You can decompile the jar, which doesn't perfectly re-create the code as compiling isn't a fully reversible process. However, some scripters like to protect their work by obfuscating the code (mixing up the layout, scrambling variable names, re-organising whitespace, all without changing the logic of the code), so watch out for this. If you run into obfuscated code, it's up to you to make a decision about whether you trust it, but if it's someone new or little known, i'd most likely avoid running that script. That being said, you can still elicit some information from obfuscated code.
-Apa