how to save my jar file from class decompiler.
Do you mean you want to protect your jar so it can't be decompiled?
There's no perfect solution, but there are ways to make it more difficult.
Google for java obfuscation
you can run it as a webservice, and only provide users the code which connects to that service, so they don't have the actual .class files locally, meaning they won't be able to decompile.