Prohibited multiple login for same user

I am using Struts frame work having Scenario like I am having one user and its password.

I can login into application through that but the provision is like that one user can login only one time in application

if same user will open other browser on same or other machine he should not be able to do the same.

Multiple login should be prohibited for the same user So how can I check that use is already logged in. I cant set the user login credentials in Servlet context because it will be static approach for a single user users can be thousand or more so how I can check it in run time.

If any of us can send code also will be beneficial to me.

With Thanks and Regards

[b]Question asked by visitor Abhishek[/b]

You can have a table for currently logged in users lets say CURRENT_USERS
when ever a user logs in check against this table to see if he/she is there in that table , if not let the user log in and add the name in it . and dont forget to remove the name once the user is logged out or session times out.
-hope it helps.