Code refactoring is a "disciplined technique for restructuring an existing body of code, altering its internal structure without changing its external behavior", undertaken in order to improve some of the nonfunctional attributes of the software. Advantages include improved code readability and reduced complexity to improve the maintainability of the source code, as well as a more expressive internal architecture or object model to improve extensibility.
By continuously improving the design of code, we make it easier and easier to work with. This is in sharp contrast to what typically happens: little refactoring and a great deal of attention paid to expediently adding new features. If you get into the hygienic habit of refactoring continuously, you'll find that it is easier to extend and maintain code.—Joshua Kerievsky, Refactoring to Patterns
Typically, refactoring is done by applying a series of standardised basic "micro-refactorings", each of which is a (usually) tiny change in a computer program's source code that either preserves the behaviour of the software or at least does not modify its conformance to functional requirements. Many development environments provide automated support for performing the mechanical aspects of these basic refactorings.
Refactoring is usually motivated by noticing a code smell. For example the method at hand may be very long, or it may be a near duplicate of another nearby method. Once recognized, such problems can be addressed by refactoring the source code, or transforming it into a new form that behaves the same as before but that no longer "smells". For a long routine, one or more smaller subroutines can be extracted; or for duplicate routines, the duplication can be removed and replaced with one shared function. Failure to perform refactoring can result in accumulating technical debt.
There are two general categories of benefits to the activity of refactoring.
- Maintainability. It is easier to fix bugs because the source code is easy to read and the intent of its author is easy to grasp. This might be achieved by reducing large monolithic routines into a set of individually concise, well-named, single-purpose methods. It might be achieved by moving a method to a more appropriate class, or by removing misleading comments.
- Extensibility. It is easier to extend the capabilities of the application if it uses recognizable design patterns, and it provides some flexibility where none before may have existed.
Before applying a refactoring to a section of code, a solid set of automatic unit tests is needed. The tests are used to demonstrate that the behavior of the module is correct before the refactoring. If it inadvertently turns out that a test fails, then it's generally best to fix the test first, because otherwise it is hard to distinguish between failures introduced by refactoring and failures that were already there. After the refactoring, the tests are run again to verify the refactoring didn't break the tests. Of course, the tests can never prove that there are no bugs, but the important point is that this process can be cost-effective: good unit tests can catch enough errors to make them worthwhile and to make refactoring safe enough.
The process is then an iterative cycle of making a small program transformation, testing it to ensure correctness, and making another small transformation. If at any point a test fails, the last small change is undone and repeated in a different way. Through many small steps the program moves from where it was to where you want it to be. In order for this very iterative process to be practical, the tests have to run very quickly, or the programmer would have to spend a large fraction of his time waiting for the tests to finish. Proponents of extreme programming and other agile methodologies describe this activity as an integral part of the software development cycle.
List of refactoring techniques 
Here are some examples of micro-refactorings; some of these may only apply to certain languages or language types. A longer list can be found in Fowler's Refactoring book and on Fowler's Refactoring Website. Many development environments provide automated support for these micro-refactorings. For instance, a programmer could click on the name of a variable and then select the "Encapsulate field" refactoring from a context menu. The IDE would then prompt for additional details, typically with sensible defaults and a preview of the code changes. After confirmation by the programmer it would carry out the required changes throughout the code.
- Techniques that allow for more abstraction
- Techniques for breaking code apart into more logical pieces
- Componentization breaks code down into reusable semantic units which present clear, well-defined, simple-to-use interfaces.
- Extract Class moves part of the code from an existing class into a new class.
- Extract Method, to turn part of a larger method into a new method. By breaking down code in smaller pieces, it is more easily understandable. This is also applicable to functions.
- Techniques for improving names and location of code
Hardware refactoring 
While the term refactoring originally referred exclusively to refactoring of software code, in recent years code written in hardware description languages (HDLs) has also been refactored. The term hardware refactoring is used as a shorthand term for refactoring of code in hardware description languages. Since HDLs are not considered to be programming languages by most hardware engineers, hardware refactoring is to be considered a separate field from traditional code refactoring.
Automated refactoring of analog hardware descriptions (in VHDL-AMS) has been proposed by Zeng and Huss. In their approach, refactoring preserves the simulated behavior of a hardware design. The non-functional measurement that improves is that refactored code can be processed by standard synthesis tools, while the original code cannot. Refactoring of digital HDLs, albeit manual refactoring, has also been investigated by Synopsys fellow Mike Keating. His target is to make complex systems easier to understand, which increases the designers' productivity.
In the summer of 2008, there was an intense discussion about refactoring of VHDL code on the news://comp.lang.vhdl newsgroup. The discussion revolved around a specific manual refactoring performed by one engineer, and the question to whether or not automated tools for such refactoring exist.
Although refactoring code has been done informally for years, William Griswold's 1991 Ph.D. dissertation is one of the first major academic works on refactoring functional and procedural programs, followed by William Opdyke's 1992 dissertation on the refactoring of object-oriented programs, although all the theory and machinery have long been available as program transformation systems. All of these resources provide a catalog of common methods for refactoring; a refactoring method has a description of how to apply the method and indicators[disambiguation needed] for when you should (or should not) apply the method.
