Recipe7.2.Tracking Client Sessions

Recipe 7.2. Tracking Client Sessions

Problem

You need to keep track of the number of clients currently using your application.

Solution

Create a class that implements the HttpSessionListener interface, like the one shown in Example 7-2, that keeps count of the total number of active sessions.

Example 7-2. Session-counting listener

package com.oreilly.strutsckbk.ch07; import javax.servlet.ServletContext; import javax.servlet.http.HttpSessionEvent; import javax.servlet.http.HttpSessionListener; public class SessionCounter implements HttpSessionListener {     public void sessionCreated(HttpSessionEvent event) {         ServletContext ctx = event.getSession( ).getServletContext( );         Integer numSessions = (Integer) ctx.getAttribute("numSessions");         if (numSessions == null) {             numSessions = new Integer(1);         }         else {             int count = numSessions.intValue( );             numSessions = new Integer(count + 1);         }         ctx.setAttribute("numSessions", numSessions);     }     public void sessionDestroyed(HttpSessionEvent event) {         ServletContext ctx = event.getSession( ).getServletContext( );         Integer numSessions = (Integer) ctx.getAttribute("numSessions");         if (numSessions == null) {             numSessions = new Integer(0);         }         else {             int count = numSessions.intValue( );             numSessions = new Integer(count - 1);         }         ctx.setAttribute("numSessions", numSessions);     } }

Declare your class in a listener element of your application's web.xml file. The listener element is supported by Version 2.3 or 2.4 of the Servlet specificationthe DTD must specify Version 2.3 or 2.4:

<?xml version="1.0" encoding="ISO-8859-1" ?> <!DOCTYPE web-app PUBLIC     "-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN"     "http://java.sun.com/dtd/web-app_2_3.dtd">      <web-app>   <display-name>Struts Cookbook - Chapter 7 Examples</display-name>      <listener>       <listener-class>com.oreilly.strutsckbk.ch07.SessionCounter       </listener-class>   </listener>   ... rest of web.xml

Discussion

In the Solution, any time a session is created, a counterstored in the servlet contextis incremented. Every time a session is destroyed, that counter is decremented. In practical terms, this custom class tracks the number of users currently using your application.

Web applications use HTTP sessions to maintain state between requests. The Servlet 2.3 specification added support for session listeners that receive notifications when any session is created or destroyed. You can write a class that receives these events by implementing the HttpSessionListener interface.

This interface defines two methods: sessionCreated( ) and sessionDestroyed( ). The container calls sessionCreated( ) after a session is created and sessionDestroyed( ) before the session is destroyed or invalidated. The container passes an HttpSessionEvent to each of these methods. This event object has a getSession( ) method that gives you a handle to the particular session.

Once you've got the HttpSession, you can do anything you want to with it. If the session was just created, you can add objects as session-scoped attributes. If destroyed, then you have the opportunity to dispose of session objects that require special handling. The HttpSession provides the getAttribute(), setAttribute( ), and removeAttribute( ) methods for manipulating session attributes. You can get a reference to your application's servlet context via the getServletContext( ) method.