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30 \subtitle{An unfinished essay}
31 \author{Erlend Kristiansen}
33 \bibliography{bibliography/master-thesis-erlenkr-bibliography}
51 \chapter{Introduction}
53 \section{What is Refactoring?}
55 This question is best answered dividing the answer into two parts. First
56 defining the concept of a refactoring, then discuss what the discipline of
57 refactoring is all about. And to make it clear already from the beginning: The
58 discussions in this report must be seen in the context of object oriented
59 programming languages. All though the techniques discussed may be applicable to
60 languages from other paradigms, they will not be the subject of this report.
62 \subsection{Defining refactoring}
63 Martin Fowler, in his masterpiece on refactoring \cite{refactoring}, defines a
64 refactoring like this:
66 \emph{Refactoring} (noun): a change made to the \todo{what does he mean by
67 internal?} internal structure of software to make it easier to understand and
68 cheaper to modify without changing its observable
69 behavior.~\cite{refactoring} % page 53
71 This definition assign additional meaning to the word \emph{refactoring}, beyond
72 the composition of the prefix \emph{re-}, usually meaning something like
73 ``again'' or ``anew'', and the word \emph{factoring}, that can mean to determine
74 the \emph{factors} of something. Where a \emph{factor} would be close to the
75 mathematical definition of something that divides a quantity, without leaving a
76 remainder. Fowler is mixing the \emph{motivation} behind refactoring into his
77 definition. Instead it could be made clean, only considering the mechanical and
78 behavioral aspects of refactoring. That is to factor the program again, putting
79 it together in a different way than before, while preserving the behavior of the
80 program. An alternative definition could then be:
82 \definition{A refactoring is a transformation
83 done to a program without altering its external behavior.}
85 From this we can conclude that a refactoring primarily changes how the
86 \emph{code} of a program is perceived by the \emph{programmer}, and not the
87 behavior experienced by any user of the program. Although the logical meaning is
88 preserved, such changes could potentially alter the program's behavior when it
89 comes to performance gain or -penalties. So any logic depending on the
90 performance of a program could make the program behave differently after a
93 In the extreme case one could argue that such a thing as \emph{software
94 obfuscation} is to refactor. If we where to define it as a refactoring, it could
95 be defined as a composite refactoring \see{intro_composite}, consisting of, for
96 instance, a series of rename refactorings. (But it could of course be much more
97 complex, and the mechanics of it would not exactly be carved in stone.) To
98 perform some serious obfuscation one would also take advantage of techniques not
99 found among established refactorings, such as removing whitespace. This might
100 not even generate a different syntax tree for languages not sensitive to
101 whitespace, placing it in the gray area of what kind of transformations is to be
102 considered refactorings.
104 Finally, to \emph{refactor} is (quoting Martin Fowler)
106 \ldots to restructure software by applying a series of refactorings without
107 changing its observable behavior.~\cite{refactoring} % page 54, definition
110 \todo{subsection with the history of refactoring?}
112 \subsection{Motivation}\todo{better headline? section vs. subsection}
113 To get a grasp of what refactoring is all about, we can answer this question:
114 \emph{Why do people refactor?} Possible answers could include: ``To remove
115 duplication'' or ``to break up long methods''. Practitioners of the art of
116 Design Patterns~\cite{dp} could say that they do it to introduce a long-needed
117 pattern into their program's design. So it's safe to say that peoples'
118 intentions are to make their programs \emph{better} in some sense. But what
119 aspects of the programs are becoming improved?
121 As already mentioned, people often refactor to get rid of duplication. Moving
122 identical or similar code into methods, and maybe pushing those up or down in
123 their class hierarchies. Making template methods for overlapping algorithms
124 \todo{better?: functionality} and so on. It's all about gathering what belongs
125 together and putting it all in one place. And the result? The code is easier to
126 maintain. When removing the implicit coupling between the code snippets, the
127 location of a bug is limited to only one place, and new functionality need only
128 to be added this one place, instead of a number of places people might not even
131 The same people find out that their program contains a lot of long and
132 hard-to-grasp methods. Then what do they do? They begin dividing their methods
133 into smaller ones, using the \emph{Extract Method}
134 refactoring~\cite{refactoring}. Then they may discover something about their
135 program that they weren't aware of before; revealing bugs they didn't know about
136 or couldn't find due to the complex structure of their program. \todo{Proof?}
137 Making the methods smaller and giving good names to the new ones clarifies the
138 algorithms and enhances the \emph{understandability} of the program. This makes
139 simple refactoring an excellent method for exploring unknown program code, or
140 code that you had forgotten that you wrote!