The first known use of the term "refactoring" in the published literature was in a September, 1990 article by William F. Opdyke and Ralph E. Johnson. Griswold's Ph.D. thesis, Opdyke's Ph.D. thesis, published in 1992, also used this term.
In extreme programming, the Extract Method refactoring technique has essentially the same meaning as factoring in Forth; to break down a "word" (or function) into smaller, more easily maintained functions.
Refactorings can also be reconstructed posthoc to produce concise descriptions of complex software changes recorded in software repositories like CVS or SVN.
Automated code refactoring 
- IntelliJ IDEA (for Java)
- NetBeans (for Java)
- JDeveloper (for Java)
- Embarcadero Delphi
- Visual Studio (for .NET)
- ReSharper (addon for Visual Studio)
- Coderush (addon for Visual Studio)
- Visual Assist (addon for Visual Studio with refactoring support for VB, VB.NET. C# and C++)
- DMS Software Reengineering Toolkit (Implements large-scale refactoring for C, C++, C#, COBOL, Java, PHP and other languages)
- Photran (a Fortran plugin for the Eclipse IDE)
- SharpSort addin for Visual Studio 2008
- Sigasi HDT (for VHDL)
- Xcode (for C and Objective-C)
- Smalltalk Refactoring Browser (for Smalltalk)
- Simplifide (for Verilog, VHDL and SystemVerilog)
- Tidier (for Erlang)
- AMIQ DVT (for e, SystemVerilog, Verilog and VHDL)
- Bicycle Repair Man! (for Python)
See also 
- Code review
- Database refactoring
- Design pattern (computer science)
- Obfuscated code
- Software peer review
- Separation of concerns
- Test-driven development
- Modular programming
- [Martin Fowler in http://refactoring.com]
- Kerievsky, Joshua (2004). Refactoring to Patterns. Addison Wesley.
- Fowler, Martin (1999). Refactoring: Improving the design of existing code. Addison Wesley.
- Martin, Robert (2009). Clean Code. Prentice Hall.
- Refactoring techniques in Fowler's refactoring Website
- Replace type-checking code with State/Strategy
- Replace conditional with polymorphism
- Hardware description languages#HDL and programming languages
- Kaiping Zeng, Sorin A. Huss, "Architecture refinements by code refactoring of behavioral VHDL-AMS models". ISCAS 2006
- M. Keating :"Complexity, Abstraction, and the Challenges of Designing Complex Systems", in DAC'08 tutorial "Bridging a Verification Gap: C++ to RTL for Practical Design"
- M. Keating, P. Bricaud: Reuse Methodology Manual for System-on-a-Chip Designs, Kluwer Academic Publishers, 1999.
- Sigasi launches its first production release for a VHDL development environment
- Griswold, William G (July 1991). Program Restructuring as an Aid to Software Maintenance. Ph.D. thesis. University of Washington. Retrieved 2011-12-24.
- Opdyke, William F (June 1992). Refactoring Object-Oriented Frameworks (compressed Postscript). Ph.D. thesis. University of Illinois at Urbana-Champaign. Retrieved 2008-02-12.
- Martin Fowler, "MF Bliki: EtymologyOfRefactoring"
- Opdyke, William F.; Johnson, Ralph E. (September 1990). "Refactoring: An Aid in Designing Application Frameworks and Evolving Object-Oriented Systems". Proceedings of the Symposium on Object Oriented Programming Emphasizing Practical Applications (SOOPPA). ACM.
- Weißgerber, Peter; Diehl, S. (2006). "Identifying Refactorings from Source-Code Changes". Proceedings of 21st IEEE/ACM International Conference on Automated Software Engineering (ASE 2006). ACM. http://www.st.uni-trier.de/~diehl/pubs/ase06.pdf.
Further reading 
- Fowler, Martin (1999). Refactoring. Improving the Design of Existing Code. Addison-Wesley. ISBN 0-201-48567-2.
- Wake, William C. (2003). Refactoring Workbook. Addison-Wesley. ISBN 0-321-10929-5.
- Mens, Tom and Tourwé, Tom (2004) A Survey of Software Refactoring, IEEE Transactions on Software Engineering, February 2004 (vol. 30 no. 2), pp. 126–139
- Feathers, Michael C (2004). Working Effectively with Legacy Code. Prentice Hall. ISBN 0-13-117705-2.
- Kerievsky, Joshua (2004). Refactoring To Patterns. Addison-Wesley. ISBN 0-321-21335-1.
- Arsenovski, Danijel (2008). Professional Refactoring in Visual Basic. Wrox. ISBN 0-470-17979-1.
- Arsenovski, Danijel (2009). Professional Refactoring in C# and ASP.NET. Wrox. ISBN 978-0-470-43452-9.
- Ritchie, Peter (2010). Refactoring with Visual Studio 2010. Packt. ISBN 978-1-84968-010-3.
- What Is Refactoring? (c2.com article)
- Martin Fowler's homepage about refactoring
- Aspect-Oriented Refactoring by Ramnivas Laddad
- A Survey of Software Refactoring by Tom Mens and Tom Tourwé
- Refactoring at the Open Directory Project
- Refactoring Java Code
- Refactoring To Patterns Catalog
- Extract Boolean Variable from Conditional (a refactoring pattern not listed in the above catalog)
- Test-Driven Development With Refactoring
- Revisiting Fowler’s Video Store: Refactoring Code, Refining Abstractions