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

in JavaScript you can use the onbeforeclose event to pass a call back to the server when the user closes the browser.

I typically use a synchronous Ajax call to do this.