug_420726}.
2124 Prefixes that originate from variable declarations within the same selection are
2125 also considered unfixes. The reason for this is that when a method is moved, it
2126 needs to be called through a variable. If this variable is also declared within
2127 the method that is to be moved, this obviously cannot be done.
2129 Also considered as unfixes are variable references that are of types that are
2130 not suitable for moving methods to. This can either be because it is not
2131 physically possible to move a method to the desired class or that it will cause
2132 compilation errors by doing so.
2134 If the type binding for a name is not resolved it is considered an unfix. The
2135 same applies to types that are only found in compiled code, so they have no
2136 underlying source that is accessible to us. (E.g. the \type{java.lang.String}
2139 Interface types are not suitable as targets. This is simply because interfaces
2140 in Java cannot contain methods with bodies. (This thesis does not deal with
2141 features of Java versions later than Java 7. Java 8 has interfaces with default
2142 implementations of methods.)
2144 Neither are local types allowed. This accounts for both local and anonymous
2145 classes. Anonymous classes are effectively the same as interface types with
2146 respect to unfixes. Local classes could in theory be used as targets, but this
2147 is not possible due to limitations of the way the \refa{Extract and Move Method}
2148 refactoring has to be implemented. The problem is that the refactoring is done
2149 in two steps, so the intermediate state between the two refactorings would not
2150 be legal Java code. In the intermediate step for the case where a local class is
2151 the move target, the extracted method would need to take the local class as a
2152 parameter. This new method would need to live in the scope of the declaring
2153 class of the originating method. The local class would then not be in the scope
2154 of the extracted method, thus bringing the source code into an illegal state.
2155 This scenario is shown in \myref{lst:extractMethodLocalClass}. One could imagine
2156 that the method was extracted and moved in one operation, without an
2157 intermediate state. Then it would make sense to include variables with types of
2158 local classes in the set of legal targets, since the local classes would then be
2159 in the scopes of the method calls. If this makes any difference for software
2160 metrics that measure coupling would be a different discussion.
2162 \todoin{highlight code!}
2164 \begin{listing}[htb]
2165 \begin{multicols}{2}
2166 \begin{minted}[frame=topline,label=Before,framesep=\mintedframesep]{java}
2167 void declaresLocalClass() {
2182 \begin{minted}[frame=topline,label={After Extract
2183 Method},framesep=\mintedframesep]{java}
2184 void declaresLocalClass() {
2195 // Illegal intermediate step
2196 void fooBar(LocalClass inst) {
2202 \caption{The \refa{Extract and Move Method} refactoring bringing the code into
2203 an illegal state with an intermediate step.}
2204 \label{lst:extractMethodLocalClass}
2207 The last class of names that are considered unfixes are names used in null
2208 tests. These are tests that read like this: if \code{<name>} equals \var{null}
2209 then do something. If allowing variables used in those kinds of expressions as
2210 targets for moving methods, we would end up with code containing boolean
2211 expressions like \code{this == null}, which would always evaluate to
2212 \code{false}, since \var{this} would never be \var{null}. The existence of a
2213 null test indicates that a variable is expected to sometimes hold the value
2214 \var{null}. By using a variable used in a null test as a move target, we could
2215 potentially end up with a
2216 null pointer exception if the method is called on a variable with a null value.
2218 \section{Finding the example selections that have possible targets}
2219 We now pick up the thread from \myref{sec:disqualifyingExample} where we have a
2220 set of text selections that need to be analyzed to find out if some of them are
2221 suitable targets for the \ExtractAndMoveMethod refactoring.
2223 We start by analyzing the text selections for nesting level 2, because these
2224 results can be used to reason about the selections for nesting level 1. First we
2225 have all the single-statement selections.
2228 \item[Selections $(6)$, $(8)$ and $(20)$.] \hfill \\
2229 All these selections have a prefix that contains a possible target, namely
2230 the variable \var{a}. The problem is that the prefixes are only one segment
2231 long, and their frequency counts are only 1 as well. None of these
2232 selections are therefore considered as suitable candidates for the
2235 \item[Selection $(7)$.] \hfill \\
2236 This selection contains the unfix \var{a}, and no other possible targets.
2237 The reason for \var{a} being an unfix is that it is assigned to within the
2238 selection. Selection $(7)$ is therefore unsuited as a refactoring candidate.
2240 \item[Selections $(16)$ and $(17)$.] \hfill \\
2241 These selections both have a possible target. The target for both selections
2242 is the variable \var{b}. Both the prefixes have frequency 1. We denote this
2243 with the new tuples $((16), \texttt{b.a}, f(1))$ and $((17), \texttt{b.a},
2244 f(1))$. They contain the selection, the prefix with the target and the
2245 frequency for this prefix.
2249 Then we have all the text selections from level 2 that are composed of multiple
2253 \item[Selections $(6,7)$, $(6,8)$ and $(7,8)$.] \hfill \\
2254 All these selections are disqualified for the reason that they contain the
2255 unfix \var{a}, due to the assignment, and no other possible move targets.
2257 \item[Selection $(16,17)$.] \hfill \\
2258 This selection is the last selection that is not analyzed on nesting level
2259 2. It contains only one possible move target, and that is the variable
2260 \var{b}. It also contains only one prefix \var{b.a}, with frequency count
2261 2. Therefore we have a new candidate $((16,17), \texttt{b.a}, f(2))$.
2265 Moving on to the text selections for nesting level 1, starting with the
2266 single-statement selections:
2269 \item[Selection $(5,9)$.] \hfill \\
2270 This selection contains two variable references that must be analyzed to see
2271 if they are possible move candidates. The first one is the variable
2272 \var{bool}. This variable is of type \type{boolean}, which is a primary type
2273 and therefore not possible to make any changes to. The second variable is
2274 \var{a}. The variable \var{a} is an unfix in $(5,9)$, for the same reason as
2275 in the selections $(6,7)$, $(7,8)$ and $(6,8)$. So selection $(5,9)$
2276 contains no possible move targets.
2278 \item[Selections $(11)$ and $(12)$.] \hfill \\
2279 These selections are disqualified for the same reasons as selections $(6)$
2280 and $(8)$. Their prefixes are one segment long and are referenced only one
2283 \item[Selection $(14,21)$] \hfill \\
2284 This is the switch statement from \myref{lst:grandExample}. It contains the
2285 relevant variable references \var{val}, \var{a} and \var{b}. The variable
2286 \var{val} is a primary type, just as \var{bool}. The variable \var{a} is
2287 only found in one statement, and in a prefix with only one segment, so it is
2288 not considered to be a possible move target. The only variable left is
2289 \var{b}. Just as in the selection $(16,17)$, \var{b} is part of the prefix
2290 \code{b.a}, which has 2 appearances. We have found a new candidate
2291 $((14,21), \texttt{b.a}, f(2))$.
2295 It remains to see if we get any new candidates by analyzing the multi-statement
2296 text selections for nesting level 1:
2299 \item[Selections $(5,11)$ and $(5,12)$.] \hfill \\
2300 These selections are disqualified for the same reason as $(5,9)$. The only
2301 possible move target \var{a} is an unfix.
2303 \item[Selection $(5,21)$.] \hfill \\
2304 This is whole of the method body in \myref{lst:grandExample}. The variables
2305 \var{a}, \var{bool} and \var{val} are either an unfix or primary types. The
2306 variable \var{b} is the only possible move target, and we have, again, the
2307 prefix \code{b.a} as the center of attention. The refactoring candidate is
2308 $((5,21), \texttt{b.a}, f(2))$.
2310 \item[Selection $(11,12)$.] \hfill \\
2311 This small selection contains the prefix \code{a} with frequency 2, and no
2312 unfixes. The candidate is $((11,12), \texttt{a}, f(2))$.
2314 \item[Selection $(11,21)$] \hfill \\
2315 This selection contains the selection $(11,12)$ in addition to the switch
2316 statement. The selection has two possible move targets. The first one is
2317 \var{b}, in a prefix with frequency 2. The second is \var{a}, although it
2318 is in a simple prefix, it is referenced 3 times, and is therefore chosen
2319 as the target for the selection. The new candidate is $((11,21),
2322 \item[Selection $(12,21)$.] \hfill \\
2323 This selection is similar to the previous $(11,21)$, only that \var{a} now
2324 has frequency count 2. This means that the prefixes \code{a} and
2325 \code{b.a} both have the count 2. Of the two, \code{b.a} is preferred,
2326 since it has more segments than \code{a}. Thus the candidate for this
2327 selection is $((12,21), \texttt{b.a}, f(2))$.
2331 For the method in \myref{lst:grandExample} we therefore have the following 8
2332 candidates for the \ExtractAndMoveMethod refactoring: $\{((16), \texttt{b.a},
2333 f(1)), ((17), \texttt{b.a}, f(1)), ((16,17), \texttt{b.a}, f(2)), ((14,21),
2334 \texttt{b.a}, f(2)), \\ ((5,21), \texttt{b.a}, f(2)), ((11,12), \texttt{a},
2335 f(2)), ((11,21), \texttt{a}, f(3)), ((12,21), \texttt{b.a}, f(2))\}$.
2337 It now only remains to specify an order for these candidates, so we can choose
2338 the most suitable one to refactor.
2341 \section{Choosing the selection}\label{sec:choosingSelection}
2342 When choosing a selection between the text selections that have possible move
2343 targets, the selections need to be ordered. The criteria below are presented in
2344 the order they are prioritized. If not one selection is favored over the other
2345 for a concrete criterion, the selections are evaluated by the next criterion.
2348 \item The first criterion that is evaluated is whether a selection contains
2349 any unfixes or not. If selection \m{A} contains no unfixes, while selection
2350 \m{B} does, selection \m{A} is favored over selection \m{B}. This is
2351 because, if we can, we want to avoid moving such as assignments and variable
2352 declarations. This is done under the assumption that, if possible, avoiding
2353 selections containing unfixes will make the code moved a little cleaner.
2355 \item The second criterion that is evaluated is whether a selection contains
2356 multiple possible targets or not. If selection \m{A} has only one possible
2357 target, and selection \m{B} has multiple, selection \m{A} is favored. If
2358 both selections have multiple possible targets, they are considered equal
2359 with respect to this criterion. The rationale for this heuristic is that we
2360 would prefer not to introduce new couplings between classes when performing
2361 the \ExtractAndMoveMethod refactoring.
2363 \item When evaluating this criterion, this is with the knowledge that
2364 selection \m{A} and \m{B} both have one possible target, or multiple
2365 possible targets. Then, if the move target candidate of selection \m{A} has
2366 a higher reference count than the target candidate of selection \m{B},
2367 selection \m{A} is favored. The reason for this is that we would like to
2368 move the selection that gets rid of the most references to another class.
2370 \item The last criterion is that if the frequencies of the targets chosen for
2371 both selections are equal, the selection with the target that is part of the
2372 prefix with highest number of segments is favored. This is done to favor
2377 If none of the above mentioned criteria favors one selection over another, the
2378 selections are considered to be equally good candidates for the
2379 \ExtractAndMoveMethod refactoring.
2381 \section{Performing changes}
2382 When a text selection and a move target is found for the \ExtractAndMoveMethod
2383 refactoring, the actual changes are executed by two existing primitive
2384 refactorings. First the \ExtractMethod refactoring is used to extract the
2385 selection into a new method. Then the \MoveMethod refactoring is used to move
2386 that new method to the class determined by the move target.
2388 If, at any point, an exception is thrown or the preconditions for one of the
2389 primitive refactorings are not satisfied, the composite refactoring is aborted,
2390 and the source code is left in its current state. This has the implication that
2391 the \ExtractAndMoveMethod refactoring could end up being partially executed.
2392 This happens if the \ExtractMethod refactoring is executed, but the \MoveMethod
2393 refactoring is being canceled. A partial execution is not considered a problem,
2394 since the code should still compile.
2396 \todoin{Pointing to implementation chapter}
2398 \section{Concluding the example}
2399 For choosing one of the remaining selections, we need to order our candidates
2400 after the criteria in the previous section. Below we have the candidates ordered
2401 in descending order, with the ``best'' candidate first:
2403 \begin{multicols}{2}
2405 \item $((16,17), \texttt{b.a}, f(2))$
2406 \item $((11,12), \texttt{a}, f(2))$
2407 \item $((16), \texttt{b.a}, f(1))$
2408 \item $((17), \texttt{b.a}, f(1))$
2411 % Many possible targets
2412 \item $((11,21), \texttt{a}, f(3))$
2413 \item $((5,21), \texttt{b.a}, f(2))$
2414 \item $((12,21), \texttt{b.a}, f(2))$
2415 \item $((14,21), \texttt{b.a}, f(2))$
2440 The candidates ordered 5-8 all have unfixes in them, therefore they are ordered
2441 last. None of the candidates ordered 1-4 have multiple possible targets. The
2442 only interesting discussion is now why $(16,17)$ was favored over $(11,12)$.
2443 This is because the prefix \code{b.a} enclosing the move target of selection
2444 $(16,17)$ has one more segment than the prefix \code{a} of $(11,12)$.
2446 The selection is now extracted into a new method \method{gen\_123} and then
2447 moved to class \type{B}, since that is the type of the variable \var{b} that is
2448 the target from the chosen refactoring candidate. The name of the method has a
2449 randomly generated suffix. In the refactoring implementation, the extracted
2450 methods follow the pattern \code{generated\_<long>}, where \code{<long>} is a
2451 pseudo-random long value. This is shortened here to make the example readable.
2452 The result after the refactoring is shown in \myref{lst:grandExampleResult}.
2454 \begin{listing}[htb]
2455 \begin{multicols}{2}
2456 \begin{minted}[linenos]{java}
2458 A a; B b; boolean bool;
2460 void method(int val) {
2483 \begin{minted}[]{java}
2487 public void gen_123(C c) {
2495 \caption{The result after refactoring the class \type{C} of
2496 \myref{lst:grandExample} with the \ExtractAndMoveMethod refactoring with
2497 $((16,17), \texttt{b.a}, f(2))$ as input.}
2498 \label{lst:grandExampleResult}
2502 \chapter{Refactorings in Eclipse JDT: Design and
2503 shortcomings}\label{ch:jdt_refactorings}
2505 This chapter will deal with some of the design behind refactoring support in
2506 \name{Eclipse}, and the JDT in specific. After which it will follow a section
2507 about shortcomings of the refactoring API in terms of composition of
2511 The refactoring world of \name{Eclipse} can in general be separated into two parts: The
2512 language independent part and the part written for a specific programming
2513 language -- the language that is the target of the supported refactorings.
2514 \todo{What about the language specific part?}
2516 \subsection{The Language Toolkit}
2517 The Language Toolkit\footnote{The content of this section is a mixture of
2518 written material from
2519 \url{https://www.eclipse.org/articles/Article-LTK/ltk.html} and
2520 \url{http://www.eclipse.org/articles/article.php?file=Article-Unleashing-the-Power-of-Refactoring/index.html},
2521 the LTK source code and my own memory.}, or LTK for short, is the framework that
2522 is used to implement refactorings in \name{Eclipse}. It is language independent and
2523 provides the abstractions of a refactoring and the change it generates, in the
2524 form of the classes \typewithref{org.eclipse.ltk.core.refactoring}{Refactoring}
2525 and \typewithref{org.eclipse.ltk.core.refactoring}{Change}.
2527 There are also parts of the LTK that is concerned with user interaction, but
2528 they will not be discussed here, since they are of little value to us and our
2529 use of the framework. We are primarily interested in the parts that can be
2532 \subsubsection{The Refactoring Class}
2533 The abstract class \type{Refactoring} is the core of the LTK framework. Every
2534 refactoring that is going to be supported by the LTK has to end up creating an
2535 instance of one of its subclasses. The main responsibilities of subclasses of
2536 \type{Refactoring} are to implement template methods for condition checking
2537 (\methodwithref{org.eclipse.ltk.core.refactoring.Refactoring}{checkInitialConditions}
2539 \methodwithref{org.eclipse.ltk.core.refactoring.Refactoring}{checkFinalConditions}),
2541 \methodwithref{org.eclipse.ltk.core.refactoring.Refactoring}{createChange}
2542 method that creates and returns an instance of the \type{Change} class.
2544 If the refactoring shall support that others participate in it when it is
2545 executed, the refactoring has to be a processor-based
2546 refactoring\typeref{org.eclipse.ltk.core.refactoring.participants.ProcessorBasedRefactoring}.
2547 It then delegates to its given
2548 \typewithref{org.eclipse.ltk.core.refactoring.participants}{RefactoringProcessor}
2549 for condition checking and change creation. Participating in a refactoring can
2550 be useful in cases where the changes done to programming source code affect
2551 other related resources in the workspace. This can be names or paths in
2552 configuration files, or maybe one would like to perform additional logging of
2553 changes done in the workspace.
2555 \subsubsection{The Change Class}
2556 This class is the base class for objects that is responsible for performing the
2557 actual workspace transformations in a refactoring. The main responsibilities for
2558 its subclasses are to implement the
2559 \methodwithref{org.eclipse.ltk.core.refactoring.Change}{perform} and
2560 \methodwithref{org.eclipse.ltk.core.refactoring.Change}{isValid} methods. The
2561 \method{isValid} method verifies that the change object is valid and thus can be
2562 executed by calling its \method{perform} method. The \method{perform} method
2563 performs the desired change and returns an undo change that can be executed to
2564 reverse the effect of the transformation done by its originating change object.