142 The word \emph{simple} came up in the last section. In fact, most basic
143 refactorings are simple. The true power of them are revealed first when they are
144 combined into larger --- higher level --- refactorings, called \emph{composite
145 refactorings} \see{intro_composite}. Often the goal of such a series of
146 refactorings is a design pattern. Thus the \emph{design} can be evolved
147 throughout the lifetime of a program, opposed to designing up-front. It's all
148 about being structured and taking small steps to improve the design.
150 Many refactorings are aimed at lowering the coupling between different classes
151 and different layers of logic. Say for instance that the coupling between the
152 user interface and the business logic of a program is lowered. Then the business
153 logic of the program could much easier be the target of automated tests,
154 increasing the productivity in the software development process. It is also
155 easier to distribute (e.g. between computers) the different components of a
156 program if they are sufficiently decoupled.
158 Another effect of refactoring is that with the increased separation of concerns
159 coming out of many refactorings, the \emph{performance} is improved. When
160 profiling programs, the problem parts are narrowed down to smaller parts of the
161 code, which are easier to tune, and optimization can be performed only where
162 needed and in a more effective way.
164 Refactoring program code --- with a goal in mind --- can give the code itself
165 more value. That is in the form of robustness to bugs, understandability and
166 maintainability. With the first as an obvious advantage, but with the following
167 two being also very important for software development. By incorporating
168 refactoring in the development process, bugs are found faster, new functionality
169 is added more easily and code is easier to understand by the next person exposed
170 to it, which might as well be the person who wrote it. The consequence of this,
171 is that refactoring can increase the average productivity of the development
172 process, and thus also add to the monetary value of a business in the long run.
173 Where this last point also should open the eyes of some nearsighted managers who
174 seldom see beyond the next milestone.
176 \todo{motivation: support new functionality?}
178 \subsection{The etymology of 'refactoring'}
179 It is a little difficult to pinpoint the exact origin of the word
180 ``refactoring'', as it seems to have evolved as part of a colloquial
181 terminology, more than a scientific term. There is no authoritative source for a
182 formal definition of it.
184 According to Martin Fowler~\cite{etymology-refactoring}, there may also be more
185 than one origin of the word. The most well-known source, when it comes to the
186 origin of \emph{refactoring}, is the Smalltalk\footnote{\emph{Smalltalk},
187 object-oriented, dynamically typed, reflective programming language.}\todo{find
188 reference to Smalltalk website or similar?} community and their infamous
190 Browser}\footnote{\url{http://st-www.cs.illinois.edu/users/brant/Refactory/RefactoringBrowser.html}}
191 described in the article \emph{A Refactoring Tool for
192 Smalltalk}~\cite{refactoringBrowser1997}, published in 1997.
193 Allegedly~\cite{etymology-refactoring}, the metaphor of factoring programs was
194 also present in the Forth\footnote{\emph{Forth} -- stack-based, extensible
195 programming language, without type-checking. See \url{http://www.forth.org}}
196 community, and the word ``refactoring'' is mentioned in a book by Leo Brodie,
197 called \emph{Thinking Forth}~\cite{brodie1984}, first published in
198 1984\footnote{\emph{Thinking Forth} was first published in 1984 by the
199 \emph{Forth Interest Group}. Then it was reprinted in 1994 with minor
200 typographical corrections, before it was transcribed into an electronic edition
201 typeset in \LaTeX\ and published under a Creative Commons licence in 2004. The
202 edition cited here is the 2004 edition, but the content should essentially be as
203 in 1984.}. The exact word is only printed one place\footnote{p. 232}, but the
204 term \emph{factoring} is prominent in the book, that also contains a whole
205 chapter dedicated to (re)factoring, and how to keep the (Forth) code clean and
208 \ldots good factoring technique is perhaps the most important skill for a
209 Forth programmer.~\cite{brodie1984}
211 Brodie also express what \emph{factoring} means to him:
213 Factoring means organizing code into useful fragments. To make a fragment
214 useful, you often must separate reusable parts from non-reusable parts. The
215 reusable parts become new definitions. The non-reusable parts become arguments
216 or parameters to the definitions.~\cite{brodie1984}
219 Fowler claims that the usage of the word \emph{refactoring} did not pass between
220 the \emph{Forth} and \emph{Smalltalk} communities, but that it emerged
221 independently in each of the communities.
