The thing with distributing software to clients, is that they always will go around poking in the inner workings of it. I know I always decompile the games I play, and look at the source code.
With jars there isn't much to do for art and assets, but you can always obfuscate your code. Obfuscation doesn't mean people won't still look at it, and potentionally figure out some things. Minecraft is obfuscated, and it's against the.. terms?.. to reverse engineer. Yet, modders still release their mods, right? Effectively proving that they have reverse engineered.
Obfuscate your code. License both code and art.