如何在tomcat实例中检索活动会话？

I was designated to implement a security requierement to mimic "messenger like" authentication, for example: if a user first logs in, then another user tries to log in with the same username, this new user will be prompted to "kick" the previously loged user and the system should invalidate the first user's web session, the web app is running on tomcat 6, i was taking a look at I was designated to implement a security requie