My requirement is to notity the use with a popup saying that the user session is about to time out in x seconds in case user does not perfrom any activity on the web page.
Addition to this requirement is to decrement the value x seconds dynamically in the popup.
The environment I use is J2EE. Please suggest.
I don't think that Java/J2EE will be really helpful here as this need to be handled on the client-side (i.e. using JavaScript). One solution I can think of would be to set a kind of timer that would notify the user a few minutes before the server's timeout.
While googling about this, I found Eric Pascarello's Update User's Session with AJAX blog post (and the reloaded version Updating User Session with Ajax - Round 2) that precisely describes such a solution (and use an XMLHttpRequest to update the session). His Ajax Session Management script is available here.
Make use of HttpSession#getMaxInactiveInterval() and setTimeout(). There's no need for Ajax in this particular purpose, unless you want to postpone the timeout on every client activity (polling).
Basic example:
<script>
var secondsBeforeExpire = ${pageContext.session.maxInactiveInterval};
var timeToDecide = 15; // Give client 15 seconds to choose.
setTimeout(function() {
alert('Your session is about to timeout in ' + timeToDecide + ' seconds!')
}, (secondsBeforeExpire - timeToDecide) * 1000);
</script>
To decrement the time inside the message magically, then instead of the basic alert() you'll need an overlay with a div wherein you have control over the content through HTML DOM tree and make use of another setTimeout() on 1 second to change the text dynamically.
Note that this script has to be served by the JspServlet to get the EL to work. Thus, you need to put the script in the HTML <head> of the JSP page, or if you really want to have all the JS in a separate *.js file, then you need to let the JspServlet handle any *.js requests as well.