Keeping track of online users

[Anders Steinlein]

I want to keep track of the number of online users in my web application.
I know when a user signs in and out, but what about those numerous users
who doesn't explicitly sign out (leaving the session hanging until it
invalidates)? What is the normal way of dealing with this?

I'm running Tomcat 5.0.19 with container managed security (if that
matters) and Struts 1.1.

Thanks for any input.

Regards,
\Anders