How to detect browser closing?

Question

In my web app, when a user logs in, I add his Id to a vector of valid Ids in the servlet, when he logs out, I remove his Id from the vector, so I can see how many current us

Answer 1

I suggest you remove the ID when the Servlet engine destroys the session. Register a HttpSessionListener that removes the user's ID when sessionDestroyed() is called.

Diodeus's idea will only help you detect that the session is over more immediately.