1 \documentclass[USenglish]{ifimaster}
3 \usepackage[utf8]{inputenc}
4 \usepackage[T1]{fontenc,url}
6 \usepackage{babel,textcomp,csquotes,ifimasterforside,varioref,graphicx}
7 \usepackage[style=numeric-comp,backend=bibtex]{biblatex}
11 \usepackage{perpage} %the perpage package
12 \MakePerPage{footnote} %the perpage package command
15 \newtheorem*{wordDef}{Definition}
17 \newcommand{\definition}[1]{\begin{wordDef}#1\end{wordDef}}
18 \newcommand{\see}[1]{(see section \ref{#1})}
19 \newcommand{\explanation}[3]{\noindent\textbf{\textit{#1}}\\*\emph{When:}
20 #2\\*\emph{How:} #3\\*[-7px]}
21 \newcommand{\type}[1]{\texttt{#1}}
22 \newcommand{\typeref}[1]{\footnote{\type{#1}}}
23 \newcommand{\typewithref}[2]{\type{#2}\typeref{#1.#2}}
24 \newcommand{\method}[1]{\type{#1}}
25 \newcommand{\methodref}[2]{\footnote{\type{#1}\method{\##2()}}}
26 \newcommand{\methodwithref}[2]{\method{#2}\footnote{\type{#1}\method{\##2()}}}
30 \subtitle{An unfinished essay}
31 \author{Erlend Kristiansen}
33 \bibliography{bibliography/master-thesis-erlenkr-bibliography}
49 To make it clear already from the beginning: The discussions in this report must
50 be seen in the context of object oriented programming languages, and Java in
51 particular, since that is the language in which most of the examples will be
52 given. All though the techniques discussed may be applicable to languages from
53 other paradigms, they will not be the subject of this report.
57 \chapter{What is Refactoring?}
59 This question is best answered by first defining the concept of a
60 \emph{refactoring}, what it is to \emph{refactor}, and then discuss what aspects
61 of programming that make people want to refactor their code.
63 \section{Defining refactoring}
64 Martin Fowler, in his masterpiece on refactoring \cite{refactoring}, defines a
65 refactoring like this:
67 \emph{Refactoring} (noun): a change made to the \todo{what does he mean by
68 internal?} internal structure of software to make it easier to understand and
69 cheaper to modify without changing its observable
70 behavior.~\cite{refactoring} % page 53
72 This definition assign additional meaning to the word \emph{refactoring}, beyond
73 the composition of the prefix \emph{re-}, usually meaning something like
74 ``again'' or ``anew'', and the word \emph{factoring}, that can mean to determine
75 the \emph{factors} of something. Where a \emph{factor} would be close to the
76 mathematical definition of something that divides a quantity, without leaving a
77 remainder. Fowler is mixing the \emph{motivation} behind refactoring into his
78 definition. Instead it could be made clean, only considering the mechanical and
79 behavioral aspects of refactoring. That is to factor the program again, putting
80 it together in a different way than before, while preserving the behavior of the
81 program. An alternative definition could then be:
83 \definition{A refactoring is a transformation
84 done to a program without altering its external behavior.}
86 From this we can conclude that a refactoring primarily changes how the
87 \emph{code} of a program is perceived by the \emph{programmer}, and not the
88 \emph{behavior} experienced by any user of the program. Although the logical
89 meaning is preserved, such changes could potentially alter the program's
90 behavior when it comes to performance gain or -penalties. So any logic depending
91 on the performance of a program could make the program behave differently after
94 In the extreme case one could argue that such a thing as \emph{software
95 obfuscation} is to refactor. If we where to define it as a refactoring, it could
96 be defined as a composite refactoring \see{intro_composite}, consisting of, for
97 instance, a series of rename refactorings. (But it could of course be much more
98 complex, and the mechanics of it would not exactly be carved in stone.) To
99 perform some serious obfuscation one would also take advantage of techniques not
100 found among established refactorings, such as removing whitespace. This might
101 not even generate a different syntax tree for languages not sensitive to
102 whitespace, placing it in the gray area of what kind of transformations is to be
103 considered refactorings.
105 Finally, to \emph{refactor} is (quoting Martin Fowler)
107 \ldots to restructure software by applying a series of refactorings without
108 changing its observable behavior.~\cite{refactoring} % page 54, definition
111 \section{The etymology of 'refactoring'}
112 It is a little difficult to pinpoint the exact origin of the word
113 ``refactoring'', as it seems to have evolved as part of a colloquial
114 terminology, more than a scientific term. There is no authoritative source for a
115 formal definition of it.
