Sometimes, using a decompiler. Alex posted a link in this thread.

Note however, that this is not guaranteed to work: there exist several
obfuscators that can mangle the Java bytecode in such a way that the
functionality remains, but automatic decompilation is not possible.

If you want to decompile such a program, manual decompilation is the only
thing that works. Rewriting it is probably cheaper in that case (that's
the intention anyway).

Luckily, such obfuscators are not / hardly used for open source projects:
if you can get the source anyway, obfuscating is pointless.