Despite the existence of some quality work, I assure you that Java obfuscation will NOT stop anyone who decides to decompile your code. Understand that all you buy is a little time. If they have class files in hand and you want to decompile them, it won't take long before you have the source code.
If you do not trust your client, do not give them class files. Come up with a different solution. More and more companies are switching to services in order to keep the source code in the house and still make their money.
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