223 \subsection{Notable contributions to the refactoring literature}
224 \todo{Update with more contributions}
226 \item[1992] William F. Opdyke submits his doctoral dissertation called
227 \emph{Refactoring Object-Oriented Frameworks}~\cite{opdyke1992}. This
228 work defines a set of refactorings, that are behavior preserving given that
229 their preconditions are met. The dissertation is focused on the automation
231 \item[1999] Martin Fowler et al.: \emph{Refactoring: Improving the Design of
232 Existing Code}~\cite{refactoring}. This is maybe the most influential text
233 on refactoring. It bares similarities with Opdykes thesis~\cite{opdyke1992}
234 in the way that it provides a catalog of refactorings. But Fowler's book is
235 more about the craft of refactoring, as he focuses on establishing a
236 vocabulary for refactoring, together with the mechanics of different
237 refactorings and when to perform them. His methodology is also founded on
238 the principles of test-driven development.
241 \section{Tool support}
242 \todo{write, section vs. subsection}
244 \section{Relation to design patterns}
245 \todo{write, section vs. subsection}
248 \section{Classification of refactorings}
249 % only interesting refactorings
250 % with 2 detailed examples? One for structured and one for intra-method?
251 % Is replacing Bubblesort with Quick Sort considered a refactoring?
253 \subsection{Structural refactorings}
255 \subsubsection{Basic refactorings}
258 \explanation{Extract Method}{You have a code fragment that can be grouped
259 together.}{Turn the fragment into a method whose name explains the purpose of
262 \explanation{Inline Method}{A method's body is just as clear as its name.}{Put
263 the method's body into the body of its callers and remove the method.}
265 \explanation{Inline Temp}{You have a temp that is assigned to once with a simple
266 expression, and the temp is getting in the way of other refactorings.}{Replace
267 all references to that temp with the expression}
269 % Moving Features Between Objects
270 \explanation{Move Method}{A method is, or will be, using or used by more
271 features of another class than the class on which it is defined.}{Create a new
272 method with a similar body in the class it uses most. Either turn the old method
273 into a simple delegation, or remove it altogether.}
275 \explanation{Move Field}{A field is, or will be, used by another class more than
276 the class on which it is defined}{Create a new field in the target class, and
277 change all its users.}
280 \explanation{Replace Magic Number with Symbolic Constant}{You have a literal
281 number with a particular meaning.}{Create a constant, name it after the meaning,
282 and replace the number with it.}
284 \explanation{Encapsulate Field}{There is a public field.}{Make it private and
287 \explanation{Replace Type Code with Class}{A class has a numeric type code that
288 does not affect its behavior.}{Replace the number with a new class.}
290 \explanation{Replace Type Code with Subclasses}{You have an immutable type code
291 that affects the behavior of a class.}{Replace the type code with subclasses.}
293 \explanation{Replace Type Code with State/Strategy}{You have a type code that
294 affects the behavior of a class, but you cannot use subclassing.}{Replace the
295 type code with a state object.}
297 % Simplifying Conditional Expressions
298 \explanation{Consolidate Duplicate Conditional Fragments}{The same fragment of
299 code is in all branches of a conditional expression.}{Move it outside of the
302 \explanation{Remove Control Flag}{You have a variable that is acting as a
303 control flag fro a series of boolean expressions.}{Use a break or return
306 \explanation{Replace Nested Conditional with Guard Clauses}{A method has
307 conditional behavior that does not make clear the normal path of
308 execution.}{Use guard clauses for all special cases.}
310 \explanation{Introduce Null Object}{You have repeated checks for a null
311 value.}{Replace the null value with a null object.}
313 \explanation{Introduce Assertion}{A section of code assumes something about the
314 state of the program.}{Make the assumption explicit with an assertion.}
316 % Making Method Calls Simpler
317 \explanation{Rename Method}{The name of a method does not reveal its
318 purpose.}{Change the name of the method}
320 \explanation{Add Parameter}{A method needs more information from its
321 caller.}{Add a parameter for an object that can pass on this information.}
323 \explanation{Remove Parameter}{A parameter is no longer used by the method
326 %\explanation{Parameterize Method}{Several methods do similar things but with
327 %different values contained in the method.}{Create one method that uses a
328 %parameter for the different values.}
330 \explanation{Preserve Whole Object}{You are getting several values from an
331 object and passing these values as parameters in a method call.}{Send the whole
334 \explanation{Remove Setting Method}{A field should be set at creation time and
335 never altered.}{Remove any setting method for that field.}
337 \explanation{Hide Method}{A method is not used by any other class.}{Make the
340 \explanation{Replace Constructor with Factory Method}{You want to do more than
341 simple construction when you create an object}{Replace the constructor with a
344 % Dealing with Generalization