117 According to Martin Fowler~\cite{etymology-refactoring}, there may also be more
118 than one origin of the word. The most well-known source, when it comes to the
119 origin of \emph{refactoring}, is the Smalltalk\footnote{\emph{Smalltalk},
120 object-oriented, dynamically typed, reflective programming language.}\todo{find
121 reference to Smalltalk website or similar?} community and their infamous
123 Browser}\footnote{\url{http://st-www.cs.illinois.edu/users/brant/Refactory/RefactoringBrowser.html}}
124 described in the article \emph{A Refactoring Tool for
125 Smalltalk}~\cite{refactoringBrowser1997}, published in 1997.
126 Allegedly~\cite{etymology-refactoring}, the metaphor of factoring programs was
127 also present in the Forth\footnote{\emph{Forth} -- stack-based, extensible
128 programming language, without type-checking. See \url{http://www.forth.org}}
129 community, and the word ``refactoring'' is mentioned in a book by Leo Brodie,
130 called \emph{Thinking Forth}~\cite{brodie1984}, first published in
131 1984\footnote{\emph{Thinking Forth} was first published in 1984 by the
132 \emph{Forth Interest Group}. Then it was reprinted in 1994 with minor
133 typographical corrections, before it was transcribed into an electronic edition
134 typeset in \LaTeX\ and published under a Creative Commons licence in 2004. The
135 edition cited here is the 2004 edition, but the content should essentially be as
136 in 1984.}. The exact word is only printed one place\footnote{p. 232}, but the
137 term \emph{factoring} is prominent in the book, that also contains a whole
138 chapter dedicated to (re)factoring, and how to keep the (Forth) code clean and
141 \ldots good factoring technique is perhaps the most important skill for a
142 Forth programmer.~\cite{brodie1984}
144 Brodie also express what \emph{factoring} means to him:
146 Factoring means organizing code into useful fragments. To make a fragment
147 useful, you often must separate reusable parts from non-reusable parts. The
148 reusable parts become new definitions. The non-reusable parts become arguments
149 or parameters to the definitions.~\cite{brodie1984}
152 Fowler claims that the usage of the word \emph{refactoring} did not pass between
153 the \emph{Forth} and \emph{Smalltalk} communities, but that it emerged
154 independently in each of the communities.
158 \section{Motivation -- Why people refactor}
159 To get a grasp of what refactoring is all about, we can try to answer this
160 question: \emph{Why do people refactor?} Possible answers could include: ``To
161 remove duplication'' or ``to break up long methods''. Practitioners of the art
162 of Design Patterns~\cite{dp} could say that they do it to introduce a
163 long-needed pattern into their program's design. So it is safe to say that
164 peoples' intentions are to make their programs \emph{better} in some sense. But
165 what aspects of the programs are becoming improved?
167 As already mentioned, people often refactor to get rid of duplication. Moving
168 identical or similar code into methods, and maybe pushing those up or down in
169 their class hierarchies. Making template methods for overlapping
170 algorithms/functionality and so on. It's all about gathering what belongs
171 together and putting it all in one place. And the result? The code is easier to
172 maintain. When removing the implicit coupling between the code snippets, the
173 location of a bug is limited to only one place, and new functionality need only
174 to be added this one place, instead of a number of places people might not even
177 The same people find out that their program contains a lot of long and
178 hard-to-grasp methods. Then what do they do? They begin dividing their methods
179 into smaller ones, using the \emph{Extract Method}
180 refactoring~\cite{refactoring}. Then they may discover something about their
181 program that they weren't aware of before; revealing bugs they didn't know about
182 or couldn't find due to the complex structure of their program. \todo{Proof?}
183 Making the methods smaller and giving good names to the new ones clarifies the
184 algorithms and enhances the \emph{understandability} of the program
185 \see{magic_number_seven}. This makes simple refactoring an excellent method for
186 exploring unknown program code, or code that you had forgotten that you wrote!
188 The word \emph{simple} came up in the last section. In fact, most basic
189 refactorings are simple. The true power of them are revealed first when they are
190 combined into larger --- higher level --- refactorings, called \emph{composite
191 refactorings} \see{intro_composite}. Often the goal of such a series of
192 refactorings is a design pattern. Thus the \emph{design} can be evolved
193 throughout the lifetime of a program, opposed to designing up-front. It's all
194 about being structured and taking small steps to improve a program's design.
196 Many refactorings are aimed at lowering the coupling between different classes
197 and different layers of logic. \todo{which refactorings?} Say for instance that
198 the coupling between the user interface and the business logic of a program is
199 lowered. Then the business logic of the program could much easier be the target
200 of automated tests, increasing the productivity in the software development
201 process. It is also easier to distribute (e.g. between computers) the different
202 components of a program if they are sufficiently decoupled.
204 Another effect of refactoring is that with the increased separation of concerns
205 coming out of many refactorings, the \emph{performance} is improved. When
206 profiling programs, the problem parts are narrowed down to smaller parts of the
207 code, which are easier to tune, and optimization can be performed only where
208 needed and in a more effective way.
210 Last, but not least, and this should probably be the best reason to refactor, is
211 to refactor to \emph{facilitate a program change}. If one has managed to keep
212 one's code clean and tidy, and the code is not bloated with design patterns that
213 is not ever going to be needed, then some refactoring might be needed to
214 introduce a design pattern that is appropriate for the change that is going to
217 Refactoring program code --- with a goal in mind --- can give the code itself
218 more value. That is in the form of robustness to bugs, understandability and
219 maintainability. With the first as an obvious advantage, but with the following
220 two being also very important for software development. By incorporating
221 refactoring in the development process, bugs are found faster, new functionality
222 is added more easily and code is easier to understand by the next person exposed
223 to it, which might as well be the person who wrote it. The consequence of this,
224 is that refactoring can increase the average productivity of the development
225 process, and thus also add to the monetary value of a business in the long run.
