Well at what level do you need to track?
Just how many hits on the site, or do you need a page breakdown/analysis as well?
You need to be able to distinguish between the clients.
If they have to log in to your application, you're sorted - you can track it via login name.
A stored cookie could also identify a client (as long as the user doesn't mind cookies)
If not, you might have to resort to something like IP address - request.getRemoteHost()?
Probably the best way to track this is via a ServletFilter that would execute on every request, and track who requested what.
http://java.sun.com/products/servlet/Filters.html