345 \explanation{Pull Up Field}{Two subclasses have the same field.}{Move the field
348 \explanation{Pull Up Method}{You have methods with identical results on
349 subclasses.}{Move them to the superclass.}
351 \explanation{Push Down Method}{Behavior on a superclass is relevant only for
352 some of its subclasses.}{Move it to those subclasses.}
354 \explanation{Push Down Field}{A field is used only by some subclasses.}{Move the
355 field to those subclasses}
357 \explanation{Extract Interface}{Several clients use the same subset of a class's
358 interface, or two classes have part of their interfaces in common.}{Extract the
359 subset into an interface.}
361 \explanation{Replace Inheritance with Delegation}{A subclass uses only part of a
362 superclasses interface or does not want to inherit data.}{Create a field for the
363 superclass, adjust methods to delegate to the superclass, and remove the
366 \explanation{Replace Delegation with Inheritance}{You're using delegation and
367 are often writing many simple delegations for the entire interface}{Make the
368 delegating class a subclass of the delegate.}
370 \subsubsection{Composite refactorings}
373 % \explanation{Replace Method with Method Object}{}{}
375 % Moving Features Between Objects
376 \explanation{Extract Class}{You have one class doing work that should be done by
377 two}{Create a new class and move the relevant fields and methods from the old
378 class into the new class.}
380 \explanation{Inline Class}{A class isn't doing very much.}{Move all its features
381 into another class and delete it.}
383 \explanation{Hide Delegate}{A client is calling a delegate class of an
384 object.}{Create Methods on the server to hide the delegate.}
386 \explanation{Remove Middle Man}{A class is doing to much simple delegation.}{Get
387 the client to call the delegate directly.}
390 \explanation{Replace Data Value with Object}{You have a data item that needs
391 additional data or behavior.}{Turn the data item into an object.}
393 \explanation{Change Value to Reference}{You have a class with many equal
394 instances that you want to replace with a single object.}{Turn the object into a
397 \explanation{Encapsulate Collection}{A method returns a collection}{Make it
398 return a read-only view and provide add/remove methods.}
400 % \explanation{Replace Array with Object}{}{}
402 \explanation{Replace Subclass with Fields}{You have subclasses that vary only in
403 methods that return constant data.}{Change the methods to superclass fields and
404 eliminate the subclasses.}
406 % Simplifying Conditional Expressions
407 \explanation{Decompose Conditional}{You have a complicated conditional
408 (if-then-else) statement.}{Extract methods from the condition, then part, an
411 \explanation{Consolidate Conditional Expression}{You have a sequence of
412 conditional tests with the same result.}{Combine them into a single conditional
413 expression and extract it.}
415 \explanation{Replace Conditional with Polymorphism}{You have a conditional that
416 chooses different behavior depending on the type of an object.}{Move each leg
417 of the conditional to an overriding method in a subclass. Make the original
420 % Making Method Calls Simpler
421 \explanation{Replace Parameter with Method}{An object invokes a method, then
422 passes the result as a parameter for a method. The receiver can also invoke this
423 method.}{Remove the parameter and let the receiver invoke the method.}
425 \explanation{Introduce Parameter Object}{You have a group of parameters that
426 naturally go together.}{Replace them with an object.}
428 % Dealing with Generalization
429 \explanation{Extract Subclass}{A class has features that are used only in some
430 instances.}{Create a subclass for that subset of features.}
432 \explanation{Extract Superclass}{You have two classes with similar
433 features.}{Create a superclass and move the common features to the
436 \explanation{Collapse Hierarchy}{A superclass and subclass are not very
437 different.}{Merge them together.}
439 \explanation{Form Template Method}{You have two methods in subclasses that
440 perform similar steps in the same order, yet the steps are different.}{Get the
441 steps into methods with the same signature, so that the original methods become
442 the same. Then you can pull them up.}
445 \subsection{Functional refactorings}
447 \explanation{Substitute Algorithm}{You want to replace an algorithm with one
448 that is clearer.}{Replace the body of the method with the new algorithm.}
452 \section{The impact on software quality}
454 \subsection{What is meant by quality?}
455 The term \emph{software quality} has many meanings. It all depends on the
456 context we put it in. If we look at it with the eyes of a software developer, it
457 usually mean that the software is easily maintainable and testable, or in other
458 words, that it is \emph{well designed}. This often correlates with the
459 management scale, where \emph{keeping the schedule} and \emph{customer
460 satisfaction} is at the center. From the customers point of view, in addition to
461 good usability, \emph{performance} and \emph{lack of bugs} is always
462 appreciated, measurements that are also shared by the software developer. (In
463 addition, such things as good documentation could be measured, but this is out
464 of the scope of this document.)