226 Where this last point also should open the eyes of some nearsighted managers who
227 seldom see beyond the next milestone.
229 \section{The magical number seven}\label{magic_number_seven}
230 \emph{The magical number seven, plus or minus two: some limits on our capacity
231 for processing information}~\cite{miller1956} is an article by George A. Miller
232 that was published in the journal \emph{Psychological Review} in 1956. It
233 presents evidence that support that the capacity of the number of objects a
234 human being can hold in its working memory is roughly seven, plus or minus two
235 objects. This number varies a bit depending on the nature and complexity of the
236 objects, but is according to Miller ``\ldots never changing so much as to be
239 Miller's article culminates in the section called \emph{Recoding}, a term he
240 borrows from communication theory. The central result in this section is that by
241 recoding information, the capacity of the amount of information that a human can
242 process at a time is increased. By \emph{recoding}, Miller means to group
243 objects together in chunks and give each chunk a new name that it can be
244 remembered by. By organizing objects into patterns of ever growing depth, one
245 can memorize and process a much larger amount of data than if it were to be
246 represented as its basic pieces. This grouping and renaming is analogous to how
247 many refactorings work, by grouping pieces of code and give them a new name.
248 Examples are the central \emph{Extract Method} and \emph{Extract Class}
249 refactorings~\cite{refactoring}.
252 \ldots recoding is an extremely powerful weapon for increasing the amount of
253 information that we can deal with.~\cite{miller1956}
255 An example from the article address the problem of memorizing a sequence of
256 binary digits. Let us say we have the following sequence\footnote{The example
257 presented here is slightly modified (and shortened) from what is presented in
258 the original article~\cite{miller1956}, but it is essentially the same.} of
259 16 binary digits: ``1010001001110011''. Most of us will have a hard time
260 memorizing this sequence by only reading it once or twice. Imagine if we instead
261 translate it to this sequence: ``A273''. If you have a background from computer
262 science, it will be obvious that the latest sequence is the first sequence
263 recoded to be represented by digits with base 16. Most people should be able to
264 memorize this last sequence by only looking at it once.
266 Another result from the Miller article is that when the amount of information a
267 human must interpret increases, it is crucial that the translation from one code
268 to another must be almost automatic for the subject to be able to remember the
269 translation, before he or she is presented with new information to recode. Thus
270 learning and understanding how to best organize certain kinds of data is
271 essential to efficiently handle that kind of data in the future. This is much
272 like when children learn to read. First they must learn how to recognize
273 letters. Then they can learn distinct words, and later read sequences of words
274 that form whole sentences. Eventually, most of them will be able to read whole
275 books and briefly retell the important parts of its content. This suggest that
276 the use of design patterns~\cite{dp} is a good idea when reasoning about
277 computer programs. With extensive use of design patterns when creating complex
278 program structures, one does not always have to read whole classes of code to
279 comprehend how they function, it may be sufficient to only see the name of a
280 class to almost fully understand its responsibilities.
283 Our language is tremendously useful for repackaging material into a few chunks
284 rich in information.~\cite{miller1956}
286 Without further evidence, these results at least indicates that refactoring
287 source code into smaller units with higher cohesion and, when needed,
288 introducing appropriate design patterns, should aid in the cause of creating
289 computer programs that are easier to maintain and has code that is easier (and
292 \section{Notable contributions to the refactoring literature}
293 \todo{Update with more contributions}
295 \item[1992] William F. Opdyke submits his doctoral dissertation called
296 \emph{Refactoring Object-Oriented Frameworks}~\cite{opdyke1992}. This
297 work defines a set of refactorings, that are behavior preserving given that
298 their preconditions are met. The dissertation is focused on the automation
300 \item[1999] Martin Fowler et al.: \emph{Refactoring: Improving the Design of
301 Existing Code}~\cite{refactoring}. This is maybe the most influential text
302 on refactoring. It bares similarities with Opdykes thesis~\cite{opdyke1992}
303 in the way that it provides a catalog of refactorings. But Fowler's book is
304 more about the craft of refactoring, as he focuses on establishing a
305 vocabulary for refactoring, together with the mechanics of different
306 refactorings and when to perform them. His methodology is also founded on
307 the principles of test-driven development.
308 \item[todo] \emph{Refactoring to Patterns}\todo{include}
311 \section{Tool support}
312 \todo{write, section vs. subsection}
314 \section{Relation to design patterns}
315 \todo{write, section vs. subsection, refactoring to patterns?}
318 \section{Classification of refactorings}
319 % only interesting refactorings
320 % with 2 detailed examples? One for structured and one for intra-method?
321 % Is replacing Bubblesort with Quick Sort considered a refactoring?
