[stevoski]

A few years ago I wrote a Java decompiler. Not to actually decompile code but to learn how to use my weapon of choice (Java) at a deeper level.

I never finished. The only code I ever bothered decompiling was code I had written to test the decompiler. But I learnt a heck of a lot about how the JVM works, about Java byte code, and about some data structures and analysis. And I learnt exactly how the Java class file is defined.

It is something I'd highly recommend trying for your language of choice.