Refactoring is merely a process of restructuring existing code so that its end result and meaning remains intact. However, this restructuring has a great potential to become far better design than the original one. (It is implied that the refactored design is easier to refactor and hence better) In rest of this post I will try to summarize how to approach and refactor a code in real life application development.
1. Get familiar with code
I am assuming here that we are almost always refactoring code written fully or partially by others, refactoring your own code should be much more simple exercise. Primarily you would be dealing with Legacy code or testable code. If you are lucky enough to get the code which is almost covered with unit tests, your job becomes substantially easy and you can start refactoring very fast without a lot of trouble. However if you are working with existing code which just exists and works somehow, you will have to start slowly. The first step is to understand the incomprehensible. There are several ways you can get familiar with code:
- Read the code
Reading code is one of the fundamental activity we developers do most of the time, if any employer is in the illusion that she is paying developers just to write code then they are paying 1/3 of the salary. Reading code is an art which develops over the time, reading good code can make you better developer (with a precondition that one possesses the ability to judge good and bad code for the language). The more you read bad code the more you familiarize yourself with the repeating mistakes in the code (duplication, absolute hostility towards testability, extremely fearful defensive checks and other hilarities). The goal is to grasp the mindset of the original developer(s) who wrote it. You will know it immediately how deeply thought out the code is in a few passes. Use any modern IDEs to navigate through files, ignore the comments which don't make sense.
- Write test
Once you have some understanding of inputs/outputs and/or interactions in code's life cycle you can write some tests just so that you can identify if you made some mistake in next step (Step 2). These tests might not be unit-tests (unless the code itself is unit testable). Writing test is incredibly useful way to get familiar with the code base.
2. Isolate the refactoring hot-spot (or "The Inflection points" as Michael Feathers calls it)
There has to be a really good reason why you are refactoring and it certainly can't be a full 'rewrite'. Irony will kill itself when it finds out that the problems in a bad code manifests itself in popular hot-spots. By isolating these hot-spots you should be able to mock out the uninteresting and untestable (or very slow) dependencies. If you have multiple inflection points; deal with them one by one in isolation. Isolation may become tricky because you may have to introduce some inevitable changes before you actually have any unit tests. For example, you might want to eliminate inherently untestable 'statics', extract methods/interfaces which you can stub with NullObjects/mocks. The tests created in earlier step will be valuable to identify any problems you introduced while isolating the code. The goal here is to not concern yourself with trivialities other than the refactoring hot-spots.
3. Write unit tests
Now that you have code which can be isolated, start writing unit tests. If you are working on Java sources I highly recommend using Mockito to mock the dependencies. Don't waste time on accidental complexities of other mock libraries. A reasonable analogy would be: if Agile software development is a good thing than other mocking tools are worse than Waterfall - Mockito is Agile.
Your primary goal is to cover code under refactoring as much as possible, there are no hard numbers on code/branch coverages (but anything beyond 90% should be good enough :)), use common sense and quality metrics like CRAP. The more unit tests you will write, more you will learn about the code base. There may be totally unused code for anti-YAGNI stronghold, or there might be bugs, Your unit test will reveal these naturally. Depending on the code in question, A good test suit generally reaches the size of the code under test.
Refactoring is particularly painful when it has to be done in an environment with highly volatile code base and for the parts of code which are critical to overall application functionality. Make small changes; make sure all tests pass.
If you are using Eclipse - turn off auto build and add a ANT or Maven builder so that it runs your tests after each compile - you will be running tests a lot.
Use a real source control system, it might be much harder for you to revert the changes otherwise. You might consider using something like git in between upstream and your local repository if your primary SCM is not good enough. In practice, this becomes a lot more important than what IDE you use, you will need the ability to identify and rollback a commit specifically in code base with many hands. Commit after each change, the smaller the commit the lesser chance of breaking a lot of things together by a long shot.
With each commit going in repository without external complains (like "You broke my build!"), you will gain confidence to make bigger changes. You should continue writing tests while refactoring because if at this point you are making changes without violating functional contracts you are evolving a new design and doing something good. If you have broken something and your justification involves the word 'refactoring', you are doing it wrong! you either don't have enough tests or you failed to understand the code entrails.
So this is refactoring in real life. For the primary audience, please consider following these simple steps before tossing the word 'refactoring' around. If you disagree, then invent the new word and let me know :).