323 \subsection{Structural refactorings}
325 \subsubsection{Basic refactorings}
328 \explanation{Extract Method}{You have a code fragment that can be grouped
329 together.}{Turn the fragment into a method whose name explains the purpose of
332 \explanation{Inline Method}{A method's body is just as clear as its name.}{Put
333 the method's body into the body of its callers and remove the method.}
335 \explanation{Inline Temp}{You have a temp that is assigned to once with a simple
336 expression, and the temp is getting in the way of other refactorings.}{Replace
337 all references to that temp with the expression}
339 % Moving Features Between Objects
340 \explanation{Move Method}{A method is, or will be, using or used by more
341 features of another class than the class on which it is defined.}{Create a new
342 method with a similar body in the class it uses most. Either turn the old method
343 into a simple delegation, or remove it altogether.}
345 \explanation{Move Field}{A field is, or will be, used by another class more than
346 the class on which it is defined}{Create a new field in the target class, and
347 change all its users.}
350 \explanation{Replace Magic Number with Symbolic Constant}{You have a literal
351 number with a particular meaning.}{Create a constant, name it after the meaning,
352 and replace the number with it.}
354 \explanation{Encapsulate Field}{There is a public field.}{Make it private and
357 \explanation{Replace Type Code with Class}{A class has a numeric type code that
358 does not affect its behavior.}{Replace the number with a new class.}
360 \explanation{Replace Type Code with Subclasses}{You have an immutable type code
361 that affects the behavior of a class.}{Replace the type code with subclasses.}
363 \explanation{Replace Type Code with State/Strategy}{You have a type code that
364 affects the behavior of a class, but you cannot use subclassing.}{Replace the
365 type code with a state object.}
367 % Simplifying Conditional Expressions
368 \explanation{Consolidate Duplicate Conditional Fragments}{The same fragment of
369 code is in all branches of a conditional expression.}{Move it outside of the
372 \explanation{Remove Control Flag}{You have a variable that is acting as a
373 control flag fro a series of boolean expressions.}{Use a break or return
376 \explanation{Replace Nested Conditional with Guard Clauses}{A method has
377 conditional behavior that does not make clear the normal path of
378 execution.}{Use guard clauses for all special cases.}
380 \explanation{Introduce Null Object}{You have repeated checks for a null
381 value.}{Replace the null value with a null object.}
383 \explanation{Introduce Assertion}{A section of code assumes something about the
384 state of the program.}{Make the assumption explicit with an assertion.}
386 % Making Method Calls Simpler
387 \explanation{Rename Method}{The name of a method does not reveal its
388 purpose.}{Change the name of the method}
390 \explanation{Add Parameter}{A method needs more information from its
391 caller.}{Add a parameter for an object that can pass on this information.}
393 \explanation{Remove Parameter}{A parameter is no longer used by the method
396 %\explanation{Parameterize Method}{Several methods do similar things but with
397 %different values contained in the method.}{Create one method that uses a
398 %parameter for the different values.}
400 \explanation{Preserve Whole Object}{You are getting several values from an
401 object and passing these values as parameters in a method call.}{Send the whole
404 \explanation{Remove Setting Method}{A field should be set at creation time and
405 never altered.}{Remove any setting method for that field.}
407 \explanation{Hide Method}{A method is not used by any other class.}{Make the
410 \explanation{Replace Constructor with Factory Method}{You want to do more than
411 simple construction when you create an object}{Replace the constructor with a
414 % Dealing with Generalization
415 \explanation{Pull Up Field}{Two subclasses have the same field.}{Move the field
418 \explanation{Pull Up Method}{You have methods with identical results on
419 subclasses.}{Move them to the superclass.}
421 \explanation{Push Down Method}{Behavior on a superclass is relevant only for
422 some of its subclasses.}{Move it to those subclasses.}
424 \explanation{Push Down Field}{A field is used only by some subclasses.}{Move the
425 field to those subclasses}
427 \explanation{Extract Interface}{Several clients use the same subset of a class's
428 interface, or two classes have part of their interfaces in common.}{Extract the
429 subset into an interface.}
431 \explanation{Replace Inheritance with Delegation}{A subclass uses only part of a
432 superclasses interface or does not want to inherit data.}{Create a field for the
433 superclass, adjust methods to delegate to the superclass, and remove the
436 \explanation{Replace Delegation with Inheritance}{You're using delegation and
437 are often writing many simple delegations for the entire interface}{Make the
438 delegating class a subclass of the delegate.}
440 \subsubsection{Composite refactorings}
443 % \explanation{Replace Method with Method Object}{}{}
445 % Moving Features Between Objects
446 \explanation{Extract Class}{You have one class doing work that should be done by
447 two}{Create a new class and move the relevant fields and methods from the old
448 class into the new class.}
450 \explanation{Inline Class}{A class isn't doing very much.}{Move all its features
451 into another class and delete it.}
453 \explanation{Hide Delegate}{A client is calling a delegate class of an
454 object.}{Create Methods on the server to hide the delegate.}
456 \explanation{Remove Middle Man}{A class is doing to much simple delegation.}{Get
457 the client to call the delegate directly.}
460 \explanation{Replace Data Value with Object}{You have a data item that needs
461 additional data or behavior.}{Turn the data item into an object.}
463 \explanation{Change Value to Reference}{You have a class with many equal