2566 \subsubsection{Executing a Refactoring}\label{executing_refactoring}
2567 The life cycle of a refactoring generally follows two steps after creation:
2568 condition checking and change creation. By letting the refactoring object be
2570 \typewithref{org.eclipse.ltk.core.refactoring}{CheckConditionsOperation} that
2571 in turn is handled by a
2572 \typewithref{org.eclipse.ltk.core.refactoring}{CreateChangeOperation}, it is
2573 assured that the change creation process is managed in a proper manner.
2575 The actual execution of a change object has to follow a detailed life cycle.
2576 This life cycle is honored if the \type{CreateChangeOperation} is handled by a
2577 \typewithref{org.eclipse.ltk.core.refactoring}{PerformChangeOperation}. If also
2578 an undo manager\typeref{org.eclipse.ltk.core.refactoring.IUndoManager} is set
2579 for the \type{PerformChangeOperation}, the undo change is added into the undo
2582 \section{Shortcomings}
2583 This section is introduced naturally with a conclusion: The JDT refactoring
2584 implementation does not facilitate composition of refactorings.
2585 \todo{refine}This section will try to explain why, and also identify other
2586 shortcomings of both the usability and the readability of the JDT refactoring
2589 I will begin at the end and work my way toward the composition part of this
2592 \subsection{Absence of generics in Eclipse source code}
2593 This section is not only concerning the JDT refactoring API, but also large
2594 quantities of the \name{Eclipse} source code. The code shows a striking absence of the
2595 Java language feature of generics. It is hard to read a class' interface when
2596 methods return objects or takes parameters of raw types such as \type{List} or
2597 \type{Map}. This sometimes results in having to read a lot of source code to
2598 understand what is going on, instead of relying on the available interfaces. In
2599 addition, it results in a lot of ugly code, making the use of typecasting more
2600 of a rule than an exception.
2602 \subsection{Composite refactorings will not appear as atomic actions}
2603 When composing primitive refactorings from the JDT, it is not possible to make
2604 them appear as being executed as one change, but only as multiple small changes.
2606 \subsubsection{Missing Flexibility from JDT Refactorings}
2607 The JDT refactorings are not made with composition of refactorings in mind. When
2608 a JDT refactoring is executed, it assumes that all conditions for it to be
2609 applied successfully can be found by reading source files that have been
2610 persisted to disk. They can only operate on the actual source material, and not
2611 (in-memory) copies thereof. This constitutes a major disadvantage when trying to
2612 compose refactorings, since if an exception occurs in the middle of a sequence
2613 of refactorings, it can leave the project in a state where the composite
2614 refactoring was only partially executed. It makes it hard to discard the changes
2615 done without monitoring and consulting the undo manager, an approach that is not
2618 \subsubsection{Broken Undo History}
2619 When designing a composed refactoring that is to be performed as a sequence of
2620 refactorings, you would like it to appear as a single change to the workspace.
2621 This implies that you would also like to be able to undo all the changes done by
2622 the refactoring in a single step. This is not the way it appears when a sequence
2623 of JDT refactorings is executed. It leaves the undo history filled up with
2624 individual undo actions corresponding to every single JDT refactoring in the
2625 sequence. This problem is not trivial to handle in \name{Eclipse}
2626 \see{hacking_undo_history}.
2630 \chapter{Composite refactorings in Eclipse}
2632 \section{A simple ad hoc model}
2633 As pointed out in \myref{ch:jdt_refactorings}, the \name{Eclipse} JDT refactoring model
2634 is not very well suited for making composite refactorings. Therefore a simple
2635 model using changer objects (of type \type{RefaktorChanger}) is used as an
2636 abstraction layer on top of the existing \name{Eclipse} refactorings, instead of
2637 extending the \typewithref{org.eclipse.ltk.core.refactoring}{Refactoring} class.
2639 The use of an additional abstraction layer is a deliberate choice. It is due to
2640 the problem of creating a composite
2641 \typewithref{org.eclipse.ltk.core.refactoring}{Change} that can handle text
2642 changes that interfere with each other. Thus, a \type{RefaktorChanger} may, or
2643 may not, take advantage of one or more existing refactorings, but it is always
2644 intended to make a change to the workspace.
2646 \subsection{A typical \type{RefaktorChanger}}
2647 The typical refaktor changer class has two responsibilities, checking
2648 preconditions and executing the requested changes. This is not too different
2649 from the responsibilities of an LTK refactoring, with the distinction that a
2650 refaktor changer also executes the change, while an LTK refactoring is only
2651 responsible for creating the object that can later be used to do the job.
2653 Checking of preconditions is typically done by an
2654 \typewithref{no.uio.ifi.refaktor.analyze.analyzers}{Analyzer}. If the
2655 preconditions validate, the upcoming changes are executed by an
2656 \typewithref{no.uio.ifi.refaktor.change.executors}{Executor}.
2658 \section{The Extract and Move Method refactoring}
2659 %The Extract and Move Method Refactoring is implemented mainly using these
2662 % \item \type{ExtractAndMoveMethodChanger}
2663 % \item \type{ExtractAndMoveMethodPrefixesExtractor}
2664 % \item \type{Prefix}
2665 % \item \type{PrefixSet}
2668 \subsection{The building blocks}
2669 This is a composite refactoring, and hence is built up using several primitive
2670 refactorings. These basic building blocks are, as its name implies, the
2671 \ExtractMethod refactoring\citing{refactoring} and the \MoveMethod
2672 refactoring\citing{refactoring}. In \name{Eclipse}, the implementations of these
2673 refactorings are found in the classes
2674 \typewithref{org.eclipse.jdt.internal.corext.refactoring.code}{ExtractMethodRefactoring}
2676 \typewithref{org.eclipse.jdt.internal.corext.refactoring.structure}{MoveInstanceMethodProcessor},
2677 where the last class is designed to be used together with the processor-based
2678 \typewithref{org.eclipse.ltk.core.refactoring.participants}{MoveRefactoring}.
2680 \subsubsection{The ExtractMethodRefactoring class}
2681 This class is quite simple in its use. The only parameters it requires for
2682 construction is a compilation
2683 unit\typeref{org.eclipse.jdt.core.ICompilationUnit}, the offset into the source
2684 code where the extraction shall start, and the length of the source to be
2685 extracted. Then you have to set the method name for the new method together with
2686 its visibility and some not so interesting parameters.
2688 \subsubsection{The MoveInstanceMethodProcessor class}
2689 For the \refa{Move Method}, the processor requires a little more advanced input than
2690 the class for the \refa{Extract Method}. For construction it requires a method
2691 handle\typeref{org.eclipse.jdt.core.IMethod} for the method that is to be moved.
2692 Then the target for the move has to be supplied as the variable binding from a
2693 chosen variable declaration. In addition to this, some parameters have to be set
2694 regarding setters/getters, as well as delegation.
2696 To make the processor a working refactoring, a \type{MoveRefactoring} must be
2697 created with it as a parameter.
2699 \subsection{The ExtractAndMoveMethodChanger class}
2701 The \typewithref{no.uio.ifi.refaktor.changers}{ExtractAndMoveMethodChanger}
2702 class is a subclass of the class
2703 \typewithref{no.uio.ifi.refaktor.changers}{RefaktorChanger}. It is responsible
2704 for analyzing and finding the best target for, and also executing, a composition
2705 of the \refa{Extract Method} and \refa{Move Method} refactorings. This particular changer is
2706 the one of my changers that is closest to being a true LTK refactoring. It can
2707 be reworked to be one if the problems with overlapping changes are resolved. The
2708 changer requires a text selection and the name of the new method, or else a
2709 method name will be generated. The selection has to be of the type
2710 \typewithref{no.uio.ifi.refaktor.utils}{CompilationUnitTextSelection}. This
2711 class is a custom extension to
2712 \typewithref{org.eclipse.jface.text}{TextSelection}, that in addition to the
2713 basic offset, length and similar methods, also carry an instance of the
2714 underlying compilation unit handle for the selection.
2717 \type{ExtractAndMoveMethodAnalyzer}}\label{extractAndMoveMethodAnalyzer}
2718 The analysis and precondition checking is done by the
2719 \typewithref{no.uio.ifi.refaktor.analyze.analyzers}{ExtractAnd\-MoveMethodAnalyzer}.
2720 First is check whether the selection is a valid selection or not, with respect
2721 to statement boundaries and that it actually contains any selections. Then it
2722 checks the legality of both extracting the selection and also moving it to
2723 another class. This checking of is performed by a range of checkers
2724 \see{checkers}. If the selection is approved as legal, it is analyzed to find
2725 the presumably best target to move the extracted method to.
2727 For finding the best suitable target the analyzer is using a
2728 \typewithref{no.uio.ifi.refaktor.analyze.collectors}{PrefixesCollector} that
2729 collects all the possible candidate targets for the refactoring. All the
2730 non-candidates are found by an
2731 \typewithref{no.uio.ifi.refaktor.analyze.collectors}{UnfixesCollector} that
2732 collects all the targets that will give some kind of error if used. (For
2733 details about the property collectors, see \myref{propertyCollectors}.) All
2734 prefixes (and unfixes) are represented by a
2735 \typewithref{no.uio.ifi.refaktor.extractors}{Prefix}, and they are collected
2736 into sets of prefixes. The safe prefixes are found by subtracting from the set
2737 of candidate prefixes the prefixes that is enclosing any of the unfixes. A
2738 prefix is enclosing an unfix if the unfix is in the set of its sub-prefixes. As
2739 an example, \code{``a.b''} is enclosing \code{``a''}, as is \code{``a''}. The
2740 safe prefixes is unified in a \type{PrefixSet}. If a prefix has only one
2741 occurrence, and is a simple expression, it is considered unsuitable as a move
2742 target. This occurs in statements such as \code{``a.foo()''}. For such
2743 statements it bares no meaning to extract and move them. It only generates an
2744 extra method and the calling of it.
2746 The most suitable target for the refactoring is found by finding the prefix with
2747 the most occurrences. If two prefixes have the same occurrence count, but they
2748 differ in the number of segments, the one with most segments is chosen.
2751 \type{ExtractAndMoveMethodExecutor}}\label{extractAndMoveMethodExecutor}
2752 If the analysis finds a possible target for the composite refactoring, it is
2754 \typewithref{no.uio.ifi.refaktor.change.executors}{ExtractAndMoveMethodExecutor}.
2755 It is composed of the two executors known as
2756 \typewithref{no.uio.ifi.refaktor.change.executors}{ExtractMethodRefactoringExecutor}
2758 \typewithref{no.uio.ifi.refaktor.change.executors}{MoveMethodRefactoringExecutor}.
2759 The \type{ExtractAndMoveMethodExecutor} is responsible for gluing the two
2760 together by feeding the \type{MoveMethod\-RefactoringExecutor} with the
2761 resources needed after executing the extract method refactoring.
2763 \subsubsection{The \type{ExtractMethodRefactoringExecutor}}
2764 This executor is responsible for creating and executing an instance of the
2765 \type{ExtractMethodRefactoring} class. It is also responsible for collecting
2766 some post execution resources that can be used to find the method handle for the
2767 extracted method, as well as information about its parameters, including the
2768 variable they originated from.
2770 \subsubsection{The \type{MoveMethodRefactoringExecutor}}
2771 This executor is responsible for creating and executing an instance of the
2772 \type{MoveRefactoring}. The move refactoring is a processor-based refactoring,
2773 and for the \refa{Move Method} refactoring it is the \type{MoveInstanceMethodProcessor}
2776 The handle for the method to be moved is found on the basis of the information
2777 gathered after the execution of the \refa{Extract Method} refactoring. The only
2778 information the \type{ExtractMethodRefactoring} is sharing after its execution,
2779 regarding finding the method handle, is the textual representation of the new
2780 method signature. Therefore it must be parsed, the strings for types of the
2781 parameters must be found and translated to a form that can be used to look up
2782 the method handle from its type handle. They have to be on the unresolved form.
2783 The name for the type is found from the original selection, since an extracted
2784 method must end up in the same type as the originating method.
2786 When analyzing a selection prior to performing the \refa{Extract Method} refactoring, a
2787 target is chosen. It has to be a variable binding, so it is either a field or a
2788 local variable/parameter. If the target is a field, it can be used with the
2789 \type{MoveInstanceMethodProcessor} as it is, since the extracted method still is
2790 in its scope. But if the target is local to the originating method, the target
2791 that is to be used for the processor must be among its parameters. Thus the
2792 target must be found among the extracted method's parameters. This is done by
2793 finding the parameter information object that corresponds to the parameter that
2794 was declared on basis of the original target's variable when the method was
2795 extracted. (The extracted method must take one such parameter for each local
2796 variable that is declared outside the selection that is extracted.) To match the
2797 original target with the correct parameter information object, the key for the
2798 information object is compared to the key from the original target's binding.
2799 The source code must then be parsed to find the method declaration for the
2800 extracted method. The new target must be found by searching through the
2801 parameters of the declaration and choose the one that has the same type as the
2802 old binding from the parameter information object, as well as the same name that
2803 is provided by the parameter information object.
2807 SearchBasedExtractAndMoveMethodChanger}\label{searchBasedExtractAndMoveMethodChanger}
2809 \typewithref{no.uio.ifi.refaktor.change.changers}{SearchBasedExtractAndMoveMethodChanger}
2810 is a changer whose purpose is to automatically analyze a method, and execute the
2811 \ExtractAndMoveMethod refactoring on it if it is a suitable candidate for the
2814 First, the \typewithref{no.uio.ifi.refaktor.analyze.analyzers}{SearchBasedExtractAndMoveMethodAnalyzer} is used
2815 to analyze the method. If the method is found to be a candidate, the result from
2816 the analysis is fed to the \type{ExtractAndMoveMethodExecutor}, whose job is to
2817 execute the refactoring \see{extractAndMoveMethodExecutor}.
2819 \subsubsection{The SearchBasedExtractAndMoveMethodAnalyzer}
2820 This analyzer is responsible for analyzing all the possible text selections of a
2821 method and then to choose the best result out of the analysis results that are,
2822 by the analyzer, considered to be the potential candidates for the
2823 \ExtractAndMoveMethod refactoring.
2825 Before the analyzer is able to work with the text selections of a method, it
2826 needs to generate them. To do this, it parses the method to obtain a
2827 \type{MethodDeclaration} for it \see{astEclipse}. Then there is a statement
2828 lists creator that creates statements lists of the different groups of
2829 statements in the body of the method declaration. A text selections generator
2830 generates text selections of all the statement lists for the analyzer to work
2833 \paragraph{The statement lists creator}
2834 is responsible for generating lists of statements for all the possible nesting
2835 levels of statements in the method. The statement lists creator is implemented
2836 as an AST visitor \see{astVisitor}. It generates lists of statements by visiting
2837 all the blocks in the method declaration and stores their statements in a
2838 collection of statement lists. In addition, it visits all of the other
2839 statements that can have a statement as a child, such as the different control
2840 structures and the labeled statement.
2842 The switch statement is the only kind of statement that is not straight forward
2843 to obtain the child statements from. It stores all of its children in a flat
2844 list. Its switch case statements are included in this list. This means that
2845 there are potential statement lists between all of these case statements. The
2846 list of statements from a switch statement is therefore traversed, and the
2847 statements between the case statements are grouped as separate lists.
2849 The highlighted part of \myref{lst:grandExample} shows an example of how the
2850 statement lists creator would separate a method body into lists of statements.
2852 \paragraph{The text selections generator} generates text selections for each
2853 list of statements from the statement lists creator. The generator generates a
2854 text selection for every sub-sequence of statements in a list. For a sequence of
2855 statements, the first statement and the last statement span out a text
2858 In practice, the text selections are calculated by only one traversal of the
2859 statement list. There is a set of generated text selections. For each statement,
2860 there is created a temporary set of selections, in addition to a text selection
2861 based on the offset and length of the statement. This text selection is added to
2862 the temporary set. Then the new selection is added with every selection from the
2863 set of generated text selections. These new selections are added to the
2864 temporary set. Then the temporary set of selections is added to the set of
2865 generated text selections. The result of adding two text selections is a new
2866 text selection spanned out by the two addends.
2870 \def\charwidth{5.7pt}
2871 \def\indent{4*\charwidth}
2872 \def\lineheight{\baselineskip}
2873 \def\mintedtop{\lineheight}
2875 \begin{tikzpicture}[overlay, yscale=-1]
2876 \tikzstyle{overlaybox}=[fill=lightgray,opacity=0.2]
2878 \draw[overlaybox] (2*\charwidth,\mintedtop) rectangle
2879 +(18*\charwidth,\lineheight);
2881 \draw[overlaybox] (2*\charwidth,\mintedtop+\lineheight) rectangle
2882 +(18*\charwidth,\lineheight);
2884 \draw[overlaybox] (2*\charwidth,\mintedtop+3*\lineheight) rectangle
2885 +(18*\charwidth,\lineheight);
2887 \draw[overlaybox] (\indent-3*\charwidth,\mintedtop) rectangle
2888 +(20*\charwidth,2*\lineheight);
2890 \draw[overlaybox] (3*\charwidth,\mintedtop+\lineheight) rectangle
2891 +(16*\charwidth,3*\lineheight);
2893 \draw[overlaybox] (\indent,\mintedtop) rectangle
2894 +(14*\charwidth,4*\lineheight);
2896 \begin{minted}{java}
2902 \caption{Example of how the text selections generator would generate text
2903 selections based on a lists of statements. Each highlighted rectangle
2904 represents a text selection.}
2905 \label{lst:textSelectionsExample}
2907 \todoin{fix \myref{lst:textSelectionsExample}? Text only? All
2908 sub-sequences\ldots}
2911 \paragraph{Finding the candidate} for the refactoring is done by analyzing all
2912 the generated text selection with the \type{ExtractAndMoveMethodAnalyzer}
2913 \see{extractAndMoveMethodAnalyzer}. If the analyzer generates a useful result,
2914 an \type{ExtractAndMoveMethodCandidate} is created from it, which is kept in a
2915 list of potential candidates. If no candidates are found, the
2916 \type{NoTargetFoundException} is thrown.
