title: How to add a licensing functionality to a java program?
body: <p>I have made an application in java. Now I want to deliver it to  several clients. I want that each client must purchase a serial key to use that software. Without that key, the software shouldn't be run just like other softwares in the market.</p>
  
  <p>How can I do that?</p>
  
  <p>I know that I can't protect it fully. I just want a simple serial key functionality so that it would be hard for non-technical persons and novice programmers to crack the security.</p>