464 instances that you want to replace with a single object.}{Turn the object into a
467 \explanation{Encapsulate Collection}{A method returns a collection}{Make it
468 return a read-only view and provide add/remove methods.}
470 % \explanation{Replace Array with Object}{}{}
472 \explanation{Replace Subclass with Fields}{You have subclasses that vary only in
473 methods that return constant data.}{Change the methods to superclass fields and
474 eliminate the subclasses.}
476 % Simplifying Conditional Expressions
477 \explanation{Decompose Conditional}{You have a complicated conditional
478 (if-then-else) statement.}{Extract methods from the condition, then part, an
481 \explanation{Consolidate Conditional Expression}{You have a sequence of
482 conditional tests with the same result.}{Combine them into a single conditional
483 expression and extract it.}
485 \explanation{Replace Conditional with Polymorphism}{You have a conditional that
486 chooses different behavior depending on the type of an object.}{Move each leg
487 of the conditional to an overriding method in a subclass. Make the original
490 % Making Method Calls Simpler
491 \explanation{Replace Parameter with Method}{An object invokes a method, then
492 passes the result as a parameter for a method. The receiver can also invoke this
493 method.}{Remove the parameter and let the receiver invoke the method.}
495 \explanation{Introduce Parameter Object}{You have a group of parameters that
496 naturally go together.}{Replace them with an object.}
498 % Dealing with Generalization
499 \explanation{Extract Subclass}{A class has features that are used only in some
500 instances.}{Create a subclass for that subset of features.}
502 \explanation{Extract Superclass}{You have two classes with similar
503 features.}{Create a superclass and move the common features to the
506 \explanation{Collapse Hierarchy}{A superclass and subclass are not very
507 different.}{Merge them together.}
509 \explanation{Form Template Method}{You have two methods in subclasses that
510 perform similar steps in the same order, yet the steps are different.}{Get the
511 steps into methods with the same signature, so that the original methods become
512 the same. Then you can pull them up.}
515 \subsection{Functional refactorings}
517 \explanation{Substitute Algorithm}{You want to replace an algorithm with one
518 that is clearer.}{Replace the body of the method with the new algorithm.}
522 \section{The impact on software quality}
524 \subsection{What is meant by quality?}
525 The term \emph{software quality} has many meanings. It all depends on the
526 context we put it in. If we look at it with the eyes of a software developer, it
527 usually mean that the software is easily maintainable and testable, or in other
528 words, that it is \emph{well designed}. This often correlates with the
529 management scale, where \emph{keeping the schedule} and \emph{customer
530 satisfaction} is at the center. From the customers point of view, in addition to
531 good usability, \emph{performance} and \emph{lack of bugs} is always
532 appreciated, measurements that are also shared by the software developer. (In
533 addition, such things as good documentation could be measured, but this is out
534 of the scope of this document.)
536 \subsection{The impact on performance}
538 Refactoring certainly will make software go more slowly, but it also makes the
539 software more amenable to performance tuning.~\cite{refactoring} % page 69
541 There is a common belief that refactoring compromises performance, due to
542 increased degree of indirection and that polymorphism is slower than
545 In a survey, Demeyer~\cite{demeyer2002} disproves this view in the case of
546 polymorphism. He is doing an experiment on, what he calls, ``Transform Self Type
547 Checks'' where you introduce a new polymorphic method and a new class hierarchy
548 to get rid of a class' type checking of a ``type attribute``. He uses this kind
549 of transformation to represent other ways of replacing conditionals with
550 polymorphism as well. The experiment is performed on the C++ programming
551 language and with three different compilers and platforms. \todo{But is the
552 result better?} Demeyer concludes that, with compiler optimization turned on,
553 polymorphism beats middle to large sized if-statements and does as well as
554 case-statements. (In accordance with his hypothesis, due to similarities
555 between the way C++ handles polymorphism and case-statements.)
557 The interesting thing about performance is that if you analyze most programs,
558 you find that they waste most of their time in a small fraction of the code.
561 So, although an increased amount of method calls could potentially slow down
562 programs, one should avoid premature optimization and sacrificing good design,
563 leaving the performance tuning until after profiling\footnote{For and example of
564 a Java profiler, check out VisualVM: \url{http://visualvm.java.net/}} the
565 software and having isolated the actual problem areas.
569 \section{Correctness of refactorings}
570 \todo{Volker's example?}
572 \section{Composite refactorings} \label{intro_composite}
573 \todo{motivation, examples, manual vs automated?, what about refactoring in a
574 very large code base?}
576 \section{Software metrics}
580 %\chapter{Planning the project}
588 \section{The problem statement}
589 \section{Choosing the language}
590 \section{Choosing the tool}
592 \chapter{Refactorings in Eclipse JDT: Design, Shortcomings and Wishful
593 Thinking}\label{ch:jdt_refactorings}
595 This chapter will deal with some of the design behind refactoring support in
596 Eclipse, and the JDT in specific. After which it will follow a section about
597 shortcomings of the refactoring API in terms of composition of refactorings. The
598 chapter will be concluded with a section telling some of the ways the
599 implementation of refactorings in the JDT could have worked to facilitate
600 composition of refactorings.