2918 Since only one of the candidates can be chosen, the analyzer must sort out which
2919 candidate to choose. The sorting is done by the static \method{sort} method of
2920 \type{Collections}. The comparison in this sorting is done by an
2921 \type{ExtractAndMoveMethodCandidateComparator}.
2922 \todoin{Write about the
2923 ExtractAndMoveMethodCandidateComparator/FavorNoUnfixesCandidateComparator}
2926 \subsection{The Prefix class}
2927 This class exists mainly for holding data about a prefix, such as the expression
2928 that the prefix represents and the occurrence count of the prefix within a
2929 selection. In addition to this, it has some functionality such as calculating
2930 its sub-prefixes and intersecting it with another prefix. The definition of the
2931 intersection between two prefixes is a prefix representing the longest common
2932 expression between the two.
2934 \subsection{The PrefixSet class}
2935 A prefix set holds elements of type \type{Prefix}. It is implemented with the
2936 help of a \typewithref{java.util}{HashMap} and contains some typical set
2937 operations, but it does not implement the \typewithref{java.util}{Set}
2938 interface, since the prefix set does not need all of the functionality a
2939 \type{Set} requires to be implemented. In addition It needs some other
2940 functionality not found in the \type{Set} interface. So due to the relatively
2941 limited use of prefix sets, and that it almost always needs to be referenced as
2942 such, and not a \type{Set<Prefix>}, it remains as an ad hoc solution to a
2945 There are two ways adding prefixes to a \type{PrefixSet}. The first is through
2946 its \method{add} method. This works like one would expect from a set. It adds
2947 the prefix to the set if it does not already contain the prefix. The other way
2948 is to \emph{register} the prefix with the set. When registering a prefix, if the
2949 set does not contain the prefix, it is just added. If the set contains the
2950 prefix, its count gets incremented. This is how the occurrence count is handled.
2952 The prefix set also computes the set of prefixes that is not enclosing any
2953 prefixes of another set. This is kind of a set difference operation only for
2956 \subsection{Hacking the refactoring undo
2957 history}\label{hacking_undo_history}
2958 \todoin{Where to put this section?}
2960 As an attempt to make multiple subsequent changes to the workspace appear as a
2961 single action (i.e. make the undo changes appear as such), I tried to alter
2962 the undo changes\typeref{org.eclipse.ltk.core.refactoring.Change} in the history
2963 of the refactorings.
2965 My first impulse was to remove the, in this case, last two undo changes from the
2966 undo manager\typeref{org.eclipse.ltk.core.refactoring.IUndoManager} for the
2967 \name{Eclipse} refactorings, and then add them to a composite
2968 change\typeref{org.eclipse.ltk.core.refactoring.CompositeChange} that could be
2969 added back to the manager. The interface of the undo manager does not offer a
2970 way to remove/pop the last added undo change, so a possible solution could be to
2971 decorate\citing{designPatterns} the undo manager, to intercept and collect the
2972 undo changes before delegating to the \method{addUndo}
2973 method\methodref{org.eclipse.ltk.core.refactoring.IUndoManager}{addUndo} of the
2974 manager. Instead of giving it the intended undo change, a null change could be
2975 given to prevent it from making any changes if run. Then one could let the
2976 collected undo changes form a composite change to be added to the manager.
2978 There is a technical challenge with this approach, and it relates to the undo
2979 manager, and the concrete implementation
2980 \typewithref{org.eclipse.ltk.internal.core.refactoring}{UndoManager2}. This
2981 implementation is designed in a way that it is not possible to just add an undo
2982 change, you have to do it in the context of an active
2983 operation\typeref{org.eclipse.core.commands.operations.TriggeredOperations}.
2984 One could imagine that it might be possible to trick the undo manager into
2985 believing that you are doing a real change, by executing a refactoring that is
2986 returning a kind of null change that is returning our composite change of undo
2987 refactorings when it is performed. But this is not the way to go.
2989 Apart from the technical problems with this solution, there is a functional
2990 problem: If it all had worked out as planned, this would leave the undo history
2991 in a dirty state, with multiple empty undo operations corresponding to each of
2992 the sequentially executed refactoring operations, followed by a composite undo
2993 change corresponding to an empty change of the workspace for rounding of our
2994 composite refactoring. The solution to this particular problem could be to
2995 intercept the registration of the intermediate changes in the undo manager, and
2996 only register the last empty change.
2998 Unfortunately, not everything works as desired with this solution. The grouping
2999 of the undo changes into the composite change does not make the undo operation
3000 appear as an atomic operation. The undo operation is still split up into
3001 separate undo actions, corresponding to the changes done by their originating
3002 refactorings. And in addition, the undo actions have to be performed separate in
3003 all the editors involved. This makes it no solution at all, but a step toward
3006 There might be a solution to this problem, but it remains to be found. The
3007 design of the refactoring undo management is partly to be blamed for this, as
3008 it is too complex to be easily manipulated.
3011 \chapter{Analyzing source code in Eclipse}
3013 \section{The Java model}\label{javaModel}
3014 The Java model of \name{Eclipse} is its internal representation of a Java project. It
3015 is light-weight, and has only limited possibilities for manipulating source
3016 code. It is typically used as a basis for the Package Explorer in \name{Eclipse}.
3018 The elements of the Java model are only handles to the underlying elements. This
3019 means that the underlying element of a handle does not need to actually exist.
3020 Hence the user of a handle must always check that it exist by calling the
3021 \method{exists} method of the handle.
3023 The handles with descriptions are listed in \myref{tab:javaModel}, while the
3024 hierarchy of the Java Model is shown in \myref{fig:javaModel}.
3027 \caption{The elements of the Java Model\citing{vogelEclipseJDT2012}.}
3028 \label{tab:javaModel}
3030 % sum must equal number of columns (3)
3031 \begin{tabularx}{\textwidth}{@{} L{0.7} L{1.1} L{1.2} @{}}
3033 \textbf{Project Element} & \textbf{Java Model element} &
3034 \textbf{Description} \\
3036 Java project & \type{IJavaProject} & The Java project which contains all other objects. \\
3038 Source folder /\linebreak[2] binary folder /\linebreak[3] external library &
3039 \type{IPackageFragmentRoot} & Hold source or binary files, can be a folder
3040 or a library (zip / jar file). \\
3042 Each package & \type{IPackageFragment} & Each package is below the
3043 \type{IPackageFragmentRoot}, sub-packages are not leaves of the package,
3044 they are listed directed under \type{IPackageFragmentRoot}. \\
3046 Java Source file & \type{ICompilationUnit} & The Source file is always below
3047 the package node. \\
3049 Types / Fields /\linebreak[3] Methods & \type{IType} / \type{IField}
3050 /\linebreak[3] \type{IMethod} & Types, fields and methods. \\
3058 \begin{tikzpicture}[%
3059 grow via three points={one child at (0,-0.7) and
3060 two children at (0,-0.7) and (0,-1.4)},
3061 edge from parent path={(\tikzparentnode.south west)+(0.5,0) |-
3062 (\tikzchildnode.west)}]
3063 \tikzstyle{every node}=[draw=black,thick,anchor=west]
3064 \tikzstyle{selected}=[draw=red,fill=red!30]
3065 \tikzstyle{optional}=[dashed,fill=gray!50]
3066 \node {\type{IJavaProject}}
3067 child { node {\type{IPackageFragmentRoot}}
3068 child { node {\type{IPackageFragment}}
3069 child { node {\type{ICompilationUnit}}
3070 child { node {\type{IType}}
3071 child { node {\type{\{ IType \}*}}
3072 child { node {\type{\ldots}}}
3075 child { node {\type{\{ IField \}*}}}
3076 child { node {\type{IMethod}}
3077 child { node {\type{\{ IType \}*}}
3078 child { node {\type{\ldots}}}
3083 child { node {\type{\{ IMethod \}*}}}
3092 child { node {\type{\{ IType \}*}}}
3103 child { node {\type{\{ ICompilationUnit \}*}}}
3116 child { node {\type{\{ IPackageFragment \}*}}}
3131 child { node {\type{\{ IPackageFragmentRoot \}*}}}
3134 \caption{The Java model of \name{Eclipse}. ``\type{\{ SomeElement \}*}'' means
3135 ``\type{SomeElement} zero or more times``. For recursive structures,
3136 ``\type{\ldots}'' is used.}
3137 \label{fig:javaModel}
3140 \section{The abstract syntax tree}
3141 \name{Eclipse} is following the common paradigm of using an abstract syntax tree for
3142 source code analysis and manipulation.
3144 When parsing program source code into something that can be used as a foundation
3145 for analysis, the start of the process follows the same steps as in a compiler.
3146 This is all natural, because the way a compiler analyzes code is no different
3147 from how source manipulation programs would do it, except for some properties of
3148 code that is analyzed in the parser, and that they may be differing in what
3149 kinds of properties they analyze. Thus the process of translation source code
3150 into a structure that is suitable for analyzing, can be seen as a kind of
3151 interrupted compilation process \see{fig:interruptedCompilationProcess}.
3156 base/.style={anchor=north, align=center, rectangle, minimum height=1.4cm},
3157 basewithshadow/.style={base, drop shadow, fill=white},
3158 outlined/.style={basewithshadow, draw, rounded corners, minimum
3160 primary/.style={outlined, font=\bfseries},
3161 dashedbox/.style={outlined, dashed},
3162 arrowpath/.style={black, align=center, font=\small},
3163 processarrow/.style={arrowpath, ->, >=angle 90, shorten >=1pt},
3165 \begin{tikzpicture}[node distance=1.3cm and 3cm, scale=1, every
3166 node/.style={transform shape}]
3167 \node[base](AuxNode1){\small source code};
3168 \node[primary, right=of AuxNode1, xshift=-2.5cm](Scanner){Scanner};
3169 \node[primary, right=of Scanner, xshift=0.5cm](Parser){Parser};
3170 \node[dashedbox, below=of Parser](SemanticAnalyzer){Semantic\\Analyzer};
3171 \node[dashedbox, left=of SemanticAnalyzer](SourceCodeOptimizer){Source
3173 \node[dashedbox, below=of SourceCodeOptimizer
3174 ](CodeGenerator){Code\\Generator};
3175 \node[dashedbox, right=of CodeGenerator](TargetCodeOptimizer){Target
3177 \node[base, right=of TargetCodeOptimizer](AuxNode2){};
3179 \draw[processarrow](AuxNode1) -- (Scanner);
3181 \path[arrowpath] (Scanner) -- node [sloped](tokens){tokens}(Parser);
3182 \draw[processarrow](Scanner) -- (tokens) -- (Parser);
3184 \path[arrowpath] (Parser) -- node (syntax){syntax
3185 tree}(SemanticAnalyzer);
3186 \draw[processarrow](Parser) -- (syntax) -- (SemanticAnalyzer);
3188 \path[arrowpath] (SemanticAnalyzer) -- node
3189 [sloped](annotated){annotated\\tree}(SourceCodeOptimizer);
3190 \draw[processarrow, dashed](SemanticAnalyzer) -- (annotated) --
3191 (SourceCodeOptimizer);
3193 \path[arrowpath] (SourceCodeOptimizer) -- node
3194 (intermediate){intermediate code}(CodeGenerator);
3195 \draw[processarrow, dashed](SourceCodeOptimizer) -- (intermediate) --
3198 \path[arrowpath] (CodeGenerator) -- node [sloped](target1){target
3199 code}(TargetCodeOptimizer);
3200 \draw[processarrow, dashed](CodeGenerator) -- (target1) --
3201 (TargetCodeOptimizer);
3203 \path[arrowpath](TargetCodeOptimizer) -- node [sloped](target2){target
3205 \draw[processarrow, dashed](TargetCodeOptimizer) -- (target2) (AuxNode2);
3207 \caption{Interrupted compilation process. {\footnotesize (Full compilation
3208 process borrowed from \emph{Compiler construction: principles and practice}
3209 by Kenneth C. Louden\citing{louden1997}.)}}
3210 \label{fig:interruptedCompilationProcess}
3213 The process starts with a \emph{scanner}, or lexer. The job of the scanner is to
3214 read the source code and divide it into tokens for the parser. Therefore, it is
3215 also sometimes called a tokenizer. A token is a logical unit, defined in the
3216 language specification, consisting of one or more consecutive characters. In
3217 the Java language the tokens can for instance be the \var{this} keyword, a curly
3218 bracket \var{\{} or a \var{nameToken}. It is recognized by the scanner on the
3219 basis of something equivalent of a regular expression. This part of the process
3220 is often implemented with the use of a finite automata. In fact, it is common to
3221 specify the tokens in regular expressions, which in turn are translated into a
3222 finite automata lexer. This process can be automated.
3224 The program component used to translate a stream of tokens into something
3225 meaningful, is called a parser. A parser is fed tokens from the scanner and
3226 performs an analysis of the structure of a program. It verifies that the syntax
3227 is correct according to the grammar rules of a language, that are usually
3228 specified in a context-free grammar, and often in a variant of the
3230 Form}\footnote{\url{https://en.wikipedia.org/wiki/Backus-Naur\_Form}}. The
3231 result coming from the parser is in the form of an \emph{Abstract Syntax Tree},
3232 AST for short. It is called \emph{abstract}, because the structure does not
3233 contain all of the tokens produced by the scanner. It only contains logical
3234 constructs, and because it forms a tree, all kinds of parentheses and brackets
3235 are implicit in the structure. It is this AST that is used when performing the
3236 semantic analysis of the code.
3238 As an example we can think of the expression \code{(5 + 7) * 2}. The root of
3239 this tree would in \name{Eclipse} be an \type{InfixExpression} with the operator
3240 \var{TIMES}, and a left operand, which is also an \type{InfixExpression} with
3241 the operator \var{PLUS}. The left operand \type{InfixExpression}, has in turn a
3242 left operand of type \type{NumberLiteral} with the value \var{``5''} and a right
3243 operand \type{NumberLiteral} with the value \var{``7''}. The root will have a
3244 right operand of type \type{NumberLiteral} and value \var{``2''}. The AST for
3245 this expression is illustrated in \myref{fig:astInfixExpression}.
3247 Contrary to the Java Model, an abstract syntax tree is a heavy-weight
3248 representation of source code. It contains information about properties like
3249 type bindings for variables and variable bindings for names.
3254 \begin{tikzpicture}[scale=0.8]
3255 \tikzset{level distance=40pt}
3256 \tikzset{sibling distance=5pt}
3257 \tikzstyle{thescale}=[scale=0.8]
3258 \tikzset{every tree node/.style={align=center}}
3259 \tikzset{edge from parent/.append style={thick}}
3260 \tikzstyle{inode}=[rectangle,rounded corners,draw,fill=lightgray,drop
3261 shadow,align=center]
3262 \tikzset{every internal node/.style={inode}}
3263 \tikzset{every leaf node/.style={draw=none,fill=none}}
3265 \Tree [.\type{InfixExpression} [.\type{InfixExpression}
3266 [.\type{NumberLiteral} \var{``5''} ] [.\type{Operator} \var{PLUS} ]
3267 [.\type{NumberLiteral} \var{``7''} ] ]
3268 [.\type{Operator} \var{TIMES} ]
3269 [.\type{NumberLiteral} \var{``2''} ]
3272 \caption{The abstract syntax tree for the expression \code{(5 + 7) * 2}.}
3273 \label{fig:astInfixExpression}
3276 \subsection{The AST in Eclipse}\label{astEclipse}
3277 In \name{Eclipse}, every node in the AST is a child of the abstract superclass
3278 \typewithref{org.eclipse.jdt.core.dom}{ASTNode}. Every \type{ASTNode}, among a
3279 lot of other things, provides information about its position and length in the
3280 source code, as well as a reference to its parent and to the root of the tree.
3282 The root of the AST is always of type \type{CompilationUnit}. It is not the same
3283 as an instance of an \type{ICompilationUnit}, which is the compilation unit
3284 handle of the Java model. The children of a \type{CompilationUnit} is an
3285 optional \type{PackageDeclaration}, zero or more nodes of type
3286 \type{ImportDecaration} and all its top-level type declarations that has node
3287 types \type{AbstractTypeDeclaration}.
3289 An \type{AbstractType\-Declaration} can be one of the types
3290 \type{AnnotationType\-Declaration}, \type{Enum\-Declaration} or
3291 \type{Type\-Declaration}. The children of an \type{AbstractType\-Declaration}
3292 must be a subtype of a \type{BodyDeclaration}. These subtypes are:
3293 \type{AnnotationTypeMember\-Declaration}, \type{EnumConstant\-Declaration},
3294 \type{Field\-Declaration}, \type{Initializer} and \type{Method\-Declaration}.
3296 Of the body declarations, the \type{Method\-Declaration} is the most interesting
3297 one. Its children include lists of modifiers, type parameters, parameters and
3298 exceptions. It has a return type node and a body node. The body, if present, is
3299 of type \type{Block}. A \type{Block} is itself a \type{Statement}, and its
3300 children is a list of \type{Statement} nodes.
3302 There are too many types of the abstract type \type{Statement} to list up, but
3303 there exists a subtype of \type{Statement} for every statement type of Java, as
3304 one would expect. This also applies to the abstract type \type{Expression}.
3305 However, the expression \type{Name} is a little special, since it is both used
3306 as an operand in compound expressions, as well as for names in type declarations
3309 There is an overview of some of the structure of an \name{Eclipse} AST in
3310 \myref{fig:astEclipse}.
