struts1 why session is not null after session.invalidate()

struts1 why session is not null after session.invalidate()

event though i invalidate the session means i called session.invalidate() in singoutACtion class (struts action) why the session obj still not null if print the session object it display the class name of the session.if session is not null how to prevent the end user after click on back button to preview the previous page and some times i checked based on the current user is null or not in user is null then i used reponse.sendRedirect() method to redirect to login page but it response.sendRedirect() method is not working plesase any body can help me