Java Development Tools

HI All

How can I distinguish session expiry and session invalidate.
I have written servlet to catch session expiry and its invoked after session invalidate also (while logging out).
Is there a way to call it only after session expiry ?
I am using the servlet to upgate my userlog table on session expiry.

(I need to differentiate session expiry and session invalidate separately because we have a column logoutstatus which should
be updated session expiry on session expiring and
updated logged out on loging out by invalidating the session)

We are using jdev 11.1.1.2.0 and WLS 10.3.2.0 to develop application.

Please advice
Susan