3314 \begin{tikzpicture}[scale=0.8]
3315 \tikzset{level distance=50pt}
3316 \tikzset{sibling distance=5pt}
3317 \tikzstyle{thescale}=[scale=0.8]
3318 \tikzset{every tree node/.style={align=center}}
3319 \tikzset{edge from parent/.append style={thick}}
3320 \tikzstyle{inode}=[rectangle,rounded corners,draw,fill=lightgray,drop
3321 shadow,align=center]
3322 \tikzset{every internal node/.style={inode}}
3323 \tikzset{every leaf node/.style={draw=none,fill=none}}
3325 \Tree [.\type{CompilationUnit} [.\type{[ PackageDeclaration ]} [.\type{Name} ]
3326 [.\type{\{ Annotation \}*} ] ]
3327 [.\type{\{ ImportDeclaration \}*} [.\type{Name} ] ]
3328 [.\type{\{ AbstractTypeDeclaration \}+} [.\node(site){\type{\{
3329 BodyDeclaration \}*}}; ] [.\type{SimpleName} ] ]
3331 \begin{scope}[shift={(0.5,-6)}]
3332 \node[inode,thescale](root){\type{MethodDeclaration}};
3333 \node[inode,thescale](modifiers) at (4.5,-5){\type{\{ IExtendedModifier \}*}
3334 \\ {\footnotesize (Of type \type{Modifier} or \type{Annotation})}};
3335 \node[inode,thescale](typeParameters) at (-6,-3.5){\type{\{ TypeParameter
3337 \node[inode,thescale](parameters) at (-5,-5){\type{\{
3338 SingleVariableDeclaration \}*} \\ {\footnotesize (Parameters)}};
3339 \node[inode,thescale](exceptions) at (5,-3){\type{\{ Name \}*} \\
3340 {\footnotesize (Exceptions)}};
3341 \node[inode,thescale](return) at (-6.5,-2){\type{Type} \\ {\footnotesize
3343 \begin{scope}[shift={(0,-5)}]
3344 \Tree [.\node(body){\type{[ Block ]} \\ {\footnotesize (Body)}};
3345 [.\type{\{ Statement \}*} [.\type{\{ Expression \}*} ]
3346 [.\type{\{ Statement \}*} [.\type{\ldots} ]]
3351 \draw[->,>=triangle 90,shorten >=1pt](root.east)..controls +(east:2) and
3352 +(south:1)..(site.south);
3354 \draw (root.south) -- (modifiers);
3355 \draw (root.south) -- (typeParameters);
3356 \draw (root.south) -- ($ (parameters.north) + (2,0) $);
3357 \draw (root.south) -- (exceptions);
3358 \draw (root.south) -- (return);
3359 \draw (root.south) -- (body);
3362 \caption{The format of the abstract syntax tree in \name{Eclipse}.}
3363 \label{fig:astEclipse}
3366 \section{The ASTVisitor}\label{astVisitor}
3367 So far, the only thing that has been addressed is how the data that is going to
3368 be the basis for our analysis is structured. Another aspect of it is how we are
3369 going to traverse the AST to gather the information we need, so we can conclude
3370 about the properties we are analyzing. It is of course possible to start at the
3371 top of the tree, and manually search through its nodes for the ones we are
3372 looking for, but that is a bit inconvenient. To be able to efficiently utilize
3373 such an approach, we would need to make our own framework for traversing the
3374 tree and visiting only the types of nodes we are after. Luckily, this
3375 functionality is already provided in \name{Eclipse}, by its
3376 \typewithref{org.eclipse.jdt.core.dom}{ASTVisitor}.
3378 The \name{Eclipse} AST, together with its \type{ASTVisitor}, follows the
3379 \pattern{Visitor} pattern\citing{designPatterns}. The intent of this design
3380 pattern is to facilitate extending the functionality of classes without touching
3381 the classes themselves.
3383 Let us say that there is a class hierarchy of elements. These elements all have
3384 a method \method{accept(Visitor visitor)}. In its simplest form, the
3385 \method{accept} method just calls the \method{visit} method of the visitor with
3386 itself as an argument, like this: \code{visitor.visit(this)}. For the visitors
3387 to be able to extend the functionality of all the classes in the elements
3388 hierarchy, each \type{Visitor} must have one visit method for each concrete
3389 class in the hierarchy. Say the hierarchy consists of the concrete classes
3390 \type{ConcreteElementA} and \type{ConcreteElementB}. Then each visitor must have
3391 the (possibly empty) methods \method{visit(ConcreteElementA element)} and
3392 \method{visit(ConcreteElementB element)}. This scenario is depicted in
3393 \myref{fig:visitorPattern}.
3397 \tikzstyle{abstract}=[rectangle, draw=black, fill=white, drop shadow, text
3398 centered, anchor=north, text=black, text width=6cm, every one node
3399 part/.style={align=center, font=\bfseries\itshape}]
3400 \tikzstyle{concrete}=[rectangle, draw=black, fill=white, drop shadow, text
3401 centered, anchor=north, text=black, text width=6cm]
3402 \tikzstyle{inheritarrow}=[->, >=open triangle 90, thick]
3403 \tikzstyle{commentarrow}=[->, >=angle 90, dashed]
3404 \tikzstyle{line}=[-, thick]
3405 \tikzset{every one node part/.style={align=center, font=\bfseries}}
3406 \tikzset{every second node part/.style={align=center, font=\ttfamily}}
3408 \begin{tikzpicture}[node distance=1cm, scale=0.8, every node/.style={transform
3410 \node (Element) [abstract, rectangle split, rectangle split parts=2]
3412 \nodepart{one}{Element}
3413 \nodepart{second}{+accept(visitor: Visitor)}
3415 \node (AuxNode01) [text width=0, minimum height=2cm, below=of Element] {};
3416 \node (ConcreteElementA) [concrete, rectangle split, rectangle split
3417 parts=2, left=of AuxNode01]
3419 \nodepart{one}{ConcreteElementA}
3420 \nodepart{second}{+accept(visitor: Visitor)}
3422 \node (ConcreteElementB) [concrete, rectangle split, rectangle split
3423 parts=2, right=of AuxNode01]
3425 \nodepart{one}{ConcreteElementB}
3426 \nodepart{second}{+accept(visitor: Visitor)}
3429 \node[comment, below=of ConcreteElementA] (CommentA) {visitor.visit(this)};
3431 \node[comment, below=of ConcreteElementB] (CommentB) {visitor.visit(this)};
3433 \node (AuxNodeX) [text width=0, minimum height=1cm, below=of AuxNode01] {};
3435 \node (Visitor) [abstract, rectangle split, rectangle split parts=2,
3438 \nodepart{one}{Visitor}
3439 \nodepart{second}{+visit(ConcreteElementA)\\+visit(ConcreteElementB)}
3441 \node (AuxNode02) [text width=0, minimum height=2cm, below=of Visitor] {};
3442 \node (ConcreteVisitor1) [concrete, rectangle split, rectangle split
3443 parts=2, left=of AuxNode02]
3445 \nodepart{one}{ConcreteVisitor1}
3446 \nodepart{second}{+visit(ConcreteElementA)\\+visit(ConcreteElementB)}
3448 \node (ConcreteVisitor2) [concrete, rectangle split, rectangle split
3449 parts=2, right=of AuxNode02]
3451 \nodepart{one}{ConcreteVisitor2}
3452 \nodepart{second}{+visit(ConcreteElementA)\\+visit(ConcreteElementB)}
3456 \draw[inheritarrow] (ConcreteElementA.north) -- ++(0,0.7) -|
3458 \draw[line] (ConcreteElementA.north) -- ++(0,0.7) -|
3459 (ConcreteElementB.north);
3461 \draw[inheritarrow] (ConcreteVisitor1.north) -- ++(0,0.7) -|
3463 \draw[line] (ConcreteVisitor1.north) -- ++(0,0.7) -|
3464 (ConcreteVisitor2.north);
3466 \draw[commentarrow] (CommentA.north) -- (ConcreteElementA.south);
3467 \draw[commentarrow] (CommentB.north) -- (ConcreteElementB.south);
3471 \caption{The Visitor Pattern.}
3472 \label{fig:visitorPattern}
3475 The use of the visitor pattern can be appropriate when the hierarchy of elements
3476 is mostly stable, but the family of operations over its elements is constantly
3477 growing. This is clearly the case for the \name{Eclipse} AST, since the
3478 hierarchy for the type \type{ASTNode} is very stable, but the functionality of
3479 its elements is extended every time someone need to operate on the AST. Another
3480 aspect of the \name{Eclipse} implementation is that it is a public API, and the
3481 visitor pattern is an easy way to provide access to the nodes in the tree.
3483 The version of the visitor pattern implemented for the AST nodes in \name{Eclipse} also
3484 provides an elegant way to traverse the tree. It does so by following the
3485 convention that every node in the tree first let the visitor visit itself,
3486 before it also makes all its children accept the visitor. The children are only
3487 visited if the visit method of their parent returns \var{true}. This pattern
3488 then makes for a prefix traversal of the AST. If postfix traversal is desired,
3489 the visitors also have \method{endVisit} methods for each node type, which is
3490 called after the \method{visit} method for a node. In addition to these visit
3491 methods, there are also the methods \method{preVisit(ASTNode)},
3492 \method{postVisit(ASTNode)} and \method{preVisit2(ASTNode)}. The
3493 \method{preVisit} method is called before the type-specific \method{visit}
3494 method. The \method{postVisit} method is called after the type-specific
3495 \method{endVisit}. The type specific \method{visit} is only called if
3496 \method{preVisit2} returns \var{true}. Overriding the \method{preVisit2} is also
3497 altering the behavior of \method{preVisit}, since the default implementation is
3498 responsible for calling it.
3500 An example of a trivial \type{ASTVisitor} is shown in
3501 \myref{lst:astVisitorExample}.
3504 \begin{minted}{java}
3505 public class CollectNamesVisitor extends ASTVisitor {
3506 Collection<Name> names = new LinkedList<Name>();
3509 public boolean visit(QualifiedName node) {
3515 public boolean visit(SimpleName node) {
3521 \caption{An \type{ASTVisitor} that visits all the names in a subtree and adds
3522 them to a collection, except those names that are children of any
3523 \type{QualifiedName}.}
3524 \label{lst:astVisitorExample}
3527 \section{Property collectors}\label{propertyCollectors}
3528 The prefixes and unfixes are found by property
3529 collectors\typeref{no.uio.ifi.refaktor.extractors.collectors.PropertyCollector}.
3530 A property collector is of the \type{ASTVisitor} type, and thus visits nodes of
3531 type \type{ASTNode} of the abstract syntax tree \see{astVisitor}.
3533 \subsection{The PrefixesCollector}
3534 The \typewithref{no.uio.ifi.refaktor.extractors.collectors}{PrefixesCollector}
3535 finds prefixes that makes up the basis for calculating move targets for the
3536 \refa{Extract and Move Method} refactoring. It visits expression
3537 statements\typeref{org.eclipse.jdt.core.dom.ExpressionStatement} and creates
3538 prefixes from its expressions in the case of method invocations. The prefixes
3539 found are registered with a prefix set, together with all its sub-prefixes.
3541 \subsection{The UnfixesCollector}\label{unfixes}
3542 The \typewithref{no.uio.ifi.refaktor.extractors.collectors}{UnfixesCollector}
3543 finds unfixes within a selection.
3544 \todoin{Give more technical detail?}
3546 \section{Checkers}\label{checkers}
3547 The checkers are a range of classes that checks that text selections comply
3548 with certain criteria. All checkers operates under the assumption that the code
3549 they check is free from compilation errors. If a
3550 \typewithref{no.uio.ifi.refaktor.analyze.analyzers}{Checker} fails, it throws a
3551 \type{CheckerException}. The checkers are managed by the
3552 \type{LegalStatementsChecker}, which does not, in fact, implement the
3553 \type{Checker} interface. It does, however, run all the checkers registered with
3554 it, and reports that all statements are considered legal if no
3555 \type{CheckerException} is thrown. Many of the checkers either extends the
3556 \type{PropertyCollector} or utilizes one or more property collectors to verify
3557 some criteria. The checkers registered with the \type{LegalStatementsChecker}
3558 are described next. They are run in the order presented below.
3560 \subsection{The CallToProtectedOrPackagePrivateMethodChecker}
3561 This checker is used to check that at selection does not contain a call to a
3562 method that is protected or package-private. Such a method either has the access
3563 modifier \code{protected} or it has no access modifier.
3565 The workings of the \type{CallToProtectedOrPackagePrivateMethod\-Checker} is
3566 very simple. It looks for calls to methods that are either protected or
3567 package-private within the selection, and throws an
3568 \type{IllegalExpressionFoundException} if one is found.
3570 \subsection{The DoubleClassInstanceCreationChecker}
3571 The \type{DoubleClassInstanceCreationChecker} checks that there are no double
3572 class instance creations where the inner constructor call takes an argument that
3573 is built up using field references.
3575 The checker visits all nodes of type \type{ClassInstanceCreation} within a
3576 selection. For all of these nodes, if its parent also is a class instance
3577 creation, it accepts a visitor that throws a
3578 \type{IllegalExpressionFoundException} if it encounters a name that is a field
3581 \subsection{The InstantiationOfNonStaticInnerClassChecker}
3582 The \type{InstantiationOfNonStaticInnerClassChecker} checks that selections
3583 do not contain instantiations of non-static inner classes. The
3584 \type{MoveInstanceMethodProcessor} in \name{Eclipse} does not handle such
3585 instantiations gracefully when moving a method. This problem is also related to
3586 bug\ldots \todoin{File Eclipse bug report}
3588 \subsection{The EnclosingInstanceReferenceChecker}
3589 The purpose of this checker is to verify that the names in a text selection are
3590 not referencing any enclosing instances. In theory, the underlying problem could
3591 be solved in some situations, but our dependency on the
3592 \type{MoveInstanceMethodProcessor} prevents this.
3595 \typewithref{no.uio.ifi.refaktor.analyze.analyzers}{EnclosingInstanceReferenceChecker}
3596 is a modified version of the
3597 \typewithref{org.eclipse.jdt.internal.corext.refactoring.structure.MoveInstanceMethod\-Processor}{EnclosingInstanceReferenceFinder}
3598 from the \type{MoveInstanceMethodProcessor}. Wherever the
3599 \type{EnclosingInstanceReferenceFinder} would create a fatal error status, the
3600 checker will throw a \type{CheckerException}.
3602 The checker works by first finding all of the enclosing types of a selection.
3603 Thereafter, it visits all the simple names of the selection to check that they
3604 are not references to variables or methods declared in any of the enclosing
3605 types. In addition, the checker visits \var{this}-expressions to verify that no
3606 such expressions are qualified with any name.
3608 \subsection{The ReturnStatementsChecker}\label{returnStatementsChecker}
3609 The checker for return statements is meant to verify that a text selection is
3610 consistent regarding return statements.
3612 If the selection is free from return statements, then the checker validates. So
3613 this is the first thing the checker investigates.
3615 If the checker proceeds any further, it is because the selection contains one
3616 or more return statements. The next test is therefore to check if the last
3617 statement of the selection ends in either a return or a throw statement. The
3618 responsibility for checking that the last statement of the selection eventually
3619 ends in a return or throw statement, is put on the
3620 \type{LastStatementOfSelectionEndsInReturnOrThrowChecker}. For every node
3621 visited, if the node is a statement, it does a test to see if the statement is a
3622 return, a throw or if it is an implicit return statement. If this is the case,
3623 no further checking is done. This checking is done in the \code{preVisit2}
3624 method \see{astVisitor}. If the node is not of a type that is being handled by
3625 its type-specific visit method, the checker performs a simple test. If the node
3626 being visited is not the last statement of its parent that is also enclosed by
3627 the selection, an \type{IllegalStatementFoundException} is thrown. This ensures
3628 that all statements are taken care of, one way or the other. It also ensures
3629 that the checker is conservative in the way it checks for legality of the
3632 To examine if a statement is an implicit return statement, the checker first
3633 finds the last statement declared in its enclosing method. If this statement is
3634 the same as the one under investigation, it is considered an implicit return
3635 statement. If the statements are not the same, the checker does a search to see
3636 if the statement examined is also the last statement of the method that can be
3637 reached. This includes the last statement of a block statement, a labeled
3638 statement, a synchronized statement or a try statement, that in turn is the last
3639 statement enclosed by one of the statement types listed. This search goes
3640 through all the parents of a statement until a statement is found that is not
3641 one of the mentioned acceptable parent statements. If the search ends in a
3642 method declaration, then the statement is considered to be the last reachable
3643 statement of the method, and thus it is an implicit return statement.
3645 There are two kinds of statements that are handled explicitly: If-statements and
3646 try-statements. Block, labeled and do-statements are handled by fall-through to
3649 If-statements are handled explicitly by overriding their type-specific visit
3650 method. If the then-part does not contain any return or throw statements an
3651 \type{IllegalStatementFoundException} is thrown. If it does contain a return or
3652 throw, its else-part is checked. If the else-part is non-existent, or it does
3653 not contain any return or throw statements an exception is thrown. If no
3654 exception is thrown while visiting the if-statement, its children are visited.
3656 A try-statement is checked very similar to an if-statement. Its body must
3657 contain a return or throw. The same applies to its catch clauses and finally
3658 body. Failure to validate produces an \type{IllegalStatementFoundException}.
3660 If the checker does not complain at any point, the selection is considered valid
3661 with respect to return statements.
