Creating encrypted JARs is not possible, since the executing JavaVM has to somehow be able to read the data it wants to execute.
And similar to a VM it would be possible for anyone with the proper tools and know-how to extract all data from the JAR.
If it would be possible to encrypt the JAR, you would also have to provide some decryption-key or facility to the client which wants to execute the JAR which defeats the purpose of encryption at all.
The best you can get is obfuscation, but that's no real security or hurdle for the ambitious attacker.
