Do you want the source code or just the compiled classes? If the case is later than:
(1) Rename the war archive to jar. (2) jar -xvf warFileName.jar
This will unpack the entire structure.
If you want the original source code then you should have some utility like Java Decompiler. But even then you will have to unpack it. The code will be cryptic though, due to optimizations that the compiler does.
Make sure you are not doing this "reverse engineering" on the copyrighted material.