3663 \subsection{The AmbiguousReturnValueChecker}
3664 This checker verifies that there are no ambiguous return values in a selection.
3666 First, the checker needs to collect some data. Those data are the binding keys
3667 for all simple names that are assigned to within the selection, including
3668 variable declarations, but excluding fields. The checker also finds out whether
3669 a return statement is found in the selection or not. No further checks of return
3670 statements are needed, since, at this point, the selection is already checked
3671 for illegal return statements \see{returnStatementsChecker}.
3673 After the binding keys of the assignees are collected, the checker searches the
3674 part of the enclosing method that is after the selection for references whose
3675 binding keys are among the collected keys. If more than one unique referral is
3676 found, or only one referral is found, but the selection also contains a return
3677 statement, we have a situation with an ambiguous return value, and an exception
3680 %\todoin{Explain why we do not need to consider variables assigned inside
3681 %local/anonymous classes. (The referenced variables need to be final and so
3684 \subsection{The IllegalStatementsChecker}
3685 This checker is designed to check for illegal statements.
3687 Notice that labels in break and continue statements need some special treatment.
3688 Since a label does not have any binding information, we have to search upwards
3689 in the AST to find the \type{LabeledStatement} that corresponds to the label
3690 from the break or continue statement, and check that it is contained in the
3691 selection. If the break or continue statement does not have a label attached to
3692 it, it is checked that its innermost enclosing loop or switch statement (break
3693 statements only) also is contained in the selection.
3695 \chapter{Technicalities}
3697 \section{Source code organization}
3698 All the parts of this master's project are under version control with
3699 \name{Git}\footnote{\url{http://git-scm.com/}}.
3701 The software written is organized as some \name{Eclipse} plugins. Writing a plugin is
3702 the natural way to utilize the API of \name{Eclipse}. This also makes it possible to
3703 provide a user interface to manually run operations on selections in program
3704 source code or whole projects/packages.
3706 When writing a plugin in \name{Eclipse}, one has access to resources such as the
3707 current workspace, the open editor and the current selection.
3709 The thesis work is contained in the following Eclipse projects:
3712 \item[no.uio.ifi.refaktor] \hfill \\ This is the main Eclipse plugin
3713 project, and contains all of the business logic for the plugin.
3715 \item[no.uio.ifi.refaktor.tests] \hfill \\
3716 This project contains the tests for the main plugin.
3718 \item[no.uio.ifi.refaktor.examples] \hfill \\
3719 Contains example code used in testing. It also contains code for managing
3720 this example code, such as creating an Eclipse project from it before a test
3723 \item[no.uio.ifi.refaktor.benchmark] \hfill \\
3724 This project contains code for running search based versions of the
3725 composite refactoring over selected Eclipse projects.
3727 \item[no.uio.ifi.refaktor.releng] \hfill \\
3728 Contains the rmap, queries and target definitions needed by Buckminster on
3729 the Jenkins continuous integration server.
3733 \subsection{The no.uio.ifi.refaktor project}
3735 \subsubsection{no.uio.ifi.refaktor.analyze}
3736 This package, and its sub-packages, contains code that is used for analyzing
3737 Java source code. The most important sub-packages are presented below.
3740 \item[no.uio.ifi.refaktor.analyze.analyzers] \hfill \\
3741 This package contains source code analyzers. These are usually responsible
3742 for analyzing text selections or running specialized analyzers for different
3743 kinds of entities. Their structures are often hierarchical. This means that
3744 you have an analyzer for text selections, that in turn is utilized by an
3745 analyzer that analyzes all the selections of a method. Then there are
3746 analyzers for analyzing all the methods of a type, all the types of a
3747 compilation unit, all the compilation units of a package, and, at last, all
3748 of the packages in a project.
3750 \item[no.uio.ifi.refaktor.analyze.checkers] \hfill \\
3751 A package containing checkers. The checkers are classes used to validate
3752 that a selection can be further analyzed and chosen as a candidate for a
3753 refactoring. Invalidating properties can be such as usage of inner classes
3754 or the need for multiple return values.
3756 \item[no.uio.ifi.refaktor.analyze.collectors] \hfill \\
3757 This package contains the property collectors. Collectors are used to gather
3758 properties from a text selection. This is mostly properties regarding
3759 referenced names and their occurrences. It is these properties that make up
3760 the basis for finding the best candidates for a refactoring.
3763 \subsubsection{no.uio.ifi.refaktor.change}
3764 This package, and its sub-packages, contains functionality for manipulate source
3768 \item[no.uio.ifi.refaktor.change.changers] \hfill \\
3769 This package contains source code changers. They are used to glue together
3770 the analysis of source code and the actual execution of the changes.
3772 \item[no.uio.ifi.refaktor.change.executors] \hfill \\
3773 The executors that are responsible for making concrete changes are found in
3774 this package. They are mostly used to create and execute one or more Eclipse
3777 \item[no.uio.ifi.refaktor.change.processors] \hfill \\
3778 Contains a refactoring processor for the \MoveMethod refactoring. The code
3779 is stolen and modified to fix a bug. The related bug is described in
3780 \myref{eclipse_bug_429416}.
3784 \subsubsection{no.uio.ifi.refaktor.handlers}
3785 This package contains handlers for the commands defined in the plugin manifest.
3787 \subsubsection{no.uio.ifi.refaktor.prefix}
3788 This package contains the \type{Prefix} type that is the data representation of
3789 the prefixes found by the \type{PrefixesCollector}. It also contains the prefix
3790 set for storing and working with prefixes.
3792 \subsubsection{no.uio.ifi.refaktor.statistics}
3793 The package contains statistics functionality. Its heart is the statistics
3794 aspect that is responsible for gathering statistics during the execution of the
3795 \ExtractAndMoveMethod refactoring.
3798 \item[no.uio.ifi.refaktor.statistics.reports] \hfill \\
3799 This package contains a simple framework for generating reports from the
3800 statistics data generated by the aspect. Currently, the only available
3801 report type is a simple text report.
3806 \subsubsection{no.uio.ifi.refaktor.textselection}
3807 This package contains the two custom text selections that are used extensively
3808 throughout the project. One of them is just a subclass of the other, to support
3809 the use of the memento pattern to optimize the memory usage during benchmarking.
3811 \subsubsection{no.uio.ifi.refaktor.debugging}
3812 The package contains a debug utility class. I addition to this, the package
3813 \code{no.uio.ifi.refaktor.utils.aspects} contains a couple of aspects used for
3816 \subsubsection{no.uio.ifi.refaktor.utils}
3817 Utility package that contains all the functionality that has to do with parsing
3818 of source code. It also has utility classes for looking up handles to methods
3819 and types et cetera.
3822 \item[no.uio.ifi.refaktor.utils.caching] \hfill \\
3823 This package contains the caching manager for compilation units, along with
3824 classes for different caching strategies.
3826 \item[no.uio.ifi.refaktor.utils.nullobjects] \hfill \\
3827 Contains classes for creating different null objects. Most of the classes
3828 are used to represent null objects of different handle types. These null
3829 objects are returned from various utility classes instead of returning a
3830 \var{null} value when other values are not available.
3834 \section{Continuous integration}
3835 The continuous integration server
3836 \name{Jenkins}\footnote{\url{http://jenkins-ci.org/}} has been set up for the
3837 project\footnote{A work mostly done by the supervisor.}. It is used as a way to
3838 run tests and perform code coverage analysis.
3840 To be able to build the \name{Eclipse} plugins and run tests for them with Jenkins, the
3841 component assembly project
3842 \name{Buckminster}\footnote{\url{http://www.eclipse.org/buckminster/}} is used,
3843 through its plugin for Jenkins. Buckminster provides for a way to specify the
3844 resources needed for building a project and where and how to find them.
3845 Buckminster also handles the setup of a target environment to run the tests in.
3846 All this is needed because the code to build depends on an \name{Eclipse}
3847 installation with various plugins.
3849 \subsection{Problems with AspectJ}
3850 The Buckminster build worked fine until introducing AspectJ into the project.
3851 When building projects using AspectJ, there are some additional steps that need
3852 to be performed. First of all, the aspects themselves must be compiled. Then the
3853 aspects need to be woven with the classes they affect. This demands a process
3854 that does multiple passes over the source code.
3856 When using AspectJ with \name{Eclipse}, the specialized compilation and the
3857 weaving can be handled by the \name{AspectJ Development
3858 Tools}\footnote{\url{https://www.eclipse.org/ajdt/}}. This works all fine, but
3859 it complicates things when trying to build a project depending on \name{Eclipse}
3860 plugins outside of \name{Eclipse}. There is supposed to be a way to specify a
3861 compiler adapter for javac, together with the file extensions for the file types
3862 it shall operate. The AspectJ compiler adapter is called
3863 \typewithref{org.aspectj.tools.ant.taskdefs}{Ajc11CompilerAdapter}, and it works
3864 with files that has the extensions \code{*.java} and \code{*.aj}. I tried to
3865 setup this in the build properties file for the project containing the aspects,
3866 but to no avail. The project containing the aspects does not seem to be built at
3867 all, and the projects that depend on it complain that they cannot find certain
3870 I then managed to write an \name{Ant}\footnote{\url{https://ant.apache.org/}}
3871 build file that utilizes the AspectJ compiler adapter, for the
3872 \code{no.uio.ifi.refaktor} plugin. The problem was then that it could no longer
3873 take advantage of the environment set up by Buckminster. The solution to this
3874 particular problem was of a ``hacky'' nature. It involves exporting the plugin
3875 dependencies for the project to an Ant build file, and copy the exported path
3876 into the existing build script. But then the Ant script needs to know where the
3877 local \name{Eclipse} installation is located. This is no problem when building
3878 on a local machine, but to utilize the setup done by Buckminster is a problem
3879 still unsolved. To get the classpath for the build setup correctly, and here
3880 comes the most ``hacky'' part of the solution, the Ant script has a target for
3881 copying the classpath elements into a directory relative to the project
3882 directory and checking it into Git. When no \code{ECLIPSE\_HOME} property is set
3883 while running Ant, the script uses the copied plugins instead of the ones
3884 provided by the \name{Eclipse} installation when building the project. This
3885 obviously creates some problems with maintaining the list of dependencies in the
3886 Ant file, as well as remembering to copy the plugins every time the list of
3887 dependencies changes.
3889 The Ant script described above is run by Jenkins before the Buckminster setup
3890 and build. When setup like this, the Buckminster build succeeds for the projects
3891 not using AspectJ, and the tests are run as normal. This is all good, but it
3892 feels a little scary, since the reason for Buckminster not working with AspectJ
3895 The problems with building with AspectJ on the Jenkins server lasted for a
3896 while, before they were solved. This is reflected in the ``Test Result Trend''
3897 and ``Code Coverage Trend'' reported by Jenkins.
3899 \chapter{Benchmarking}\label{sec:benchmarking}
3900 This part of the master's project is located in the \name{Eclipse} project
3901 \code{no.uio.ifi.refaktor.benchmark}. The purpose of it is to run the equivalent
3902 of the \type{SearchBasedExtractAndMoveMethodChanger}
3903 \see{searchBasedExtractAndMoveMethodChanger} over a larger software project,
3904 both to test its robustness but also its effect on different software metrics.
3906 \section{The benchmark setup}
3907 The benchmark itself is set up as a \name{JUnit} test case. This is a convenient
3908 setup, and utilizes the \name{JUnit Plugin Test Launcher}. This provides us with
3909 a fully functional \name{Eclipse} workbench. Most importantly, this gives us
3910 access to the Java Model of \name{Eclipse} \see{javaModel}.
3912 \subsection{The ProjectImporter}
3913 The Java project that is going to be used as the data for the benchmark, must be
3914 imported into the JUnit workspace. This is done by the
3915 \typewithref{no.uio.ifi.refaktor.benchmark}{ProjectImporter}. The importer
3916 requires the absolute path to the project description file. This file is named
3917 \code{.project} and is located at the root of the project directory.
3919 The project description is loaded to find the name of the project to be
3920 imported. The project that shall be the destination for the import is created in
3921 the workspace, on the base of the name from the description. Then an import
3922 operation is created, based on both the source and destination information. The
3923 import operation is run to perform the import.
3925 I have found no simple API call to accomplish what the importer does, which
3926 tells me that it may not be too many people performing this particular action.
3927 The solution to the problem was found on \name{Stack
3928 Overflow}\footnote{\url{https://stackoverflow.com/questions/12401297}}. It
3929 contains enough dirty details to be considered inconvenient to use, if not
3930 wrapping it in a class like my \type{ProjectImporter}. One would probably have
3931 to delve into the source code for the import wizard to find out how the import
3932 operation works, if no one had already done it.
3934 \section{Statistics}
3935 Statistics for the analysis and changes is captured by the
3936 \typewithref{no.uio.ifi.refaktor.aspects}{StatisticsAspect}. This an
3937 \emph{aspect} written in \name{AspectJ}.
3939 \subsection{AspectJ}
3940 \name{AspectJ}\footnote{\url{http://eclipse.org/aspectj/}} is an extension to
3941 the Java language, and facilitates combining aspect-oriented programming with
3942 the object-oriented programming in Java.
3944 Aspect-oriented programming is a programming paradigm that is meant to isolate
3945 so-called \emph{cross-cutting concerns} into their own modules. These
3946 cross-cutting concerns are functionalities that span over multiple classes, but
3947 may not belong naturally in any of them. It can be functionality that does not
3948 concern the business logic of an application, and thus may be a burden when
3949 entangled with parts of the source code it does not really belong. Examples
3950 include logging, debugging, optimization and security.
3952 Aspects are interacting with other modules by defining advices. The concept of
3953 an \emph{advice} is known from both aspect-oriented and functional
3954 programming\citing{wikiAdvice2014}. It is a function that modifies another
3955 function when the latter is run. An advice in AspectJ is somewhat similar to a
3956 method in Java. It is meant to alter the behavior of other methods, and contains
3957 a body that is executed when it is applied.
3959 An advice can be applied at a defined \emph{pointcut}. A pointcut picks out one
3960 or more \emph{join points}. A join point is a well-defined point in the
3961 execution of a program. It can occur when calling a method defined for a
3962 particular class, when calling all methods with the same name,
3963 accessing/assigning to a particular field of a given class and so on. An advice
3964 can be declared to run both before, after returning from a pointcut, when there
3965 is thrown an exception in the pointcut or after the pointcut either returns or
3966 throws an exception. In addition to picking out join points, a pointcut can
3967 also bind variables from its context, so they can be accessed in the body of an
3968 advice. An example of a pointcut and an advice is found in
3969 \myref{lst:aspectjExample}.
3972 \begin{minted}{aspectj}
3973 pointcut methodAnalyze(
3974 SearchBasedExtractAndMoveMethodAnalyzer analyzer) :
3975 call(* SearchBasedExtractAndMoveMethodAnalyzer.analyze())
3976 && target(analyzer);
3978 after(SearchBasedExtractAndMoveMethodAnalyzer analyzer) :
3979 methodAnalyze(analyzer) {
3980 statistics.methodCount++;
3981 debugPrintMethodAnalysisProgress(analyzer.method);
3984 \caption{An example of a pointcut named \method{methodAnalyze},
3985 and an advice defined to be applied after it has occurred.}
3986 \label{lst:aspectjExample}
3989 \subsection{The Statistics class}
3990 The statistics aspect stores statistical information in an object of type
3991 \type{Statistics}. As of now, the aspect needs to be initialized at the point in
3992 time where it is desired that it starts its data gathering. At any point in time
3993 the statistics aspect can be queried for a snapshot of the current statistics.
3995 The \type{Statistics} class also includes functionality for generating a report
3996 of its gathered statistics. The report can be given either as a string or it can
3997 be written to a file.
3999 \subsection{Advices}
4000 The statistics aspect contains advices for gathering statistical data from
4001 different parts of the benchmarking process. It captures statistics from both
4002 the analysis part and the execution part of the composite \ExtractAndMoveMethod
4005 For the analysis part, there are advices to count the number of text selections
4006 analyzed and the number of methods, types, compilation units and packages
4007 analyzed. There are also advices that counts for how many of the methods there
4008 are found a selection that is a candidate for the refactoring, and for how many
4009 methods there are not.
4011 There exist advices for counting both the successful and unsuccessful executions
4012 of all the refactorings. Both for the \ExtractMethod and \MoveMethod
4013 refactorings in isolation, as well as for the combination of them.
4015 \section{Optimizations}
4016 When looking for possible optimizations for the benchmarking process, I used the
4017 \name{VisualVM}\footnote{\url{http://visualvm.java.net/}} \gloss{profiler} for
4018 the Java Virtual Machine to both profile the application and also to make memory
4021 \subsection{Caching}
4022 When \gloss{profiling} the benchmark process before making any optimizations, it
4023 early became apparent that the parsing of source code was a place to direct
4024 attention towards. This discovery was done when only \emph{analyzing} source
4025 code, before trying to do any \emph{manipulation} of it. Caching of the parsed
4026 ASTs seemed like the best way to save some time, as expected. With only a simple
4027 cache of the most recently used AST, the analysis time was speeded up by a
4028 factor of around 20. This number depends a little upon which type of system the
4031 The caching is managed by a cache manager, that now, by default, utilizes the
4032 not so well known feature of Java called a \emph{soft reference}. Soft
4033 references are best explained in the context of weak references. A \emph{weak
4034 reference} is a reference to an object instance that is only guaranteed to
4035 persist as long as there is a \emph{strong reference} or a soft reference
4036 referring the same object. If no such reference is found, its referred object is
4037 garbage collected. A strong reference is basically the same as a regular Java
4038 reference. A soft reference has the same guarantees as a week reference when it
4039 comes to its relation to strong references, but it is not necessarily garbage
4040 collected if there are no strong references to it. A soft reference \emph{may}
4041 reside in memory as long as the JVM has enough free memory in the heap. A soft
4042 reference will therefore usually perform better than a weak reference when used
4043 for simple caching and similar tasks. The way to use a soft/weak reference is to
4044 as it for its referent. The return value then has to be tested to check that it
4045 is not \var{null}. For the basic usage of soft references, see
4046 \myref{lst:softReferenceExample}. For a more thorough explanation of weak
4047 references in general, see\citing{weakRef2006}.
