Refactoring is "the process of changing a software system in such a way that it does not alter the external behavior of the code yet improves its internal structure," according to Martin Fowler, the "father" of refactoring. The concept of refactoring covers practically any revision or cleaning up of source code, but Fowler consolidated many best practices from across the software development industry into a specific list of "refactorings" and described methods to implement them in his book, Refactoring: Improving the Design of Existing Code.
One approach to refactoring is to improve the structure of source code at one point and then extend the same changes systematically to all applicable references throughout the program. The result is to make the code more efficient, scalable, maintainable or reusable, without actually changing any functions of the program itself. In his book, Fowler describes a methodology for cleaning up code while minimizing the chance of introducing bugs.
While refactoring can be applied to any programming language, the majority of refactoring current tools were originally developed for the Java language. In 2001, automated refactoring tools began to appear in earnest, including the IntelliJ IDEA Java IDE (Integrated Development Environment), the X-ref plug-in tool for the EMACS editor and the Instantiations jFactor stand-alone refactoring tool.
Many basic editing environments support simple refactorings like renaming a function or variable across an entire code base. Eric Raymond, a leading philosopher about program development, maintains that the concept of refactoring is consistent with the idea of get-something-working-now-and-perfect-it-later approach long familiar to Unix and open source programmers and hackers. The idea is also embodied in the approach known as extreme programming.
Today, refactoring plays an important role in application modernization and moving legacy apps from a monolithic structure to microservices.