Session Binding Question

Question

Hello All,
I might be missing something, but I am trying to bind a session when a user logs
in and out of my web application in order to be able to actively track online usage,
however I can't seem to get it to work.
I would appreciate any help that may shed some light on the problem.
I invoke the bind listener in a servlet thus:
session.setAttribute("listener", listener);
I have a simple BindListener Class:
import javax.servlet.http.*;
public class BindListener implements HttpSessionBindingListener
public int numSessions = 0;
public BindListener()
public synchronized void valueBound(HttpSessionBindingEvent event)
numSessions++;
public synchronized void valueUnbound(HttpSessionBindingEvent event)
numSessions--;
public int getNumSessions()
return numSessions;
However when I call a jsp page to monitor the online connections with the command
BindListener listener = new BindListener();
listener.getNumSessions()
It always reports a 0 value even though I have serveral sessions running.
Can anyone help explain why the counter is not working?
I would extremely grateful of any assistance on this issue.
Thanks in advance.
Regards
Les,

Answer 1

Greetings,
I would appreciate any help that may shed some light
on the problem.First, it's important to understand a couple things about binding listeners:
1. A session binding listener is an object that listens for its own binding; IOW, it receives notifications when it specifically is bound to or unbound from the session; and
2. A session binding listener is specific to the session to which it is bound.
I invoke the bind listener in a servlet thus:
session.setAttribute("listener", listener);After the container binds the specified listener instance to this particular session, it calls the object's valueBound method...
Conversely, when (after) the object is removed from the session by calling
    session.removeAttribute( "listener" );or
    session.invalidate();  // Which implicitly calls the above ;)then it's valueUnbound method is called.
I have a simple BindListener Class:
import javax.servlet.http.*;
public class BindListener implements
HttpSessionBindingListener
public int numSessions = 0;
public BindListener()
public synchronized void
valueBound(HttpSessionBindingEvent event)Synchronizing this method only keeps multiple threads from concurrently calling it on this instance... Since it's the current container instance that calls these (container callback) methods, there will (should) be only one thread calling it...
numSessions++;
}numSessions will only ever be 1 (at most), since this method only gets called when the instance is bound to the session - which is only when your user logs in. Again, session objects ('listener' or otherwise), unless persisted, exist only for the life of the current session...
public synchronized void
valueUnbound(HttpSessionBindingEvent event)
numSessions--;
}Refer again to the above. Incidentally, what are you trying to achieve with this?
public int getNumSessions()
return numSessions;
}Refer again to the above. :)
However when I call a jsp page to monitor the online
connections with the command
BindListener listener = new BindListener();This simply invokes the class' constructor and - most importantly in this case ;) - creates a new instance of the class, and...
listener.getNumSessions();
It always reports a 0 value even though I haveSince this is a new instance whose 'numSessions' member is initialized to '0', the accessor is reporting correctly.
To get an existing listener object for the current session use:
<jsp:useBean id="listener" scope="session" class="BindListener"/>...then one of:
<jsp:getProperty name="listener" property="numSessions"/>
<%-- or --%>
<%= listener.getNumSessions() %>Keep in mind, however, that the above only retrieves the listener object for, again, the current session and whose 'numSessions' will still only be '1' (as currently implemented ;).
serveral sessions running.A session is a reference to a client's life in the application (refer to thread http://forum.java.sun.com/thread.jsp?forum=13&thread=249978 for further discussion on "session life"); one session cannot access the (in memory ;) information of other sessions...
Can anyone help explain why the counter is not
working?Refer again to all the above. :)
I would extremely grateful of any assistance on this
issue.If your goal is to track "online usage" then you're on the right track. However, as explained, the current implementation will only count the current session. To keep track of all sessions for a user your bean needs to persist and re-load the current count statistics for the user. The following illustrates:
import java.io.*;
import java.util.*;
import javax.servlet.http.*;
* <p>Tracks user login counts.  A count of user's accesses is stored
* in a file within the webapp space.</p>
* <p>NOTE: This class is intended only to illustrate a method for
* managing persistent session tracking.</p>
* @author     Tony "Vee Schade" Cook, [email protected]
public class SessionTracker implements HttpSessionBindingListener
     * <p>Creates a new session tracking instance.
     * @param  sessionFile     the user specific session file.
    public SessionTracker( File sessionFile )
        this.sessionFile = sessionFile;
     * <p>Loads current session count.
    public void valueBound( HttpSessionBindingEvent event )
        Properties data = new Properties();
        try {
            data.load( new FileInputStream( sessionFile ) );
            numSessions = Integer.parseInt( data.getProperty( "count" ) );
        catch( Exception ex ) {
            /* Assume no persistent data, or corrupt/invalid...
             * either way, accept default numSessions = 0
        /* Remember this session too! ;)
        numSessions++;
     * <p>Stores current session count for later retrieval.
    public void valueUnbound( HttpSessionBindingEvent event )
        Properties data = new Properties();
        data.setProperty( "count", Integer.toString( numSessions ) );
        try {
            data.store( new FileOutputStream( sessionFile ), sessionFile.toString() );
        catch( Exception ex ) {
            /* Sorry, Charley - "log" the error for the administrator.
            ex.printStackTrace();
     * <p>Returns current session count for this user.
    public int getNumSessions()
        return numSessions;
    private File sessionFile;
    private int numSessions;
}The above should actually be using a database for the persistence if you also want to be able to efficiently access the count statistics from a separate session. Yet, the above illustrates what is needed (session persistence) and should get you started in the right direction. :)
I hope this helps.
Thanks in advance.
Regards
Les,Regards,
Tony "Vee Schade" Cook