JODE is a java package containing a decompiler and an optimizer for Java.
The decompiler reads in class files and produces something similar to the original java file. Of course this can't be perfect: There is no way to produce the comments or the names of local variables (except when compiled with debuging) and there are often more ways to write the same thing. However, JODE does its job quite well, so you should give it a try.
These details are provided for information only. No information here is legal advice and should not be used as such.