It depends on how heavily the jar has been obfuscated and with what. The least you can do is post the jar so we know what we are dealing with, because, in general terms, anyone can decompile a jar and restore it, since the decompiling itself is basically restoration.
Let me put it this way - had the code's class names and elements been obfuscated, think ProGuard, you are not getting the original code back, no matter what. The best you would get is an interpretation.