1. A Survey of Software Refactoring Tom Mens, Tom Tourwé
IEEE TRANSACTIONS ON SOFTWARE ENGINEERING, VOL. XX, NO. Y, MONTH 2004
Presented by: Kishen Maloor

2. Overview Restructuring, refactoring and associated activities
Behavior preservation – Assertions, Formalisms…
Types of software artifacts
Tool support
Qualities of refactoring tools
Process support
Conclusion

3. Restructuring and refactoring
Restructuring is a transformation from one form of presentation to another
Refactoring is the object-oriented variant of restructuring
The subjects external behavior is preserved
Idea is to make existing code more extensible

4. Refactoring activities
Identify where the software should be refactored
Determine which refactoring(s) should be applied to the identified places
Guarantee that the applied refactoring preserves behaviour
Apply the refactoring
Assess the effect of the refactoring on quality characteristics of the software
Maintain the consistency between the refactored program code and other software artifacts

5. Behavior Preservation
A refactoring is a parameterized behavior-preserving program transformation
Approaches to behavior preservation may perform checks either statically or dynamically
Input
Original
System 2 …
1+1
2-3
5+3
4*6 =
Refactored
System 2 …
Source: spic.kaist.ac.kr/~selab/html/Study/Lab%20Seminar/A%20Survey%20of%20Software%20Refactoring.ppt

6. Behavior Preservation
Static: - Preconditions, graph transformations, (access, update and call) preservations, type checking…
Dynamic: - Take dynamic information into account - Checks more aspects of the program behavior - WSL comes along with a tool that preserves dynamic behavior of programs [Bennett, 1995]

7. Behavior Preservation
Ideally, the behavior preservation of refactorings should be proven formally.
But, proving semantic correctness is an undecidable problem [Gupta, 1996]
Therefore, a more conservative, engineering approach is needed
We could do rigorous testing of the code
We could use Assertions and formalisms to check for behavior preservation

8. Assertions
Identify a set of invariants in the program that preserve its behavior for refactorings
Create a set of conditions which guarantee that these invariants hold
Pre- and postconditions are therefore formulated and checked before and after refactorings are applied
Are a lightweight and automatic means of verification

9. Example of use of assertions
Before refactoring
After refactoring
Consider the refactoring:
Add Class (new name, package, superclasses, subclasses)
Source:

10. Example of use of assertions
Preconditions
no class or global variable exists with new name in the same scope
subclasses are all subclasses of all superclasses
Postconditions
new class is added into the hierarchy with superclasses as superclasses and subclasses as subclasses
new class has name new name
subclasses inherit from new class and not anymore from superclasses
Invariant: Classes A, B & F are defined before and after the refactoring

11. Problems with assertions
Opdyke proposed a set of seven invariants to preserve behavior for refactorings [Opdyke, 1992]
Ex: Distinct class, Type safe assignments, Compatible signatures in member function definition, Inherited member variables not defined, Distinct member names, Distinct class names etc.
A more complex language may need more preconditions
Opdyke’s invariants were observed to be insufficient for C++ [Tokuda, 2001]

12. Problems with assertions
Static checking of preconditions can sometimes be very expensive
Preconditions do not consider the size or structure of programs
[Roberts, 1999] suggests a way to augment refactorings with postconditions

13. Formalisms
Graph Transformations - Programs represented as graphs - Refactorings correspond to graph production rules - Application of refactorings correspond to a graph transformation - Assertions specified as application pre- and postconditions
Provides formal support for refactoring

14. Formalisms
[Mens, 2002] uses graph rewriting to show that certain kinds of program behavior are preserved using static analysis
Graph transformations can be used to reason about dependence between refactorings Ex. Move method and Rename method
Analysis of sequential dependencies between refactorings can be useful

15. Other useful techniques
Program Slicing
Formal concept analysis
Software metrics
Software visualization
Dynamic program analysis

16. Types of software artifacts
Program source code - Supported by a variety of programming paradigms (Imperative, OO, Functional, AOP) - Formal support (WSL)
Non-OO languages are as such more difficult to restructure
More complex the language, harder it is to automate the refactoring process

17. Types of software artifacts
Refactoring of designs, a recent research trend
Independent of the underlying programming language
Refactoring of UML diagrams [Boger, 2002]
Refactoring pre- and post-conditions can be expressed using OCL

18. Types of software artifacts
Design patterns describe program structure at a higher level
Refactorings are used to introduce new design pattern instances into software

19. Types of software artifacts
Restructuring software architectures - based on graphical representation of the architecture [Philipps, 1997] - Behavior specified by casual relationship between two components is preserved
Software requirements - [Russo, 1998] restructure natural language requirements

20. Tool support Semiautomatic approach ? ? ? Refactoring Move Method Add
Class
Op1
Op2
Opn ? ?
Refactored System ?
Refactoring
operations
Source: spic.kaist.ac.kr/~selab/html/Study/Lab%20Seminar/A%20Survey%20of%20Software%20Refactoring.ppt

21. Tool support
In the semiautomatic approach, the developer: - Identifies the part of the software to be refactored and selects an appropriate refactoring to apply
Application of the refactoring is automatic
Fully automated refactoring has been demonstrated to be feasible [Moore, 1996]

22. Tool support
Fully automatic approach: - Refactorings can be easily undone - Can make the code difficult to understand
Semiautomatic approach: - For large software, needs lots of human interaction; time consuming! - More useful-most knowledge required to perform refactoring cannot be extracted from software

23. Qualities of refactoring tools
Reliability - Is it possible to check if the provided refactorings are truly behavior preserving?
Tools check preconditions before applying them and perform tests afterwards
Tools provide an undo mechanism to make undesired changes undone
Coverage specifies the number of refactoring activities supported by the tool

24. Qualities of refactoring tools
Configurability - Modifying bad smell specifications - Changing the links between bad smells and refactorings
[Leitao, 2002] specifies a pattern language to express refactorings
[Munoz, 2003] provides configurable threshold values to specify conditions for a bad smell

25. Qualities of refactoring tools
Scalability - Combine frequently used primitive refactorings into composite refactorings - Better capture intent of software change - Performance gain - Weaken behavior preservation requirements for primitive refactorings - Support for cascaded refactorings [Tourwe, 2003]

26. Qualities of refactoring tools
Language Independence - Applicability to different languages - Provide hooks to add language specific behavior
[Lammel, 2002] proposes generic program refactoring – a language parametric framework, can be used with Java, XML…
[Mens, 2002] Meta-model based approach
Use of an intermediate formal language [Bennett, 1995]

27. Process Support
An important activity in the software development process
Software Reengineering - To restructure legacy software to implement a new solution - To assist a MDA tool

28. Process Support Agile Software Development (XP)
- Develops and refactor software in small iterations
Framework-Based or Product Line Development

29. Conclusion
Provided an overview of refactoring; spoke about the different refactoring activities, prevalent problems and open issues, tool support for refactoring
Spoke about how refactoring fits into the software engineering process
Cited several relevant publications