Counting active sessions of web application

[Christian Hauser]

Hello
I wrote a JSP and a Servlet some time ago that both count the active sessions of a web application and display them.
To do that I wrote a class SessionCounter which implements HttpSessionListener and thus is being informed when a new session is created or a session has been destroyed.
Unfortunately there seems to be a problem with my program, because the web application says that there are currently -16 active sessions.
Maybe there is a problem within my code or some resources within the servlet engine (ServletExec 4.1) are not reset when restarted.
I hope someone can give me a hint.
Thank you in advance,
Christian
Code of JSP (similar in Servlets in other web applications):

  <tr>
    <td>Concurrent sessions:</td>
    <td><%= SessionCounter.getActiveSessions() %></td>
  </tr>
  <tr>
    <td>Last refresh:</td>
    <td><%= new Date() %></td>
  </tr>
  <tr>
    <td>Last session change:</td>
    <td><%= SessionCounter.getLastChange() %></td>
  </tr>

Here's the class SessionCounter, which belongs to a utility JAR that is situated in the lib directory of each web application.

public class SessionCounter implements HttpSessionListener {
  /** Static variable to keep track of the current number of active sessions. */
  private static int activeSessions = 0;
  /** Static variable to keep track of the time of the last change. */
  private static Date lastChangeDate = new Date();
  public void sessionCreated(HttpSessionEvent event) {
    activeSessions++;
    lastChangeDate = new Date();
  }
  public void sessionDestroyed(HttpSessionEvent event) {
    activeSessions--;
    lastChangeDate = new Date();
  }
  public static int getActiveSessions() {
    return activeSessions;
  }
  public static Date getLastChange() {
    return lastChangeDate;
  }
}

[Ryan Lowe]

In your sessionDestroyed...
public void sessionDestroyed ( HttpSessionEvent se )
{
if ( activeSessions > 0 )
{
activeSessions--;
}
}
Your code continues to decrement even if activeSessions is a negative number.
onlyOOD

[Christian Hauser]

Okey, but why is it that I get negative numbers? Is this a problem of my ServletEngine or does it have to be this way?
Christian (a little confused about HttpSessionEvents)

[Rangarajan Suresh]

Originally posted by Christian Hauser:
Okey, but why is it that I get negative numbers? Is this a problem of my ServletEngine or does it have to be this way?
Christian (a little confused about HttpSessionEvents)

what is the scope of the bean?
I remember tomcat having problems in invalidating sessions.

[steve souza]

JAMon is a performance tuning API, but it also does much more than that. Here is some sample code using JAMon that I have successfully used to count http sessions. You can also use it to co unt the number of times any particular user logs in as well as if they are currently logged in. You can also tell maximum and average number of sessions and more. JAMon can also track hits, average max and min execution times per page and more.
Sample session and page hit tracking code can be found at
http://www.javaperformancetuning.com/tools/jamon/#SampleCode
JAMon documentation and the download can be found at http://www.jamonapi.com
Steve

[Christian Hauser]

Thank you Steve for pointing me to your JAMon API. I'll have a look at it.
However, I still do not understand WHY it is possible that I get a negative session count. Why should there be more sessionDestroyed() calls than sessionCreated() (overall)?
Christian (still confused).

[Mike Curwen]

For the benefit of those not on the tomcat-user list...

Tomcat will restore an active session after restart, but your static variable will not. So if you have 20 active sessions and the server bounces, your sessions are re-created without the sessionCreated event being called.

When one of these sessions eventually gets destroyed, your counter will go negative.

[Christian Hauser]

Thank you Mike.
In fact, I was just about to start updating this thread when I read your post.
Christian