466 \subsection{The impact on performance}
468 Refactoring certainly will make software go more slowly, but it also makes the
469 software more amenable to performance tuning.~\cite{refactoring} % page 69
471 There is a common belief that refactoring compromises performance, due to
472 increased degree of indirection and that polymorphism is slower than
475 In a survey, Demeyer~\cite{demeyer2002} disproves this view in the case of
476 polymorphism. He is doing an experiment on, what he calls, ``Transform Self Type
477 Checks'' where you introduce a new polymorphic method and a new class hierarchy
478 to get rid of a class' type checking of a ``type attribute``. He uses this kind
479 of transformation to represent other ways of replacing conditionals with
480 polymorphism as well. The experiment is performed on the C++ programming
481 language and with three different compilers and platforms. \todo{But is the
482 result better?} Demeyer concludes that, with compiler optimization turned on,
483 polymorphism beats middle to large sized if-statements and does as well as
484 case-statements. (In accordance with his hypothesis, due to similarities
485 between the way C++ handles polymorphism and case-statements.)
487 The interesting thing about performance is that if you analyze most programs,
488 you find that they waste most of their time in a small fraction of the code.
491 So, although an increased amount of method calls could potentially slow down
492 programs, one should avoid premature optimization and sacrificing good design,
493 leaving the performance tuning until after profiling\footnote{For and example of
494 a Java profiler, check out VisualVM: \url{http://visualvm.java.net/}} the
495 software and having isolated the actual problem areas.
499 \section{Correctness of refactorings}
502 \section{Composite refactorings} \label{intro_composite}
503 % motivation, examples
504 % manual vs automated?
505 % what about refactoring in a very large code base?
507 \section{Software metrics}
511 %\chapter{Planning the project}
519 \section{The problem statement}
520 \section{Choosing the language}
521 \section{Choosing the tool}
523 \chapter{Refactorings in Eclipse JDT: Design, Shortcomings and Wishful
524 Thinking}\label{ch:jdt_refactorings}
526 This chapter will deal with some of the design behind refactoring support in
527 Eclipse, and the JDT in specific. After which it will follow a section about
528 shortcomings of the refactoring API in terms of composition of refactorings. The
529 chapter will be concluded with a section telling some of the ways the
530 implementation of refactorings in the JDT could have worked to facilitate
531 composition of refactorings.
534 The refactoring world of Eclipse can in general be separated into two parts: The
535 language independent part and the part written for a specific programming
536 language -- the language that is the target of the supported refactorings.
537 \todo{What about the language specific part?}
539 \subsection{The Language Toolkit}
540 The Language Toolkit, or LTK for short, is the framework that is used to
541 implement refactorings in Eclipse. It is language independent and provides the
542 abstractions of a refactoring and the change it generates, in the form of the
543 classes \typewithref{org.eclipse.ltk.core.refactoring}{Refactoring} and
544 \typewithref{org.eclipse.ltk.core.refactoring}{Change}. (There is also parts of
545 the LTK that is concerned with user interaction, but they will not be discussed
546 here, since they are of little value to us and our use of the framework.)
548 \subsubsection{The Refactoring Class}
549 The abstract class \type{Refactoring} is the core of the LTK framework. Every
550 refactoring that is going to be supported by the LTK have to end up creating an
551 instance of one of its subclasses. The main responsibilities of subclasses of
552 \type{Refactoring} is to implement template methods for condition checking
553 (\methodwithref{org.eclipse.ltk.core.refactoring.Refactoring}{checkInitialConditions}
555 \methodwithref{org.eclipse.ltk.core.refactoring.Refactoring}{checkFinalConditions}),
557 \methodwithref{org.eclipse.ltk.core.refactoring.Refactoring}{createChange}
558 method that creates and returns an instance of the \type{Change} class.
560 If the refactoring shall support that others participate in it when it is
561 executed, the refactoring has to be a processor-based
562 refactoring\typeref{org.eclipse.ltk.core.refactoring.participants.ProcessorBasedRefactoring}.
563 It then delegates to its given
564 \typewithref{org.eclipse.ltk.core.refactoring.participants}{RefactoringProcessor}
565 for condition checking and change creation.
