Some errors with my ActiveSessions counter

[Jenkin Zheng]

Hi, everyone! I want to count active sessions on server. So I implement javax.servlet.http.HttpSessionListener, Here is a part code of SessionCounter

public void sessionCreated(HttpSessionEvent event) {
		// TODO Auto-generated method stub
		ServletContext context = event.getSession().getServletContext();
		Integer activeSessions = (Integer) context
				.getAttribute("activeSessions");
		System.out.println(activeSessions + ".created");
		if (activeSessions == null) {
			activeSessions = new Integer(1);
		} else {
			int count = activeSessions.intValue();
			activeSessions = new Integer(count + 1);
		}
		context.setAttribute("activeSessions", activeSessions);
	}

	/**
	 * @see HttpSessionListener#sessionDestroyed(HttpSessionEvent)
	 */
	public void sessionDestroyed(HttpSessionEvent event) {
		// TODO Auto-generated method stub
		ServletContext context = event.getSession().getServletContext();
		Integer activeSessions = (Integer) context
				.getAttribute("activeSessions");
		System.out.println(activeSessions + ".destoryed");
		if (activeSessions == null) {
			activeSessions = new Integer(0);
		} else {
			if (activeSessions > 0) {
				int count = activeSessions.intValue();
				activeSessions = new Integer(count - 1);
			}
		}
		context.setAttribute("activeSessions", activeSessions);
	}

Here is my test the counter code:

<span>[<%=application.getAttribute("activeSessions") %>]active sessions on server</span>

Sometimes, the result is not right, like:[null]active sessions on server.
Why does it show null?I'm a newer!
thanks, everyone!

[Piyush Joshi]

may be because you are printing it too early..
print it after the if/else block after you set the attribute in your code..

[syed aq]

Donot use listener, in rare cases listeners are not fired

[Piyush Joshi]

syed aq wrote:Donot use listener, in rare cases listeners are not fired

Can you please explain what are the cases when listeners will not be fired. (except when the server crashes)