Have you heard of a program allowing to disassemble a java class file?
By disassembling i mean to find out the VM instructions, not the source code.

Thanks a lot.

IDA will do just what you're asking.

If that isn't what you want, I'm sure this link will provide you with some nice utils to play around with:
http://www.meurrens.org/ip-Links/Java/CodeEngineering/