567 \subsubsection{The Change Class}
568 This class is the base class for objects that is responsible for performing the
569 actual workspace transformations in a refactoring. The main responsibilities for
570 its subclasses is to implement the
571 \methodwithref{org.eclipse.ltk.core.refactoring.Change}{perform} and
572 \methodwithref{org.eclipse.ltk.core.refactoring.Change}{isValid} methods. The
573 \method{isValid} method verifies that the change object is valid and thus can be
574 executed by calling its \method{perform} method. The \method{perform} method
575 performs the desired change and returns an undo change that can be executed to
576 reverse the effect of the transformation done by its originating change object.
578 \subsubsection{Executing a Refactoring}\label{executing_refactoring}
579 The life cycle of a refactoring generally follows two steps after creation:
580 condition checking and change creation. By letting the refactoring object be
582 \typewithref{org.eclipse.ltk.core.refactoring}{CheckConditionsOperation} that
583 in turn is handled by a
584 \typewithref{org.eclipse.ltk.core.refactoring}{CreateChangeOperation}, it is
585 assured that the change creation process is managed in a proper manner.
587 The actual execution of a change object has to follow a detailed life cycle.
588 This life cycle is honored if the \type{CreateChangeOperation} is handled by a
589 \typewithref{org.eclipse.ltk.core.refactoring}{PerformChangeOperation}. If also
590 an undo manager\typeref{org.eclipse.ltk.core.refactoring.IUndoManager} is set
591 for the \type{PerformChangeOperation}, the undo change is added into the undo
594 \section{Shortcomings}
595 This section is introduced naturally with a conclusion: The JDT refactoring
596 implementation does not facilitate composition of refactorings.
597 \todo{refine}This section will try to explain why, and also identify other
598 shortcomings of both the usability and the readability of the JDT refactoring
601 I will begin at the end and work my way toward the composition part of this
604 \subsection{Absence of Generics in Eclipse Source Code}
605 This section is not only concerning the JDT refactoring API, but also large
606 quantities of the Eclipse source code. The code shows a striking absence of the
607 Java language feature of generics. It is hard to read a class' interface when
608 methods return objects or takes parameters of raw types such as \type{List} or
609 \type{Map}. This sometimes results in having to read a lot of source code to
610 understand what is going on, instead of relying on the available interfaces. In
611 addition, it results in a lot of ugly code, making the use of typecasting more
612 of a rule than an exception.
614 \subsection{Composite Refactorings Will Not Appear as Atomic Actions}
616 \subsubsection{Missing Flexibility from JDT Refactorings}
617 The JDT refactorings are not made with composition of refactorings in mind. When
618 a JDT refactoring is executed, it assumes that all conditions for it to be
619 applied successfully can be found by reading source files that has been
620 persisted to disk. They can only operate on the actual source material, and not
621 (in-memory) copies thereof. This constitutes a major disadvantage when trying to
622 compose refactorings, since if an exception occur in the middle of a sequence of
623 refactorings, it can leave the project in a state where the composite
624 refactoring was executed only partly. It makes it hard to discard the changes
625 done without monitoring and consulting the undo manager, an approach that is not
628 \subsubsection{Broken Undo History}
629 When designing a composed refactoring that is to be performed as a sequence of
630 refactorings, you would like it to appear as a single change to the workspace.
631 This implies that you would also like to be able to undo all the changes done by
632 the refactoring in a single step. This is not the way it appears when a sequence
633 of JDT refactorings is executed. It leaves the undo history filled up with
634 individual undo actions corresponding to every single JDT refactoring in the
635 sequence. This problem is not trivial to handle in Eclipse. (See section
636 \ref{hacking_undo_history}.)
638 \section{Wishful Thinking}
642 \chapter{Composite Refactorings in Eclipse}
644 \section{A Simple Ad Hoc Model}
645 As pointed out in chapter \ref{ch:jdt_refactorings}, the Eclipse JDT refactoring
646 model is not very well suited for making composite refactorings. Therefore a
647 simple model using changer objects (of type \type{RefaktorChanger}) is used as
648 an abstraction layer on top of the existing Eclipse refactorings.