4050 \begin{minted}{java}
4052 Object strongRef = new Object();
4055 SoftReference<Object> softRef =
4056 new SoftReference<Object>(new Object());
4058 // Using the soft reference
4059 Object obj = softRef.get();
4064 \caption{Showing the basic usage of soft references. Weak references is used the
4065 same way. {\footnotesize (The references are part of the \code{java.lang.ref}
4067 \label{lst:softReferenceExample}
4070 The cache based on soft references has no limit for how many ASTs it caches. It
4071 is generally not advisable to keep references to ASTs for prolonged periods of
4072 time, since they are expensive structures to hold on to. For regular plugin
4073 development, \name{Eclipse} recommends not creating more than one AST at a time to
4074 limit memory consumption. Since the benchmarking has nothing to do with user
4075 experience, and throughput is everything, these advices are intentionally
4076 ignored. This means that during the benchmarking process, the target \name{Eclipse}
4077 application may very well work close to its memory limit for the heap space for
4078 long periods during the benchmark.
4080 \subsection{Candidates stored as mementos}
4081 When performing large scale analysis of source code for finding candidates to
4082 the \ExtractAndMoveMethod refactoring, memory is an issue. One of the inputs to
4083 the refactoring is a variable binding. This variable binding indirectly retains
4084 a whole AST. Since ASTs are large structures, this quickly leads to an
4085 \type{OutOfMemoryError} if trying to analyze a large project without optimizing
4086 how we store the candidates' data. This means that the JVM cannot allocate more
4087 memory for our benchmark, and it exits disgracefully.
4089 A possible solution could be to just allow the JVM to allocate even more memory,
4090 but this is not a dependable solution. The allocated memory could easily
4091 supersede the physical memory of a machine, which would make the benchmark go
4094 Thus, the candidates' data must be stored in another format. Therefore, we use
4095 the \gloss{mementoPattern} to store variable binding information. This is done
4096 in a way that makes it possible to retrieve a variable binding at a later point.
4097 The data that is stored to achieve this, is the key to the original variable
4098 binding. In addition to the key, we know which method and text selection the
4099 variable is referenced in, so that we can find it by parsing the source code and
4100 search for it when it is needed.
4102 \section{Handling failures}
4106 \chapter{Case studies}
4108 In this chapter I am going to present a few case studies. This is done to give
4109 an impression of how the search-based \ExtractAndMoveMethod refactoring
4110 performs when giving it a larger project to take on. I will try to answer where
4111 it lacks, in terms of completeness, as well as showing its effect on refactored
4114 The first and primary case, is refactoring source code from the \name{Eclipse
4115 JDT UI} project. The project is chosen because it is a well-known open-source
4116 project, still in development, with a large code base that is written by many
4117 different people over several years. The code is installed in a large number of
4118 \name{Eclipse} applications worldwide, and many other projects build on the
4119 Eclipse platform. For a long time, it was even the official IDE for Android
4120 development. All this means that Eclipse must be seen as a good representative
4121 for professionally written Java source code. It is also the home for most of the
4122 JDT refactoring code.
4124 For the second case, the \ExtractAndMoveMethod refactoring is fed the
4125 \code{no.uio.ifi.refaktor} project. This is done as a variation of the
4126 ``dogfooding'' methodology.
4129 For conducting these experiments, three software tools are used. Two of the
4130 tools both use Eclipse as their platform. The first is our own tool, described
4131 in \myref{sec:benchmarking}, written to be able to run the \ExtractAndMoveMethod
4132 refactoring as a batch process. It analyzes and refactors all the methods of a
4133 project in sequence. The second is JUnit, which is used for running the
4134 project's own unit tests on the target code both before and after it is
4135 refactored. The last tool that is used is a code quality management tool, called
4136 \name{SonarQube}. It can be used to perform different tasks for assuring code
4137 quality, but we are only going to take advantage of one of its main features,
4138 namely quality profiles.
4140 A quality profile is used to define a set of coding rules that a project is
4141 supposed to comply with. Failure to following these rules will be recorded as
4142 so-called ``issues'', marked as having one of several degrees of severities,
4143 ranging from ``info'' to ``blocker'', where the latter one is the most severe.
4144 The measurements done for these case studies are therefore not presented as
4145 fine-grained software metrics results, but rather as the number of issues for
4148 In its analysis, \name{SonarQube} discriminates between functions and accessors.
4149 Accessors are methods that are recognized as setters or getters.
4151 In addition to the coding rules defined through quality profiles,
4152 \name{SonarQube} calculates the complexity of source code. The metric that is
4153 used is cyclomatic complexity, developed by Thomas J. McCabe in
4154 1976\citing{mccabeComplexity1976}. In this metric, functions have an initial
4155 complexity of 1, and whenever the control flow of a function splits, the
4156 complexity increases by
4157 one\footnote{\url{http://docs.codehaus.org/display/SONAR/Metric+definitions}}.
4158 Accessors are not counted in the complexity analysis.
4160 Specifications for the computer used during the experiments are shown in
4161 \myref{tab:experimentComputerSpecs}.
4164 \caption{Specifications for experiment computer.}
4165 \label{tab:experimentComputerSpecs}
4167 \begin{tabularx}{\textwidth}{@{}>{\bfseries}L{0.35}R{1.65}@{}}
4169 \spancols{2}{Hardware} \\
4171 Model & Lenovo ThinkPad Edge S430 \\
4172 Processor & Intel\textregistered{} Core\texttrademark{}
4173 i5-3210M\linebreak[4] (2.5 GHz/3.1 GHz (turbo),
4174 2 cores, 4 threads, 3 MB Cache) \\
4175 Memory & 8 GB DDR3 1600 MHz \\
4176 Storage & 500 GB HDD (7200 RPM) + 16 GB SSD Cache for Lenovo Hard Disk Drive
4177 Performance Booster \\
4179 \spancols{2}{Operating system} \\
4181 Distribution & Ubuntu 12.10 \\
4182 Kernel & Linux 3.5.0-49-generic (x86\_64) \\
4189 \section{The \name{SonarQube} quality profile}
4190 The quality profile that is used with \name{SonarQube} in these case studies has got
4191 the name \name{IFI Refaktor Case Study} (version 6). The rules defined in the
4192 profile are chosen because they are the available rules found in \name{SonarQube} that
4193 measures complexity and coupling. Now follows a description of the rules in the
4194 quality profile. The values that are set for these rules are listed in
4195 \myref{tab:qualityProfile1}.
4198 \item[Avoid too complex class] is a rule that measures cyclomatic complexity
4199 for every statement in the body of a class, except for setters and getter.
4200 The threshold value set is its default value of 200.
4202 \item[Classes should not be coupled to too many other classes ] is a rule that
4203 measures how many other classes a class depends upon. It does not count the
4204 dependencies of nested classes. It is meant to promote the Single
4205 Responsibility Principle. The metric for the rule resembles the CBO metric
4206 that is described in \myref{sec:CBO}, but is only considering outgoing
4207 dependencies. The max value for the rule is chosen on the basis of an
4208 empirical study by Raed Shatnawi, which concludes that the number 9 is the
4209 most useful threshold for the CBO metric\citing{shatnawiQuantitative2010}.
4210 This study is also performed on Eclipse source code, so this threshold value
4211 should be particularly well suited for the Eclipse JDT UI case in this
4214 \item[Control flow statements \ldots{} should not be nested too deeply] is
4215 a rule that is meant to counter ``Spaghetti code''. It measures the nesting
4216 level of \emph{if}, \emph{for}, \emph{while}, \emph{switch} and \emph{try}
4217 statements. The nesting levels start at 1. The max value set is its default
4220 \item[Methods should not be too complex] is a rule that measures cyclomatic
4221 complexity the same way as the ``Avoid too complex class'' rule. The max
4222 value used is 10, which ``seems like a reasonable, but not magical, upper
4223 limit``\citing{mccabeComplexity1976}.
4225 \item[Methods should not have too many lines] is a rule that simply measures
4226 the number of lines in methods. A threshold value of 20 is used for this
4227 metric. This is based on my own subjective opinions, as the default value of
4228 100 describes method bodies that do not even fit on most screens.
4230 \item[NPath Complexity] is a rule that measures the number of possible
4231 execution paths through a function. The value used is the default value of
4232 200, which seems like a recognized threshold for this metric.
4234 \item[Too many methods] is a rule that measures the number of methods in a
4235 class. The threshold value used is the default value of 10.
4241 \caption{The \name{IFI Refaktor Case Study} quality profile (version 6).}
4242 \label{tab:qualityProfile1}
4244 \begin{tabularx}{\textwidth}{@{}>{\bfseries}L{1.5}R{0.5}@{}}
4246 \textbf{Rule} & \textbf{Max value} \\
4248 Avoid too complex class & 200 \\
4249 Classes should not be coupled to too many other classes (Single
4250 Responsibility Principle) & 9 \\
4251 Control flow statements \ldots{} should not be nested too deeply &
4253 Methods should not be too complex & 10 \\
4254 Methods should not have too many lines & 20 \\
4255 NPath Complexity & 200 \\
4256 Too many methods & 10 \\
4263 A precondition for the source code that is going to be the target for a series
4264 of \ExtractAndMoveMethod refactorings, is that it is organized as an Eclipse
4265 project. It is also assumed that the code is free from compilation errors.
4267 \section{The experiment}
4268 For a given project, the first job that is done, is to refactor its source code.
4269 The refactoring batch job produces three things: The refactored project,
4270 statistics gathered during the execution of the series of refactorings, and an
4271 error log describing any errors happening during this execution. See
4272 \myref{sec:benchmarking} for more information about how the refactorings are
4275 After the refactoring process is done, the before- and after-code is analyzed
4276 with \name{SonarQube}. The analysis results are then stored in a database and
4277 displayed through a \name{SonarQube} server with a web interface.
4279 The before- and after-code is also tested with their own unit tests. This is
4280 done to discover any changes in the semantic behavior of the refactored code,
4281 within the limits of these tests.
4283 \section{Case 1: The Eclipse JDT UI project}
4284 This case is the ultimate test for our \ExtractAndMoveMethod refactoring. The
4285 target source code is massive. With its over 300,000 lines of code\footnote{For
4286 all uses of ``lines of code'' we follow the definition from \name{SonarQube}.
4287 LOC = the number of physical lines containing a character which is neither
4288 whitespace or part of a comment.} and over 25,000 methods, it is a formidable
4289 task to perform automated changes on it. There should be plenty of situations
4290 where things can go wrong, and, as we shall see later, they do.
4292 I will start by presenting some statistics from the refactoring execution,
4293 before I pick apart the \name{SonarQube} analysis and conclude by commenting on
4294 the results from the unit tests. The configuration for the experiment is
4295 specified in \myref{tab:configurationCase1}.
4298 \caption{Configuration for Case 1.}
4299 \label{tab:configurationCase1}
4301 \begin{tabularx}{\textwidth}{@{}>{\bfseries}L{0.67}L{1.33}@{}}
4303 \spancols{2}{Benchmark data} \\
4305 Launch configuration & CaseStudy.launch \\
4306 Project & no.uio.ifi.refaktor.benchmark \\
4307 Repository & gitolite@git.uio.no:ifi-stolz-refaktor \\
4308 Commit & 43c16c04520746edd75f8dc2a1935781d3d9de6c \\
4310 \spancols{2}{Input data} \\
4312 Project & org.eclipse.jdt.ui \\
4313 Repository & git://git.eclipse.org/gitroot/jdt/eclipse.jdt.ui.git \\
4314 Commit & f218388fea6d4ec1da7ce22432726c244888bb6b \\
4315 Branch & R3\_8\_maintenance \\
4316 Tests suites & org.eclipse.jdt.ui.tests.AutomatedSuite,
4317 org.eclipse.jdt.ui.tests.refactoring.all.\-AllAllRefactoringTests \\
4322 \subsection{Statistics}
4323 The statistics gathered during the refactoring execution is presented in
4324 \myref{tab:case1Statistics}.
4327 \caption{Statistics after batch refactoring the Eclipse JDT UI project with
4328 the \ExtractAndMoveMethod refactoring.}
4329 \label{tab:case1Statistics}
4331 \begin{tabularx}{\textwidth}{@{}>{\bfseries}L{1.5}R{0.5}@{}}
4333 \spancols{2}{Time used} \\
4335 Total time & 98m38s \\
4336 Analysis time & 14m41s (15\%) \\
4337 Change time & 74m20s (75\%) \\
4338 Miscellaneous tasks & 9m37s (10\%) \\
4340 \spancols{2}{Numbers of each type of entity analyzed} \\
4343 Compilation units & 2,097 \\
4346 Text selections & 591,500 \\
4348 \spancols{2}{Numbers for \ExtractAndMoveMethod refactoring candidates} \\
4350 Methods chosen as candidates & 2,552 \\
4351 Methods NOT chosen as candidates & 25,115 \\
4352 Candidate selections (multiple per method) & 36,843 \\
4354 \spancols{2}{\ExtractAndMoveMethod refactorings executed} \\
4356 Fully executed & 2,469 \\
4357 Not fully executed & 83 \\
4358 Total attempts & 2,552 \\
4360 \spancols{2}{Primitive refactorings executed} \\
4361 \spancols{2}{\small \ExtractMethod refactorings} \\
4363 Performed & 2,483 \\
4364 Not performed & 69 \\
4365 Total attempts & 2,552 \\
4367 \spancols{2}{\small \MoveMethod refactorings} \\
4370 Not performed & 14 \\
4371 Total attempts & 2,483 \\
4377 \subsubsection{Execution time}
4378 I consider the total execution time of approximately 1.5 hours, on a regular
4379 laptop computer, as being acceptable. It clearly makes the batch process
4380 unsuitable for doing any on-demand analysis or changes, but it is good enough
4381 for running periodic jobs, like over-night analysis.
4383 As the statistics show, 75\% of the total time goes into making the actual code
4384 changes. The time consumers are here the primitive \ExtractMethod and
4385 \MoveMethod refactorings. Included in the change time is the parsing and
4386 precondition checking done by the refactorings, as well as textual changes done
4387 to files on disk. All this parsing and disk access is time-consuming, and
4388 constitutes a large part of the change time.
4390 In comparison, the pure analysis time, used to find suitable candidates, only
4391 makes up for 15\% of the total time consumed. This includes analyzing almost
4392 600,000 text selections, while the number of attempted executions of the
4393 \ExtractAndMoveMethod refactoring is only about 2,500. So the number of executed
4394 primitive refactorings is approximately 5,000. Assuming the time used on
4395 miscellaneous tasks are used mostly for parsing source code for the analysis, we
4396 can say that the time used for analyzing code is at most 25\% of the total time.
4397 This means that for every primitive refactoring executed, we can analyze around
4398 360 text selections. So, with an average of about 21 text selections per method,
4399 it is reasonable to say that we can analyze over 15 methods in the time it
4400 takes to perform a primitive refactoring.
4402 \subsubsection{Refactoring candidates}
4403 Out of the 27,667 methods that were analyzed, 2,552 methods contained selections
4404 that were considered candidates for the \ExtractAndMoveMethod refactoring. This
4405 is roughly 9\% off the methods in the project. These 9\% of the methods had on
4406 average 14.4 text selections that were considered possible refactoring
4409 \subsubsection{Executed refactorings}
4410 2,469 out of 2,552 attempts on executing the \ExtractAndMoveMethod refactoring
4411 were successful, giving a success rate of 96.7\%. The failure rate of 3.3\%
4412 stems from situations where the analysis finds a candidate selection, but the
4413 change execution fails. This failure could be an exception that was thrown, and
4414 the refactoring aborts. It could also be the precondition checking for one of
4415 the primitive refactorings that gives us an error status, meaning that if the
4416 refactoring proceeds, the code will contain compilation errors afterwards,
4417 forcing the composite refactoring to abort. This means that if the
4418 \ExtractMethod refactoring fails, no attempt is done for the \MoveMethod
4419 refactoring. \todo{Redundant information? Put in benchmark chapter?}
4421 Out of the 2,552 \ExtractMethod refactorings that were attempted executed, 69 of
4422 them failed. This gives a failure rate of 2.7\% for the primitive refactoring.
4423 In comparison, the \MoveMethod refactoring had a failure rate of 0.6 \% of the
4424 2,483 attempts on the refactoring.
4426 The failure rates for the refactorings are not that bad, if we also take into
4427 account that the pre-refactoring analysis is incomplete.\todo{see \ldots}
4429 \subsection{\name{SonarQube} analysis}
4430 Results from the \name{SonarQube} analysis are shown in
4431 \myref{tab:case1ResultsProfile1}.