603 The refactoring world of Eclipse can in general be separated into two parts: The
604 language independent part and the part written for a specific programming
605 language -- the language that is the target of the supported refactorings.
606 \todo{What about the language specific part?}
608 \subsection{The Language Toolkit}
609 The Language Toolkit, or LTK for short, is the framework that is used to
610 implement refactorings in Eclipse. It is language independent and provides the
611 abstractions of a refactoring and the change it generates, in the form of the
612 classes \typewithref{org.eclipse.ltk.core.refactoring}{Refactoring} and
613 \typewithref{org.eclipse.ltk.core.refactoring}{Change}. (There is also parts of
614 the LTK that is concerned with user interaction, but they will not be discussed
615 here, since they are of little value to us and our use of the framework.)
617 \subsubsection{The Refactoring Class}
618 The abstract class \type{Refactoring} is the core of the LTK framework. Every
619 refactoring that is going to be supported by the LTK have to end up creating an
620 instance of one of its subclasses. The main responsibilities of subclasses of
621 \type{Refactoring} is to implement template methods for condition checking
622 (\methodwithref{org.eclipse.ltk.core.refactoring.Refactoring}{checkInitialConditions}
624 \methodwithref{org.eclipse.ltk.core.refactoring.Refactoring}{checkFinalConditions}),
626 \methodwithref{org.eclipse.ltk.core.refactoring.Refactoring}{createChange}
627 method that creates and returns an instance of the \type{Change} class.
629 If the refactoring shall support that others participate in it when it is
630 executed, the refactoring has to be a processor-based
631 refactoring\typeref{org.eclipse.ltk.core.refactoring.participants.ProcessorBasedRefactoring}.
632 It then delegates to its given
633 \typewithref{org.eclipse.ltk.core.refactoring.participants}{RefactoringProcessor}
634 for condition checking and change creation.
636 \subsubsection{The Change Class}
637 This class is the base class for objects that is responsible for performing the
638 actual workspace transformations in a refactoring. The main responsibilities for
639 its subclasses is to implement the
640 \methodwithref{org.eclipse.ltk.core.refactoring.Change}{perform} and
641 \methodwithref{org.eclipse.ltk.core.refactoring.Change}{isValid} methods. The
642 \method{isValid} method verifies that the change object is valid and thus can be
643 executed by calling its \method{perform} method. The \method{perform} method
644 performs the desired change and returns an undo change that can be executed to
645 reverse the effect of the transformation done by its originating change object.
647 \subsubsection{Executing a Refactoring}\label{executing_refactoring}
648 The life cycle of a refactoring generally follows two steps after creation:
649 condition checking and change creation. By letting the refactoring object be
651 \typewithref{org.eclipse.ltk.core.refactoring}{CheckConditionsOperation} that
652 in turn is handled by a
653 \typewithref{org.eclipse.ltk.core.refactoring}{CreateChangeOperation}, it is
654 assured that the change creation process is managed in a proper manner.
656 The actual execution of a change object has to follow a detailed life cycle.
657 This life cycle is honored if the \type{CreateChangeOperation} is handled by a
658 \typewithref{org.eclipse.ltk.core.refactoring}{PerformChangeOperation}. If also
659 an undo manager\typeref{org.eclipse.ltk.core.refactoring.IUndoManager} is set
660 for the \type{PerformChangeOperation}, the undo change is added into the undo
663 \section{Shortcomings}
664 This section is introduced naturally with a conclusion: The JDT refactoring
665 implementation does not facilitate composition of refactorings.
666 \todo{refine}This section will try to explain why, and also identify other
667 shortcomings of both the usability and the readability of the JDT refactoring
670 I will begin at the end and work my way toward the composition part of this
673 \subsection{Absence of Generics in Eclipse Source Code}
674 This section is not only concerning the JDT refactoring API, but also large
675 quantities of the Eclipse source code. The code shows a striking absence of the
676 Java language feature of generics. It is hard to read a class' interface when
677 methods return objects or takes parameters of raw types such as \type{List} or
678 \type{Map}. This sometimes results in having to read a lot of source code to
679 understand what is going on, instead of relying on the available interfaces. In
680 addition, it results in a lot of ugly code, making the use of typecasting more
681 of a rule than an exception.
683 \subsection{Composite Refactorings Will Not Appear as Atomic Actions}
685 \subsubsection{Missing Flexibility from JDT Refactorings}
686 The JDT refactorings are not made with composition of refactorings in mind. When
687 a JDT refactoring is executed, it assumes that all conditions for it to be
688 applied successfully can be found by reading source files that has been
689 persisted to disk. They can only operate on the actual source material, and not
690 (in-memory) copies thereof. This constitutes a major disadvantage when trying to
691 compose refactorings, since if an exception occur in the middle of a sequence of
692 refactorings, it can leave the project in a state where the composite
693 refactoring was executed only partly. It makes it hard to discard the changes
694 done without monitoring and consulting the undo manager, an approach that is not
697 \subsubsection{Broken Undo History}
698 When designing a composed refactoring that is to be performed as a sequence of
699 refactorings, you would like it to appear as a single change to the workspace.
