Introduction
  Session timeout client-side notification has always been an issue in web development. 
  Many applications just make it in a "user-self-service" manner. That is, users have no 
  idea about whether the http session is timeout until they make their first submission after 
  the session timeout. Usability is low in this way: users may enter a lot of data 
  without realizing the session is timeout and may lose all the data they enter when submiting
  in case the form is not cached locally.
        
  With AJAX+Javascript, Session timeout notification is made easy. The result of my approach
  is that almost "as soon as" server session timeout, browser will pop up a message alert to indicate
  what is going on in the server. It is done in a "in parallel timing" style.  That is, client 
  and server counting timeout in parallel while client wait a little bit longer. When client side
  timeout browser sends an AJAX request to server to verify if the session is over. Any client side
  submission will reset the client side counter just like web container does at the server side.
  In this low overhead way, no client polling  nor server pushing, only one verification request 
  is needed per HTTPSession timeout.
  
  Here are some key steps you may want to pay attention to when you read the source codes:
        
read session timeout from web.xml and write into javascript to be used as
timeout period in JSP
          
provide a server side session timeout checker
          
launch a timeout thread in Javascript with given period (a little bit longer) 
from web.xml on page load
          
when javascript timeout is trigged, make an AJAX call to the session timeout checker
          
handle the timeout
        
        
   
   
Demo
    This is a shopping page where session timeout is set to 2 minutes. Please read the "Introduction"
    in the popped up page to learn what to try.

Demo of Client-Side HTTP Session Timeout