we have obfuscated our java source code and currently working on it, so make our source fool proof what are all thing to be done while obfuscation ?, how it can be prevented from de-obfuscation? and
What are all the de-obfuscation open source tool available right now?
There is no such thing as a deobfuscation tool. What you are actually talking about is a decompiler.
A good obfuscator will remove most of the clues that a decompiler needs to produce readable source code. However, it will not prevent someone with enough skill and determination from reverse engineering your code anyway. In fact, there is NOTHING you can do to prevent that.
If you want to make a judgement on how easy / hard it would be, then the best answer is to attempt to do the reverse engineering yourself ... using all of the decompilers you can get your hands on. (I imagine that you are really worried about someone attacking a critical part ... like the license key checking. So focus on reverse engineering / circumventing that.)