700 This implies that you would also like to be able to undo all the changes done by
701 the refactoring in a single step. This is not the way it appears when a sequence
702 of JDT refactorings is executed. It leaves the undo history filled up with
703 individual undo actions corresponding to every single JDT refactoring in the
704 sequence. This problem is not trivial to handle in Eclipse. (See section
705 \ref{hacking_undo_history}.)
707 \section{Wishful Thinking}
711 \chapter{Composite Refactorings in Eclipse}
713 \section{A Simple Ad Hoc Model}
714 As pointed out in chapter \ref{ch:jdt_refactorings}, the Eclipse JDT refactoring
715 model is not very well suited for making composite refactorings. Therefore a
716 simple model using changer objects (of type \type{RefaktorChanger}) is used as
717 an abstraction layer on top of the existing Eclipse refactorings.
719 \section{The Extract and Move Method Refactoring}
720 %The Extract and Move Method Refactoring is implemented mainly using these
723 % \item \type{ExtractAndMoveMethodChanger}
724 % \item \type{ExtractAndMoveMethodPrefixesExtractor}
725 % \item \type{Prefix}
726 % \item \type{PrefixSet}
729 \subsection{The Building Blocks}
730 This is a composite refactoring, and hence is built up using several primitive
731 refactorings. These basic building blocks are, as its name implies, the Extract
732 Method Refactoring \cite{refactoring} and the Move Method Refactoring
733 \cite{refactoring}. In Eclipse, the implementations of these refactorings are
735 \typewithref{org.eclipse.jdt.internal.corext.refactoring.code}{ExtractMethodRefactoring}
737 \typewithref{org.eclipse.jdt.internal.corext.refactoring.structure}{MoveInstanceMethodProcessor},
738 where the last class is designed to be used together with the processor-based
739 \typewithref{org.eclipse.ltk.core.refactoring.participants}{MoveRefactoring}.
741 \subsubsection{The ExtractMethodRefactoring Class}
742 This class is quite simple in its use. The only parameters it requires for
743 construction is a compilation
744 unit\typeref{org.eclipse.jdt.core.ICompilationUnit}, the offset into the source
745 code where the extraction shall start, and the length of the source to be
746 extracted. Then you have to set the method name for the new method together with
747 which access modifier that shall be used and some not so interesting parameters.
749 \subsubsection{The MoveInstanceMethodProcessor Class}
750 For the Move Method the processor requires a little more advanced input than
751 the class for the Extract Method. For construction it requires a method
752 handle\typeref{org.eclipse.jdt.core.IMethod} from the Java Model for the method
753 that is to be moved. Then the target for the move have to be supplied as the
754 variable binding from a chosen variable declaration. In addition to this, one
755 have to set some parameters regarding setters/getters and delegation.
757 To make a whole refactoring from the processor, one have to construct a
758 \type{MoveRefactoring} from it.
760 \subsection{The ExtractAndMoveMethodChanger Class}
761 The \typewithref{no.uio.ifi.refaktor.changers}{ExtractAndMoveMethodChanger}
762 class, that is a subclass of the class
763 \typewithref{no.uio.ifi.refaktor.changers}{RefaktorChanger}, is the class
764 responsible for composing the \type{ExtractMethodRefactoring} and the
765 \type{MoveRefactoring}. Its constructor takes a project
766 handle\typeref{org.eclipse.core.resources.IProject}, the method name for the new
767 method and a \typewithref{no.uio.ifi.refaktor.utils}{SmartTextSelection}.
769 A \type{SmartTextSelection} is basically a text
770 selection\typeref{org.eclipse.jface.text.ITextSelection} object that enforces
771 the providing of the underlying document during creation. I.e. its
772 \methodwithref{no.uio.ifi.refaktor.utils.SmartTextSelection}{getDocument} method
773 will never return \type{null}.
775 Before extracting the new method, the possible targets for the move operation is
776 found with the help of an
777 \typewithref{no.uio.ifi.refaktor.extractors}{ExtractAndMoveMethodPrefixesExtractor}.
778 The possible targets is computed from the prefixes that the extractor returns
780 \methodwithref{no.uio.ifi.refaktor.extractors.ExtractAndMoveMethodPrefixesExtractor}{getSafePrefixes}
781 method. The changer then choose the most suitable target by finding the most
782 frequent occurring prefix among the safe ones. The target is the type of the
783 first part of the prefix.
785 After finding a suitable target, the \type{ExtractAndMoveMethodChanger} first
786 creates an \type{ExtractMethodRefactoring} and performs it as explained in
787 section \ref{executing_refactoring} about the execution of refactorings. Then it
788 creates and performs the \type{MoveRefactoring} in the same way, based on the
789 changes done by the Extract Method refactoring.
