package de.spieleck.servlets;

import java.util.TreeSet;
import java.util.Iterator;
import java.util.Comparator;

import javax.servlet.http.*;

/**
 * Watch all Sessions in the current Application:
 * Setup in the web.xml of a Servlet 2.3 conforming container via
 * <PRE>
      /lt;listener>
        /lt;listener-class>de.spieleck.servlets.SessionAccountingListener/lt;/listener-class>
      /lt;/listener>
 * </PRE>
 * then use the API's like
 * <PRE>
 * ..
 * SessionAccountingListener salt = SessionAccountingListener.getInstance();
 * int sessionsCount = salt.getSessionCount();
 * Iterator sessions = salt.getSessions();
 * ...
 * </PRE>
 *
 */
public class SessionAccountingListener 
    implements HttpSessionListener
{
    protected static SessionAccountingListener instance = null;

    protected TreeSet sessions;

    protected int totalSessions = 0;

    public SessionAccountingListener()
    {
        if ( instance != null )
            System.err.println("Overwrite previous listener: "+this);
        instance = this;
        sessions = new TreeSet(SessionAccessComparator);
    }

    public static SessionAccountingListener getInstance()
    {
        return instance;
    }

    public void sessionCreated(HttpSessionEvent ev)
    {
        synchronized ( sessions )
        {
            sessions.add(ev.getSession());
        }
        totalSessions++;
    }

    public void sessionDestroyed(HttpSessionEvent ev)
    {
        synchronized ( sessions )
        {
            sessions.remove(ev.getSession());
        }
    }

    public int getTotalSessions()
    {
        return totalSessions;
    }

    public int getSessionCount()
    {
        return sessions.size();
    }

    public Iterator getSessions()
    {
        synchronized ( sessions )
        {
            // Copy sessions, to make iterator failsafe!
            return ((TreeSet)sessions.clone()).iterator();
        }
    }

    public static Comparator SessionAccessComparator =
                new Comparator()
                {
                    public int compare(Object o1, Object o2)
                    {
                        if ( o1 instanceof HttpSession
                            && o2 instanceof HttpSession )
                            return (int)( 
                                ((HttpSession)o2).getLastAccessedTime()
                                -((HttpSession)o1).getLastAccessedTime()
                            );
                        return 0;
                    }
                    public boolean equals(Object o1, Object o2)
                    {
                        return o1 == o2;
                    }
                }
        ;
}