4434 \caption{Results for analyzing the Eclipse JDT UI project, before and after
4435 the refactoring, with \name{SonarQube} and the \name{IFI Refaktor Case Study}
4436 quality profile. (Bold numbers are better.)}
4437 \label{tab:case1ResultsProfile1}
4439 \begin{tabularx}{\textwidth}{@{}>{\bfseries}L{1.5}R{0.25}R{0.25}@{}}
4441 \textnormal{Number of issues for each rule} & Before & After \\
4443 Avoid too complex class & 81 & \textbf{79} \\
4444 Classes should not be coupled to too many other classes (Single
4445 Responsibility Principle) & \textbf{1,098} & 1,199 \\
4446 Control flow statements \ldots{} should not be nested too deeply & 1,375 &
4448 Methods should not be too complex & 1,518 & \textbf{1,452} \\
4449 Methods should not have too many lines & 3,396 & \textbf{3,291} \\
4450 NPath Complexity & 348 & \textbf{329} \\
4451 Too many methods & \textbf{454} & 520 \\
4453 Total number of issues & 8,270 & \textbf{8,155} \\
4456 \spancols{3}{Complexity} \\
4458 Per function & 3.6 & \textbf{3.3} \\
4459 Per class & \textbf{29.5} & 30.4 \\
4460 Per file & \textbf{44.0} & 45.3 \\
4462 Total complexity & \textbf{84,765} & 87,257 \\
4465 \spancols{3}{Numbers of each type of entity analyzed} \\
4467 Files & 1,926 & 1,926 \\
4468 Classes & 2,875 & 2,875 \\
4469 Functions & 23,744 & 26,332 \\
4470 Accessors & 1,296 & 1,019 \\
4471 Statements & 162,768 & 165,145 \\
4472 Lines of code & 320,941 & 329,112 \\
4474 Technical debt (in days) & \textbf{1,003.4} & 1,032.7 \\
4479 \subsubsection{Diversity in the number of entities analyzed}
4480 The analysis performed by \name{SonarQube} is reporting fewer methods than found
4481 by the pre-refactoring analysis. \name{SonarQube} discriminates between
4482 functions (methods) and accessors, so the 1,296 accessors play a part in this
4483 calculation. \name{SonarQube} also has the same definition as our plugin when
4484 it comes to how a class is defined. Therefore it seems like \name{SonarQube}
4485 misses 277 classes that our plugin handles. This can explain why the {SonarQube}
4486 report differs from our numbers by approximately 2,500 methods,
4488 \subsubsection{Complexity}
4489 On all complexity rules that works on the method level, the number of issues
4490 decreases with between 3.1\% and 6.5\% from before to after the refactoring. The
4491 average complexity of a method decreases from 3.6 to 3.3, which is an
4492 improvement of about 8.3\%. So, on the method level, the refactoring must be
4493 said to have a slightly positive impact. This is due to the extraction of a lot
4494 of methods, making the average method size smaller.
4496 The improvement in complexity on the method level is somewhat traded for
4497 complexity on the class level. The complexity per class metric is worsened by
4498 3\% from before to after. The issues for the ``Too many methods'' rule also
4499 increases by 14.5\%. These numbers indicate that the refactoring makes quite a
4500 lot of the classes a little more complex overall. This is the expected outcome,
4501 since the \ExtractAndMoveMethod refactoring introduces almost 2,500 new methods
4504 The only number that can save the refactoring's impact on complexity on the
4505 class level, is the ``Avoid too complex class'' rule. It improves with 2.5\%,
4506 thus indicating that the complexity is moderately better distributed between the
4507 classes after the refactoring than before.
4509 \subsubsection{Coupling}
4510 One of the hopes when starting this project, was to be able to make a
4511 refactoring that could lower the coupling between classes. Better complexity at
4512 the method level is a not very unexpected byproduct of dividing methods into
4513 smaller parts. Lowering the coupling on the other hand, is a far greater task.
4514 This is also reflected in the results for the only coupling rule defined in the
4515 \name{SonarQube} quality profile, namely the ``Classes should not be coupled to
4517 other classes (Single Responsibility Principle)'' rule.
4519 The number of issues for the coupling rule is 1,098 before the refactoring, and
4520 1,199 afterwards. This is an increase in issues of 9.2\%. These numbers can be
4521 interpreted two ways. The first possibility is that our assumptions are wrong,
4522 and that increasing indirection does not decrease coupling between classes. The
4523 other possibility is that our analysis and choices of candidate text selections
4524 are not good enough. I vote for the second possibility. (Voting against the
4525 public opinion may also be a little bold.)
4527 \subsubsection{An example of what makes the number of dependency issues grow}
4528 \Myref{lst:sonarJDTExampleBefore} shows a portion of the class
4529 \typewithref{org.eclipse.jdt.ui.actions}{ShowActionGroup} from the JDT UI
4530 project before it is refactored with the search-based \ExtractAndMoveMethod
4531 refactoring. Before the refactoring, the \type{ShowActionGroup} class has 12
4532 outgoing dependencies (reported by \name{SonarQube}).
4534 \begin{listing}[htb]
4535 \begin{minted}[linenos,samepage]{java}
4536 public class ShowActionGroup extends ActionGroup {
4538 private void initialize(IWorkbenchSite site,
4539 boolean isJavaEditor) {
4541 ISelectionProvider provider= fSite.getSelectionProvider();
4542 ISelection selection= provider.getSelection();
4543 fShowInPackagesViewAction.update(selection);
4544 if (!isJavaEditor) {
4545 provider.addSelectionChangedListener(
4546 fShowInPackagesViewAction);
4551 \caption{Portion of the \type{ShowActionGroup} class before refactoring.}
4552 \label{lst:sonarJDTExampleBefore}
4555 During the benchmark process, the search-based \ExtractAndMoveMethod refactoring
4556 extracts the lines 6 to 12 of the code in \myref{lst:sonarJDTExampleBefore}, and
4557 moves the new method to the move target, which is the field
4558 \var{fShowInPackagesViewAction} with type
4559 \typewithref{org.eclipse.jdt.ui.actions}{ShowInPackageViewAction}. The result is
4560 shown in \myref{lst:sonarJDTExampleAfter}.
4562 \begin{listing}[htb]
4563 \begin{minted}[linenos,samepage]{java}
4564 public class ShowActionGroup extends ActionGroup {
4566 private void initialize(IWorkbenchSite site,
4567 boolean isJavaEditor) {
4569 fShowInPackagesViewAction.generated_8019497110545412081(
4570 this, isJavaEditor);
4575 \begin{minted}[linenos,samepage]{java}
4576 public class ShowInPackageViewAction
4577 extends SelectionDispatchAction {
4579 public void generated_8019497110545412081(
4580 ShowActionGroup showactiongroup, boolean isJavaEditor) {
4581 ISelectionProvider provider=
4582 showactiongroup.fSite.getSelectionProvider();
4583 ISelection selection= provider.getSelection();
4585 if (!isJavaEditor) {
4586 provider.addSelectionChangedListener(this);
4591 \caption{Portions of the classes \type{ShowActionGroup} and
4592 \type{ShowInPackageViewAction} after refactoring.}
4593 \label{lst:sonarJDTExampleAfter}
4596 After the refactoring, the \type{ShowActionGroup} has only 11 outgoing
4597 dependencies. It no longer depends on the
4598 \typewithref{org.eclipse.jface.viewers}{ISelection} type. So our refactoring
4599 managed to get rid of one dependency, which is exactly what we wanted. The only
4600 problem is, that now the \type{ShowInPackageViewAction} class has got two new
4601 dependencies, in the \type{ISelectionProvider} and the \type{ISelection} types.
4602 The bottom line is that we eliminated one dependency, but introduced two more,
4603 ending up with a program that has more dependencies now than when we started.
4605 What can happen in many situations where the \ExtractAndMoveMethod refactoring
4606 is performed, is that the \MoveMethod refactoring ``drags'' with it references
4607 to classes that are unknown to the method destination. If the refactoring
4608 happens to be so lucky that it removes a dependency from one class, it might as
4609 well introduce a couple of new dependencies to another class, as shown in the
4610 previous example. In those situations where a destination class does not know
4611 about the originating class of a moved method, the \MoveMethod refactoring most
4612 certainly will introduce a dependency. This is because there is a
4613 bug\footnote{\href{https://bugs.eclipse.org/bugs/show\_bug.cgi?id=228635}{Eclipse
4614 Bug 228635 - [move method] unnecessary reference to source}} in the refactoring,
4615 making it pass an instance of the originating class as a reference to the moved
4616 method, regardless of whether the reference is used in the method body or not.
4618 There is also the possibility that the heuristics used to find candidate text
4619 selections are not good enough. There is work to be done with fine-tuning the
4620 heuristics and to complete the analysis part of this project.
4622 \subsubsection{Totals}
4623 On the bright side, the total number of issues is lower after the refactoring
4624 than it was before. Before the refactoring, the total number of issues was
4625 8,270, and after it is 8,155. This is an improvement of 1.4\%.
4627 The down side is that \name{SonarQube} shows that the total cyclomatic
4628 complexity has increased by 2.9\%, and that the (more questionable) ``technical
4629 debt'' has increased from 1,003.4 to 1,032.7 days, also a deterioration of
4630 2.9\%. Although these numbers are similar, no correlation has been found
4633 \subsection{Unit tests}
4634 The tests that have been run for the \name{Eclipse JDT UI} project, are the
4635 test suites specified as the main test suites on the JDT UI wiki page on how to
4637 project\footnote{\url{https://wiki.eclipse.org/JDT\_UI/How\_to\_Contribute\#Unit\_Testing}}.
4638 The results from these tests are shown in \myref{tab:case1UnitTests}.
4641 \caption{Results from the unit tests run for the Eclipse JDT UI project,
4642 before and after the refactoring.}
4643 \label{tab:case1UnitTests}
4645 \begin{tabularx}{\textwidth}{@{}>{\bfseries}L{1}R{0.5}R{0.5}@{}}
4647 \textnormal{AutomatedSuite} & Before & After \\
4649 Runs & 2007/2007 & 2007/2007 \\
4653 \spancols{2}{AllAllRefactoringTests} \\
4655 Runs & 3815/3816 & 3815/3816 \\
4656 Errors & 2 & 2257 \\
4662 \subsubsection{Before the refactoring}
4663 Running the tests for the before-code of Eclipse JDT UI yielded 4 errors and 3
4664 failures for the \type{AutomatedSuite} test suite (2,007 test cases), and 2
4665 errors and 3 failures for the \type{AllAllRefactoringTests} test suite (3,816
4668 \subsubsection{After the refactoring}
4669 For the after-code of the Eclipse JDT UI project, Eclipse reports that the
4670 project contains 322 compilation errors, and a lot of other errors that
4671 follow from these. All of the errors are caused by the \ExtractAndMoveMethod
4672 refactoring. Had these errors originated from only one bug, it would not have
4673 been much of a problem, but this is not the case. By only looking at some random
4674 compilation problems in the refactored code, I came up with at least four
4675 different bugs \todo{write bug reports?} that caused those problems. I then
4676 stopped looking for more, since some of the bugs would take more time to fix
4677 than I could justify using on them at this point.
4679 One thing that can be said in my defense, is that all the compilation errors
4680 could have been avoided if the types of situations that cause them were properly
4681 handled by the primitive refactorings, which again are supplied by the Eclipse
4682 JDT UI project. All four bugs that I mentioned before are weaknesses of the
4683 \MoveMethod refactoring. If the primitive refactorings had detected the
4684 up-coming errors in their precondition checking phase, the refactorings would
4685 have been aborted, since this is how the \ExtractAndMoveMethod refactoring
4686 handles such situations. This shows that it is not safe to completely rely upon
4687 the primitive refactorings to save us if our own pre-refactoring analysis fails
4688 to detect that a compilation error will be introduced. A problem is that the
4689 source code analysis done by both the JDT refactorings and our own tool is
4692 Of course, taking into account all possible situations that could lead to
4693 compilation errors is an immense task. A complete analysis of these situations
4694 is too big of a problem for this master's project to solve. Looking at it now,
4695 this comes as no surprise, since the task is obviously also too big for the
4696 creators of the primitive \MoveMethod refactoring.
4698 Considering all these problems, it is difficult to know how to interpret the
4699 unit test results from after refactoring the Eclipse JDT UI. The
4700 \type{AutomatedSuite} reported 565 errors and 5 failures, which means that 1437,
4701 or 71.6\%, of the tests still passed. Three of the failures were the same as
4702 reported before the refactoring took place, so two of them are new. For these
4703 two cases it is not immediately apparent what makes them behave differently. The
4704 program is so complex that to analyze it to find this out, we might need more
4705 powerful methods than just manually analyzing its source code. This is somewhat
4706 characteristic for imperative programming: The programs are often hard to
4707 analyze and understand.
4709 For the \type{AllAllRefactoringTests} test suite, the three failures are gone,
4710 but the two errors have grown to 2,257 errors. I will not try to analyze those
4713 What I can say at this point, is that it is likely that the
4714 \ExtractAndMoveMethod refactoring has introduced some unintentional behavioral
4715 changes. Let us say that the refactoring introduces at least two
4716 behavior-altering changes for every 2,500 executions. More than that is
4717 difficult to say about the behavior-preserving properties of the
4718 \ExtractAndMoveMethod refactoring, at this point.
4720 \subsection{Conclusions}
4721 After automatically analyzing and executing the \ExtractAndMoveMethod
4722 refactoring for all the methods in the Eclipse JDT UI project, the results do
4723 not look that promising. For this case, the refactoring seems almost unusable as
4724 it is now. The error rate and measurements tell us this.
4726 The refactoring makes the code a little less complex at the method level. But
4727 this is merely a side effect of extracting methods. When it comes to the overall
4728 complexity, it is increased, although it is slightly better spread among the
4731 The analysis done before the \ExtractAndMoveMethod refactoring, is currently not
4732 complete enough to make the refactoring useful. It introduces too many errors in
4733 the code, and the code may change its behavior. It also remains to prove that
4734 large scale refactoring with it can decrease coupling between classes. A better
4735 analysis may prove this, but in its present state, the opposite is the fact. The
4736 coupling measurements done by \name{SonarQube} show this.
4738 On the bright side, the performance of the refactoring process is not that bad.
4739 It shows that it is possible to make a tool the way we do, if we can make the
4740 tool do anything useful. As long as the analysis phase is not going to involve
4741 anything that uses too much disk access, a lot of analysis can be done in a
4742 reasonable amount of time.
4744 The time used on performing the actual changes excludes a trial and error
4745 approach with the tools used in this master's project. In a trial and error
4746 approach, you could for instance be using the primitive refactorings used in
4747 this project to refactor code, and only then make decisions based on the effect,
4748 possibly shown by traditional software metrics. The problem with the approach
4749 taken in this project, compared to a trial and error approach, is that using
4750 heuristics beforehand is much more complicated. But on the other hand, a trial
4751 and error approach would still need to face the challenges of producing code
4752 that does compile without errors. If using refactorings that could produce
4753 in-memory changes, a trial and error approach could be made more efficient.
4755 \section{Case 2: The \type{no.uio.ifi.refaktor} project}
4756 In this case we will see a form of the ``dogfooding'' methodology used, when
4757 refactoring our own \type{no.uio.ifi.refaktor} project with the
4758 \ExtractAndMoveMethod refactoring.
4760 In this case I will try to point out some differences from the first case, and
4761 how they impact the execution of the benchmark. The refaktor project is 39 times
4762 smaller than the Eclipse JDT UI project, measured in lines of code. This will
4763 make things a bit more transparent. It will therefore be interesting to see if
4764 this case can shed light on any aspect of our project that were lost in the
4767 The configuration for the experiment is specified in
4768 \myref{tab:configurationCase2}.
4771 \caption{Configuration for Case 2.}
4772 \label{tab:configurationCase2}
4774 \begin{tabularx}{\textwidth}{@{}>{\bfseries}L{0.67}L{1.33}@{}}
4776 \spancols{2}{Benchmark data} \\
4778 Launch configuration & CaseStudyDogfooding.launch \\
4779 Project & no.uio.ifi.refaktor.benchmark \\
4780 Repository & gitolite@git.uio.no:ifi-stolz-refaktor \\
4781 Commit & 43c16c04520746edd75f8dc2a1935781d3d9de6c \\
4783 \spancols{2}{Input data} \\
4785 Project & no.uio.ifi.refaktor \\
4786 Repository & gitolite@git.uio.no:ifi-stolz-refaktor \\
4787 Commit & 43c16c04520746edd75f8dc2a1935781d3d9de6c \\
4789 Test configuration & no.uio.ifi.refaktor.tests/ExtractTest.launch \\
4794 \subsection{Statistics}
4795 The statistics gathered during the refactoring execution is presented in
4796 \myref{tab:case2Statistics}.
4799 \caption{Statistics after batch refactoring the \type{no.uio.ifi.refaktor}
4800 project with the \ExtractAndMoveMethod refactoring.}
4801 \label{tab:case2Statistics}
4803 \begin{tabularx}{\textwidth}{@{}>{\bfseries}L{1.5}R{0.5}@{}}
4805 \spancols{2}{Time used} \\
4807 Total time & 1m15s \\
4808 Analysis time & 0m18s (24\%) \\
4809 Change time & 0m47s (63\%) \\
4810 Miscellaneous tasks & 0m10s (14\%) \\
4812 \spancols{2}{Numbers of each type of entity analyzed} \\
4815 Compilation units & 154 \\
4818 Text selections & 8,609 \\
4820 \spancols{2}{Numbers for \ExtractAndMoveMethod refactoring candidates} \\
4822 Methods chosen as candidates & 58 \\
4823 Methods NOT chosen as candidates & 1,012 \\
4824 Candidate selections (multiple per method) & 227 \\
4826 \spancols{2}{\ExtractAndMoveMethod refactorings executed} \\
4828 Fully executed & 53 \\
4829 Not fully executed & 5 \\
4830 Total attempts & 58 \\
4832 \spancols{2}{Primitive refactorings executed} \\
4833 \spancols{2}{\small \ExtractMethod refactorings} \\
4836 Not performed & 2 \\
4837 Total attempts & 58 \\
4839 \spancols{2}{\small \MoveMethod refactorings} \\
4842 Not performed & 3 \\
4843 Total attempts & 56 \\
4849 \subsubsection{Differences}
4850 There are some differences between the two projects that make them a little
4851 difficult to compare by performance.
