Hi Bloo,
You can invalidate the session using:
(Note that this is a standard
J2EE function, not specific to Struts)
If a session is still valid, then calling:
will return a non-null session. However I have a feeling Struts will automatically create the session (if it does not exist) before any Action is called. You may need to create a custom RequestProcessor or ServletRequestListener to intercept the call before this happens.
Hope that helps.