Hi ,
I had created a new session in loginpage.I invalidated that session in logout page.if i goback by back button previous page is displaying.how can i rectify that problem.Its my requirement to display session expired message when i tries to go back throw back button after logout.please tell m ehow can i do that??????????????

That page is displayed from the browsercache, there's very little you can do about it.

There are some hacks that disable backbuttons and stuff but those are easily circumvented (and will just **** off your users).
Another option is to set some headers that cause the page to expire immediately (which should but doesn't always prevent caching).

Rest assured, if your users try to do anything with that page they should (if you coded your webapp correctly) get an error.