650 \section{The Extract and Move Method Refactoring}
651 %The Extract and Move Method Refactoring is implemented mainly using these
654 % \item \type{ExtractAndMoveMethodChanger}
655 % \item \type{ExtractAndMoveMethodPrefixesExtractor}
656 % \item \type{Prefix}
657 % \item \type{PrefixSet}
660 \subsection{The Building Blocks}
661 This is a composite refactoring, and hence is built up using several primitive
662 refactorings. These basic building blocks are, as its name implies, the Extract
663 Method Refactoring \cite{refactoring} and the Move Method Refactoring
664 \cite{refactoring}. In Eclipse, the implementations of these refactorings are
666 \typewithref{org.eclipse.jdt.internal.corext.refactoring.code}{ExtractMethodRefactoring}
668 \typewithref{org.eclipse.jdt.internal.corext.refactoring.structure}{MoveInstanceMethodProcessor},
669 where the last class is designed to be used together with the processor-based
670 \typewithref{org.eclipse.ltk.core.refactoring.participants}{MoveRefactoring}.
672 \subsubsection{The ExtractMethodRefactoring Class}
673 This class is quite simple in its use. The only parameters it requires for
674 construction is a compilation
675 unit\typeref{org.eclipse.jdt.core.ICompilationUnit}, the offset into the source
676 code where the extraction shall start, and the length of the source to be
677 extracted. Then you have to set the method name for the new method together with
678 which access modifier that shall be used and some not so interesting parameters.
680 \subsubsection{The MoveInstanceMethodProcessor Class}
681 For the Move Method the processor requires a little more advanced input than
682 the class for the Extract Method. For construction it requires a method
683 handle\typeref{org.eclipse.jdt.core.IMethod} from the Java Model for the method
684 that is to be moved. Then the target for the move have to be supplied as the
685 variable binding from a chosen variable declaration. In addition to this, one
686 have to set some parameters regarding setters/getters and delegation.
688 To make a whole refactoring from the processor, one have to construct a
689 \type{MoveRefactoring} from it.
691 \subsection{The ExtractAndMoveMethodChanger Class}
692 The \typewithref{no.uio.ifi.refaktor.changers}{ExtractAndMoveMethodChanger}
693 class, that is a subclass of the class
694 \typewithref{no.uio.ifi.refaktor.changers}{RefaktorChanger}, is the class
695 responsible for composing the \type{ExtractMethodRefactoring} and the
696 \type{MoveRefactoring}. Its constructor takes a project
697 handle\typeref{org.eclipse.core.resources.IProject}, the method name for the new
698 method and a \typewithref{no.uio.ifi.refaktor.utils}{SmartTextSelection}.
700 A \type{SmartTextSelection} is basically a text
701 selection\typeref{org.eclipse.jface.text.ITextSelection} object that enforces
702 the providing of the underlying document during creation. I.e. its
703 \methodwithref{no.uio.ifi.refaktor.utils.SmartTextSelection}{getDocument} method
704 will never return \type{null}.
706 Before extracting the new method, the possible targets for the move operation is
707 found with the help of an
708 \typewithref{no.uio.ifi.refaktor.extractors}{ExtractAndMoveMethodPrefixesExtractor}.
709 The possible targets is computed from the prefixes that the extractor returns
711 \methodwithref{no.uio.ifi.refaktor.extractors.ExtractAndMoveMethodPrefixesExtractor}{getSafePrefixes}
712 method. The changer then choose the most suitable target by finding the most
713 frequent occurring prefix among the safe ones. The target is the type of the
714 first part of the prefix.
716 After finding a suitable target, the \type{ExtractAndMoveMethodChanger} first
717 creates an \type{ExtractMethodRefactoring} and performs it as explained in
718 section \ref{executing_refactoring} about the execution of refactorings. Then it
719 creates and performs the \type{MoveRefactoring} in the same way, based on the
720 changes done by the Extract Method refactoring.
722 \subsection{The ExtractAndMoveMethodPrefixesExtractor Class}
723 This extractor extracts properties needed for building the Extract and Move
724 Method refactoring. It searches through the given selection to find safe
725 prefixes, and those prefixes form a base that can be used to compute possible
726 targets for the move part of the refactoring. It finds both the candidates, in
727 the form of prefixes, and the non-candidates, called unfixes. All prefixes (and
728 unfixes) are represented by a
729 \typewithref{no.uio.ifi.refaktor.extractors}{Prefix}, and they are collected
730 into prefix sets.\typeref{no.uio.ifi.refaktor.extractors.PrefixSet}.
732 The prefixes and unfixes are found by property
733 collectors\typeref{no.uio.ifi.refaktor.extractors.collectors.PropertyCollector}.
