HttpSessionListener: Negative session count

[Christian Hauser]

Hello J2EE programmers

I wrote a JSP some time ago that uses a class SessionCounter (which 
implements HttpSessionListener) to count the active sessions of a web 
application and to display them.

Unfortunately there seems to be a problem with my program, because the 
web application says that there are currently -18 active sessions. 
However, the lowest possible count should be 0.

Of course I could implement the sessionDestroyed the following way (to 
prohibit a negative session count, but then the current session count 
would not be correct, not?
    public void sessionDestroyed(HttpSessionEvent event) {
      if (activeSessions > 0)
        activeSessions--;
    }

Code of JSP file:
  <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 The web application.

  public class SessionCounter implements HttpSessionListener {

    /** Static variable to keep track of the current number of active 
sessions. */
    private static int activeSessions = 0;

    public void sessionCreated(HttpSessionEvent event) {
      activeSessions++;
    }
    public void sessionDestroyed(HttpSessionEvent event) {
      activeSessions--;
    }
    public static int getActiveSessions() {
      return activeSessions;
    }
  }
I'm using Tomcat 4.1.24 and hope that someone of you might give me a 
hint on how I can get a reliable session counter.

Christian

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