Notify user about session timeout in Java EE
My requirement is to notify the user with a popup saying that the user session is about to time out in x seconds in case user does not perform 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 Java EE.
Answer
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.