734 A property collector follows the visitor pattern \cite{dp} and is of the
735 \typewithref{org.eclipse.jdt.core.dom}{ASTVisitor} type. An \type{ASTVisitor}
736 visits nodes in an abstract syntax tree that forms the Java document object
737 model. The tree consists of nodes of type
738 \typewithref{org.eclipse.jdt.core.do}{ASTNode}.
740 \subsubsection{The PrefixesCollector}
741 The \typewithref{no.uio.ifi.refaktor.extractors.collectors}{PrefixesCollector}
742 is of type \type{PropertyCollector}. It visits expression
743 statements\typeref{org.eclipse.jdt.core.dom.ExpressionStatement} and creates
744 prefixes from its expressions in the case of method invocations. The prefixes
745 found is registered with a prefix set, together with all its sub-prefixes.
746 \todo{Rewrite in the case of changes to the way prefixes are found}
748 \subsubsection{The UnfixesCollector}
749 The \typewithref{no.uio.ifi.refaktor.extractors.collectors}{UnfixesCollector}
750 finds unfixes within the selection. An unfix is a name that is assigned to
751 within the selection. The reason that this cannot be allowed, is that the result
752 would be an assignment to the \type{this} keyword, which is not valid in Java.
754 \subsubsection{Computing Safe Prefixes}
755 A safe prefix is a prefix that does not enclose an unfix. A prefix is enclosing
756 an unfix if the unfix is in the set of its sub-prefixes. As an example,
757 \texttt{``a.b''} is enclosing \texttt{``a''}, as is \texttt{``a''}. The safe
758 prefixes is unified in a \type{PrefixSet} and can be fetched calling the
759 \method{getSafePrefixes} method of the
760 \type{ExtractAndMoveMethodPrefixesExtractor}.
762 \subsection{The Prefix Class}
764 \subsection{The PrefixSet Class}
766 \subsection{Hacking the Refactoring Undo
767 History}\label{hacking_undo_history}
768 \todo{Where to put this section?}
770 As an attempt to make multiple subsequent changes to the workspace appear as a
771 single action (i.e. make the undo changes appear as such), I tried to alter
772 the undo changes\typeref{org.eclipse.ltk.core.refactoring.Change} in the history
775 My first impulse was to remove the, in this case, last two undo changes from the
776 undo manager\typeref{org.eclipse.ltk.core.refactoring.IUndoManager} for the
777 Eclipse refactorings, and then add them to a composite
778 change\typeref{org.eclipse.ltk.core.refactoring.CompositeChange} that could be
779 added back to the manager. The interface of the undo manager does not offer a
780 way to remove/pop the last added undo change, so a possible solution could be to
781 decorate \cite{dp} the undo manager, to intercept and collect the undo changes
782 before delegating to the \method{addUndo}
783 method\methodref{org.eclipse.ltk.core.refactoring.IUndoManager}{addUndo} of the
784 manager. Instead of giving it the intended undo change, a null change could be
785 given to prevent it from making any changes if run. Then one could let the
786 collected undo changes form a composite change to be added to the manager.
788 There is a technical challenge with this approach, and it relates to the undo
789 manager, and the concrete implementation
790 UndoManager2\typeref{org.eclipse.ltk.internal.core.refactoring.UndoManager2}.
791 This implementation is designed in a way that it is not possible to just add an
792 undo change, you have to do it in the context of an active
793 operation\typeref{org.eclipse.core.commands.operations.TriggeredOperations}.
794 One could imagine that it might be possible to trick the undo manager into
795 believing that you are doing a real change, by executing a refactoring that is
796 returning a kind of null change that is returning our composite change of undo
797 refactorings when it is performed.
799 Apart from the technical problems with this solution, there is a functional
800 problem: If it all had worked out as planned, this would leave the undo history
801 in a dirty state, with multiple empty undo operations corresponding to each of
802 the sequentially executed refactoring operations, followed by a composite undo
803 change corresponding to an empty change of the workspace for rounding of our
804 composite refactoring. The solution to this particular problem could be to
805 intercept the registration of the intermediate changes in the undo manager, and
806 only register the last empty change.
808 Unfortunately, not everything works as desired with this solution. The grouping
809 of the undo changes into the composite change does not make the undo operation
810 appear as an atomic operation. The undo operation is still split up into
811 separate undo actions, corresponding to the change done by its originating
812 refactoring. And in addition, the undo actions has to be performed separate in
813 all the editors involved. This makes it no solution at all, but a step toward
816 There might be a solution to this problem, but it remains to be found. The
817 design of the refactoring undo management is partly to be blamed for this, as it
818 it is to complex to be easily manipulated.