If you have invalidated the session once the user logs out, and even still you see the page when you press back button, it is most likely a cached page shown by the browser.. You can change this behaviour by adding the following code to your jsp..
SCJP 5 (93%)
Joined: Feb 23, 2011
thanku so much Prasad Krishnegowda fro your reply but i have already added these lines on my page
Actually this code works allright for Internet Explorer but for Mozilla it does not work