4853 \paragraph{Different complexity.}
4854 Although the JDT UI project is 39 times greater than the refaktor project in
4855 terms of lines of code, it is only about 26 times its size measured in numbers
4856 of methods. This means that the methods in the refaktor project are smaller in
4857 average than in the JDT project. This is also reflected in the \name{SonarQube}
4858 report, where the complexity per method for the JDT project is 3.6, while the
4859 refaktor project has a complexity per method of 2.1.
4861 \paragraph{Number of selections per method.}
4862 The analysis for the JDT project processed 21 text selections per method in
4863 average. This number for the refaktor project is only 8 selections per method
4864 analyzed. This is a direct consequence of smaller methods.
4866 \paragraph{Different candidates to methods ratio.}
4867 The differences in how the projects are factored are also reflected in the
4868 ratios for how many methods that are chosen as candidates compared to the total
4869 number of methods analyzed. For the JDT project, 9\% of the methods were
4870 considered to be candidates, while for the refaktor project, only 5\% of the
4871 methods were chosen.
4873 \paragraph{The average number of possible candidate selection.}
4874 For the methods that are chosen as candidates, the average number of possible
4875 candidate selections for these methods differ quite much. For the JDT project,
4876 the number of possible candidate selections for these methods was 14.44
4877 selections per method, while the candidate methods in the refaktor project had
4878 only 3.91 candidate selections to choose from, in average.
4880 \subsubsection{Execution time}
4881 The differences in complexity, and the different candidate methods to total
4882 number of methods ratios, is shown in the distributions of the execution times.
4883 For the JDT project, 75\% of the total time was used on the actual changes,
4884 while for the refaktor project, this number was only 63\%.
4886 For the JDT project, the benchmark used on average 0.21 seconds per method in
4887 the project, while for the refaktor project it used only 0.07 seconds per
4888 method. So the process used 3 times as much time per method for the JDT project
4889 than for the refaktor project.
4891 While the JDT project is 39 times larger than the refaktor project measured in
4892 lines of code, the benchmark used about 79 times as long time on it than for the
4893 refaktor project. Relatively, this is about twice as long.
4895 Since the details of these execution times are not that relevant to this
4896 master's project, only their magnitude, I will leave them here.
4898 \subsubsection{Executed refactorings}
4899 For the composite \ExtractAndMoveMethod refactoring performed in case 2, 53
4900 successful attempts out of 58 gives a success rate of 91.4\%. This is 5.3
4901 percentage points worse than for the first case.
4903 \subsection{\name{SonarQube} analysis}
4904 Results from the \name{SonarQube} analysis are shown in
4905 \myref{tab:case2ResultsProfile1}.
4907 Not much is to be said about these results. The trends in complexity and
4908 coupling are the same. We end up a little worse after the refactoring process
4912 \caption{Results for analyzing the \var{no.uio.ifi.refaktor} project, before
4913 and after the refactoring, with \name{SonarQube} and the \name{IFI Refaktor
4914 Case Study} quality profile. (Bold numbers are better.)}
4915 \label{tab:case2ResultsProfile1}
4917 \begin{tabularx}{\textwidth}{@{}>{\bfseries}L{1.5}R{0.25}R{0.25}@{}}
4919 \textnormal{Number of issues for each rule} & Before & After \\
4921 Avoid too complex class & 1 & 1 \\
4922 Classes should not be coupled to too many other classes (Single
4923 Responsibility Principle) & \textbf{29} & 34 \\
4924 Control flow statements \ldots{} should not be nested too deeply & 24 &
4926 Methods should not be too complex & 17 & \textbf{15} \\
4927 Methods should not have too many lines & 41 & \textbf{40} \\
4928 NPath Complexity & 3 & 3 \\
4929 Too many methods & \textbf{13} & 15 \\
4931 Total number of issues & \textbf{128} & 129 \\
4934 \spancols{3}{Complexity} \\
4936 Per function & 2.1 & 2.1 \\
4937 Per class & \textbf{12.5} & 12.9 \\
4938 Per file & \textbf{13.8} & 14.2 \\
4940 Total complexity & \textbf{2,089} & 2,148 \\
4943 \spancols{3}{Numbers of each type of entity analyzed} \\
4945 Files & 151 & 151 \\
4946 Classes & 167 & 167 \\
4947 Functions & 987 & 1,045 \\
4948 Accessors & 35 & 30 \\
4949 Statements & 3,355 & 3,416 \\
4950 Lines of code & 8,238 & 8,460 \\
4952 Technical debt (in days) & \textbf{19.0} & 20.7 \\
4957 \subsection{Unit tests}
4958 The tests used for this case are the same that has been developed throughout
4959 this master's project.
4961 The code that was refactored for this case suffered from some of the problems
4962 discovered in the first case. This means that the after-code for this case also
4963 contained compilation errors, but they were not as many. The code contained only
4964 6 errors that made the code not compile.
4966 All of the six errors originated from the same bug. The bug arises in a
4967 situation where a class instance creation is moved between packages, and the
4968 class for the instance is package-private. The \MoveMethod refactoring does not
4969 detect that there will be a visibility problem, and neither does it promote the
4970 package-private class to be public.
4972 Since the errors in the refactored refaktor code were easy to fix manually, I
4973 corrected them and ran the unit tests as planned. The unit test results are
4974 shown in \myref{tab:case2UnitTests}. Before the refactoring, all tests passed.
4975 All tests also passed after the refactoring, with the six error corrections.
4976 Since the corrections done are not of a kind that could make the behavior of the
4977 program change, it is likely that the refactorings done to the
4978 \type{no.uio.ifi.refaktor} project did not change its behavior. This is also
4979 supported by the informal experiment presented next.
4982 \caption{Results from the unit tests run for the \type{no.uio.ifi.refaktor}
4983 project, before and after the refactoring (with 6 corrections done to the
4985 \label{tab:case2UnitTests}
4987 \begin{tabularx}{\textwidth}{@{}>{\bfseries}L{1}R{0.5}R{0.5}@{}}
4991 Runs & 148/148 & 148/148 \\
4998 \subsection{An additional experiment}
4999 To complete the task of ``eating my own dog food'', I conducted an experiment
5000 where I used the refactored version of the \type{no.uio.ifi.refaktor} project,
5001 with the corrections, to again refaktor ``itself''.
5003 The experiment produced code containing the same six errors as after the
5004 previous experiment. I also compared the after-code from the two experiments
5005 with a diff-tool. The only differences found were different method names. This
5006 is expected, since the method names are randomly generated by the
5007 \ExtractAndMoveMethod refactoring.
5009 The outcome of this simple experiment makes me more confident that the
5010 \ExtractAndMoveMethod refactoring made only behavior-preserving changes to the
5011 \type{no.uio.ifi.refaktor} project, apart from the compilation errors.
5013 \subsection{Conclusions}
5014 The differences in complexity between the Eclipse JDT UI project and the
5015 \type{no.uio.ifi.refaktor} project, clearly influenced the differences in their
5016 execution times. This is mostly because fewer of the methods were chosen to be
5017 refactored for the refaktor project than for the JDT project. This makes it
5018 difficult to know if there are any severe performance penalties associated with
5019 refactoring on a large project compared to a small one.
5021 The trends in the \name{SonarQube} analysis are the same for this case as for
5022 the previous one. This gives more confidence in the these results.
5024 By refactoring our own code and using it again to refactor our code, we showed
5025 that it is possible to write an automated composite refactoring that works for
5026 many cases. That it probably did not alter the behavior of a smaller project
5027 shows us nothing more than that though, and might just be a coincidence.
5030 \todoin{Write? Or wrap up in final conclusions?}
5031 \todoin{``Threats to validity''}
5034 \chapter{Conclusions and future work}
5035 This chapter will conclude this master's thesis. It will try to give justified
5036 answers to the research questions posed \see{sec:researchQuestions} and present
5037 some future work that could be done to take this project to the next level.
5039 \section{Conclusions}
5040 One of the motivations for this thesis was to create a fully automated composite
5041 refactoring that could be used to make program source code better in terms of
5042 coupling between classes. Earlier, in \mysimpleref{sec:CBO}, it was shown that a
5043 composition of the \ExtractMethod and the \MoveMethod refactorings reduces the
5044 coupling between two classes in an ideal situation. The Eclipse IDE implements
5045 both these refactorings, as well as providing a framework for analyzing source
5046 code, so it was considered a suitable tool to build upon for our project.
5048 The search-based \ExtractAndMoveMethod refactoring was created by utilizing the
5049 analysis and refactoring support of Eclipse, and a small framework was built
5050 for executing large scale refactoring with it. The refactoring was set up to
5051 analyze and execute changes on the Eclipse JDT UI project. Statistics was
5052 gathered during this process and the resulting code was analyzed through
5053 SonarQube. The project's own unit tests were also performed to find out if our
5054 refactoring introduces any behavior-altering changes in the code it refactor.
5056 \paragraph{Answering the main research question.}
5057 The first and greatest challenge was to find out if the \ExtractAndMoveMethod
5058 refactoring could be automated, in all tasks ranging from analysis to executing
5059 changes. It is now confirmed that this can be done, since it has been
5060 implemented as a part of the work done for this project. It has also been shown
5061 that the refactoring can be used to refactor large code bases, through the case
5062 study done on the Eclipse JDT UI project.
5064 If we ask if using the existing Eclipse refactorings for this task is
5065 \emph{easy}, this is another question. The refactorings provided by the JDT UI
5066 project are clearly not meant to be combined in any way. The preconditions for
5067 one refactoring are not always easily retrievable after the execution of
5068 another. Also, the refactorings are all assuming that the code they shall
5069 refactor is textualized. This means that the source code must be parsed between
5070 the executions of each refactoring. Another problem with this dependency on
5071 textual changes is that you cannot make a composition of two refactorings appear
5072 as one change if their changes overlap. This will make the undo-history of the
5073 refactoring show two changes instead of one, and is not nice for usability it
5074 the refactoring would be used as an on-demand refactoring in an IDE.
5076 Apart from the problems with implementing the actual refactoring, the analysis
5077 framework is quite nicely solved in Eclipse. The AST generated when parsing
5078 source code supports using visitors to traverse it, and this works without
5081 \paragraph{Is the refactoring efficient enough?}
5082 Since we have concluded that the search-based \ExtractAndMoveMethod refactoring
5083 is not suitable for on-demand large scale refactoring, but may be put to better
5084 use as a kind of analysis tool, superb performance is not crucial. By being able
5085 to process over 300,000 pure lines of code in about 1.5 hours on a mid-level
5086 laptop computer, the refactoring must be said to perform well enough for this
5087 purpose. In comparison, the \name{SonarQube} analysis consumes about the same
5088 amount of time. If performed on demand for a single method, the performance of
5089 the \ExtractAndMoveMethod refactoring is no issue.
5091 \paragraph{What about breaking the source code?}
5092 The case studies showed that our safety measures that rely on the precondition
5093 checking of the existing primitive refactorings are not good enough in practice.
5094 If we were going to assure that code we change compiles, we would need to
5095 consider all possible situations where the refactoring could fail and search for
5096 them in our analysis. It is an open question if this is even feasible. Our
5097 analysis is incomplete, and so is the analysis for the \ExtractMethod and the
5098 \MoveMethod refactorings.
5100 Our refactoring does not take any precautions to preserve behavior. A few
5101 running and failing unit test for the JDT UI project after the refactoring
5102 indicate that our refactoring probably causes some changes to the way a program
5105 \paragraph{Is the quality of the code improved?}
5106 For coupling, there is no evidence that the refactoring improves the quality of
5107 source code. Shall we believe the SonarQube analysis from the case studies, our
5108 refactoring makes classes more coupled after the refactoring than before, in the
5109 general case. This is probably because our analysis and heuristics for finding
5110 the best candidates for the refactoring are not adequate.
5112 \paragraph{Is the refactoring useful?}
5113 In its present state, the refactoring cannot be said to be very useful. It
5114 generates too many compilation errors for it to fall into that category. On the
5115 other hand, if the problems with the search-based \ExtractAndMoveMethod
5116 refactoring were to be solved it could be useful in some situations.
5118 If the refactoring was perfected, it could of course be used as a regular
5119 on-demand automated refactoring on a per method base (or per class, package or
5122 As it is now, the refactoring is not very well suited to be set to perform
5123 unattended refactoring. But if we could find a way to filter out the changes
5124 that create compilation errors, we could use the refactoring to look for
5125 improvement points in a software project. This process could for instance be
5126 scheduled to run at regular times, possibly after a nightly build or the like.
5127 Then the results could be made available, and an administrator could be set to
5128 review them and choose whether or not they should be performed.
5130 \section{Future work}
5131 \todoin{Find out if a complete analysis is feasible}
5132 \todoin{Complete the analysis}
5133 \todoin{Make refactorings safer (behavior)}
5134 \todoin{Improve heuristics/introduce metrics}
5140 \chapter{Eclipse bugs submitted}
5141 \newcommand{\submittedBugReport}[1]{The submitted bug report can be found on
5144 \section{Eclipse bug 420726: Code is broken when moving a method that is
5145 assigning to the parameter that is also the move
5146 destination}\label{eclipse_bug_420726}
5148 was found when analyzing what kinds of names that were to be considered as
5149 \emph{unfixes} \see{unfixes}.
5152 The bug emerges when trying to move a method from one class to another, and when
5153 the target for the move (must be a variable, local or field) is both a parameter
5154 variable and also is assigned to within the method body. \name{Eclipse} allows this to
5155 happen, although it is the sure path to a compilation error. This is because we
5156 would then have an assignment to a \var{this} expression, which is not allowed
5158 \submittedBugReport{https://bugs.eclipse.org/bugs/show\_bug.cgi?id=420726}
5160 \paragraph{The solution}
5161 The solution to this problem is to add all simple names that are assigned to in
5162 a method body to the set of unfixes.
5164 \section{Eclipse bug 429416: IAE when moving method from anonymous
5165 class}\label{eclipse_bug_429416}
5167 this bug during a batch change on the \type{org.eclipse.jdt.ui} project.
5170 This bug surfaces when trying to use the \refa{Move Method} refactoring to move a
5171 method from an anonymous class to another class. This happens both for my
5172 simulation as well as in \name{Eclipse}, through the user interface. It only occurs
5173 when \name{Eclipse} analyzes the program and finds it necessary to pass an
5174 instance of the originating class as a parameter to the moved method. I.e. it
5175 wants to pass a \var{this} expression. The execution ends in an
5176 \typewithref{java.lang}{IllegalArgumentException} in
5177 \typewithref{org.eclipse.jdt.core.dom}{SimpleName} and its
5178 \method{setIdentifier(String)} method. The simple name is attempted created in
5180 \methodwithref{org.eclipse.jdt.internal.corext.refactoring.structure.\\MoveInstanceMethodProcessor}{createInlinedMethodInvocation}
5181 so the \type{MoveInstanceMethodProcessor} was early a clear suspect.
5183 The \method{createInlinedMethodInvocation} is the method that creates a method
5184 invocation where the previous invocation to the method that was moved was
5185 located. From its code it can be read that when a \var{this} expression is going
5186 to be passed in to the invocation, it shall be qualified with the name of the
5187 original method's declaring class, if the declaring class is either an anonymous
5188 class or a member class. The problem with this, is that an anonymous class does
5189 not have a name, hence the term \emph{anonymous} class! Therefore, when its
5190 name, an empty string, is passed into
5191 \methodwithref{org.eclipse.jdt.core.dom.AST}{newSimpleName} it all ends in an
5192 \type{IllegalArgumentException}.
5193 \submittedBugReport{https://bugs.eclipse.org/bugs/show\_bug.cgi?id=429416}
5195 \paragraph{How I solved the problem}
5196 Since the \type{MoveInstanceMethodProcessor} is instantiated in the
5197 \typewithref{no.uio.ifi.refaktor.change.executors}{MoveMethod\-RefactoringExecutor},
5198 and only need to be a
5199 \typewithref{org.eclipse.ltk.core.refactoring.participants}{MoveProcessor}, I
5200 was able to copy the code for the original move processor and modify it so that
5201 it works better for me. It is now called
5202 \typewithref{no.uio.ifi.refaktor.change.processors}{ModifiedMoveInstanceMethodProcessor}.
5203 The only modification done (in addition to some imports and suppression of
5204 warnings), is in the \method{createInlinedMethodInvocation}. When the declaring
5205 class of the method to move is anonymous, the \var{this} expression in the
5206 parameter list is not qualified with the declaring class' (empty) name.
5208 \section{Eclipse bug 429954: Extracting statement with reference to local type
5209 breaks code}\label{eclipse_bug_429954}
5210 The bug was discovered when doing some changes to the way unfixes is computed.
5213 The problem is that \name{Eclipse} is allowing selections that references variables of
5214 local types to be extracted. When this happens the code is broken, since the
5215 extracted method must take a parameter of a local type that is not in the
5216 methods scope. The problem is illustrated in
5217 \myref{lst:extractMethodLocalClass}, but there in another setting.
5218 \submittedBugReport{https://bugs.eclipse.org/bugs/show\_bug.cgi?id=429954}
5220 \paragraph{Actions taken}
5221 There are no actions directly springing out of this bug, since the Extract
5222 Method refactoring cannot be meant to be this way. This is handled on the
5223 analysis stage of our \refa{Extract and Move Method} refactoring. So names
5224 representing variables of local types are considered unfixes \see{unfixes}.