Joining WordPress and Java for validation of user

Here's the scenario. Suppose, we have 1 company
1. I am using WordPress for website.
2. Using Java for developing one application (paid application).
Now I want to make this application such that, only the valid customers (only buyers) can make use of it.
We can mail unique key. We can use online method for "one-time" registration or anything else.
How to proceed? Any suggestions?
Please Guide me.