The only way I can see to solve your problem is to sign an aggrement with your customer about how to use the source code. If they want to maintain / do future development by themself , they need the source code.
If you do not even give your customer class files, how are they going to use your application? :-)
One thing to prevent decompile is to use obfuscate tools. Check this out
http://www.retrologic.com/retroguard-faq.html Lin