License Key for my applications

Question

I have created an application and would like to distribute as freeware from my product website.
I would like to enforce users of my application to continually upgrade to the next version of the application on expiry of a license key.
How can I create license keys for my application that will enable me to specify an expiry date etc.. That way when the license expires the users are required to input a new license key of the application.
Thanks for any information !

Answer 1

Remember, you are developing in Java. The Java VM executes byte code. Byte code can be modified in place, or reverse engineered, modified at the source level and re-compiled.
You can not stop clever people form working around whatever scheme you go for.
You may come up with a very cunning scheme indeed and congratulate yourself on a job well done. Then along comes some guy/gal who's brighter and/or more experienced than you and in two shakes your scheme is compromised.
Consider the information in this link: http://www.cigital.com/hostile-applets/maginot.html
Large, expensive, commercial systems have large, expensive licensing mechnisms. They can all be compromised. Take BEA's scheme for WLS, they keep chaning it between version, but you just need to be dilligent and figure it out. That is, I'm sure you could do it. I, of course, would never consider attempting such a thing.
One word of warning - messing with commercial vendors code will, at the very least, invalidate your license agreement and, at worst, end up with you spending time behind bars.
Let me say here that I do not advocate reverse engineering commercial code. There, I think I've covered my ass on that one ;)
Having said all this. Even the simplest scheme will work for the vast majority of users/developers who have dificulty working out how to set their own class path.
My advice? If you insist on putting this key scheme into your system, spend as little time as possible and make it real simple. As Albert Einstein once said "Everything should be made as simple as possible, but no simpler".