What should I keep in mind in order to refactor huge code base?

Question

I'm going to refactor certain parts in a huge code base (18000+ Java classes). Goal is to be able to extract lower layers as independent libraries to be reused in other projects that currently use duplicate of this code base. Especially one part is of interest to be refactored into a framework independent of business logic. Ultimately I would like the code to have a clean architectural layering.

I've looked at the code with a tool called Structure 101 for java and found lots (!) of architectural layering issues where lower layers are referencing upper layers.

I don't want to simply start messing with the code but try to come up with a reasonable strategy to go about this problem. What things should I keep in mind?

I'm thinking about at least taking small steps. I'm also thinking about have unit tests in place, but that requires creating them, since there are none.

Any thoughts on this?

Answer 1

You should also take a look at Working with legacy code by Michael Feathers:

http://www.amazon.com/Working-Effectively-Legacy-Robert-Martin/dp/0131177052/ref=sr_1_1?ie=UTF8&s=books&qid=1242430219&sr=8-1

I think one of the most important things you can put in place to facilitate this are tests to ensure that everything still works post refactoring/pulling out into separate modules. Add to this by introducing a continuous integration system that runs your tests when you check something in.