791 \subsection{The ExtractAndMoveMethodPrefixesExtractor Class}
792 This extractor extracts properties needed for building the Extract and Move
793 Method refactoring. It searches through the given selection to find safe
794 prefixes, and those prefixes form a base that can be used to compute possible
795 targets for the move part of the refactoring. It finds both the candidates, in
796 the form of prefixes, and the non-candidates, called unfixes. All prefixes (and
797 unfixes) are represented by a
798 \typewithref{no.uio.ifi.refaktor.extractors}{Prefix}, and they are collected
799 into prefix sets.\typeref{no.uio.ifi.refaktor.extractors.PrefixSet}.
801 The prefixes and unfixes are found by property
802 collectors\typeref{no.uio.ifi.refaktor.extractors.collectors.PropertyCollector}.
803 A property collector follows the visitor pattern \cite{dp} and is of the
804 \typewithref{org.eclipse.jdt.core.dom}{ASTVisitor} type. An \type{ASTVisitor}
805 visits nodes in an abstract syntax tree that forms the Java document object
806 model. The tree consists of nodes of type
807 \typewithref{org.eclipse.jdt.core.do}{ASTNode}.
809 \subsubsection{The PrefixesCollector}
810 The \typewithref{no.uio.ifi.refaktor.extractors.collectors}{PrefixesCollector}
811 is of type \type{PropertyCollector}. It visits expression
812 statements\typeref{org.eclipse.jdt.core.dom.ExpressionStatement} and creates
813 prefixes from its expressions in the case of method invocations. The prefixes
814 found is registered with a prefix set, together with all its sub-prefixes.
815 \todo{Rewrite in the case of changes to the way prefixes are found}
817 \subsubsection{The UnfixesCollector}
818 The \typewithref{no.uio.ifi.refaktor.extractors.collectors}{UnfixesCollector}
819 finds unfixes within the selection. An unfix is a name that is assigned to
820 within the selection. The reason that this cannot be allowed, is that the result
821 would be an assignment to the \type{this} keyword, which is not valid in Java.
823 \subsubsection{Computing Safe Prefixes}
824 A safe prefix is a prefix that does not enclose an unfix. A prefix is enclosing
825 an unfix if the unfix is in the set of its sub-prefixes. As an example,
826 \texttt{``a.b''} is enclosing \texttt{``a''}, as is \texttt{``a''}. The safe
827 prefixes is unified in a \type{PrefixSet} and can be fetched calling the
828 \method{getSafePrefixes} method of the
829 \type{ExtractAndMoveMethodPrefixesExtractor}.
831 \subsection{The Prefix Class}
833 \subsection{The PrefixSet Class}
835 \subsection{Hacking the Refactoring Undo
836 History}\label{hacking_undo_history}
837 \todo{Where to put this section?}
839 As an attempt to make multiple subsequent changes to the workspace appear as a
840 single action (i.e. make the undo changes appear as such), I tried to alter
841 the undo changes\typeref{org.eclipse.ltk.core.refactoring.Change} in the history
844 My first impulse was to remove the, in this case, last two undo changes from the
845 undo manager\typeref{org.eclipse.ltk.core.refactoring.IUndoManager} for the
846 Eclipse refactorings, and then add them to a composite
847 change\typeref{org.eclipse.ltk.core.refactoring.CompositeChange} that could be
848 added back to the manager. The interface of the undo manager does not offer a
849 way to remove/pop the last added undo change, so a possible solution could be to
850 decorate \cite{dp} the undo manager, to intercept and collect the undo changes
851 before delegating to the \method{addUndo}
852 method\methodref{org.eclipse.ltk.core.refactoring.IUndoManager}{addUndo} of the
853 manager. Instead of giving it the intended undo change, a null change could be
854 given to prevent it from making any changes if run. Then one could let the
855 collected undo changes form a composite change to be added to the manager.
857 There is a technical challenge with this approach, and it relates to the undo
858 manager, and the concrete implementation
859 UndoManager2\typeref{org.eclipse.ltk.internal.core.refactoring.UndoManager2}.
860 This implementation is designed in a way that it is not possible to just add an
861 undo change, you have to do it in the context of an active
862 operation\typeref{org.eclipse.core.commands.operations.TriggeredOperations}.
863 One could imagine that it might be possible to trick the undo manager into
864 believing that you are doing a real change, by executing a refactoring that is
865 returning a kind of null change that is returning our composite change of undo
866 refactorings when it is performed.
868 Apart from the technical problems with this solution, there is a functional
869 problem: If it all had worked out as planned, this would leave the undo history
870 in a dirty state, with multiple empty undo operations corresponding to each of
871 the sequentially executed refactoring operations, followed by a composite undo
872 change corresponding to an empty change of the workspace for rounding of our
873 composite refactoring. The solution to this particular problem could be to
874 intercept the registration of the intermediate changes in the undo manager, and
875 only register the last empty change.
877 Unfortunately, not everything works as desired with this solution. The grouping
878 of the undo changes into the composite change does not make the undo operation
879 appear as an atomic operation. The undo operation is still split up into
880 separate undo actions, corresponding to the change done by its originating
881 refactoring. And in addition, the undo actions has to be performed separate in
882 all the editors involved. This makes it no solution at all, but a step toward
885 There might be a solution to this problem, but it remains to be found. The
886 design of the refactoring undo management is partly to be blamed for this, as it
887 it is to complex to be easily manipulated.