package moreservlets.listeners;

import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
import java.util.*;

/** Listener that keeps track of the number of sessions
 *  that the Web application is currently using.
 *  <P>
 *  Taken from More Servlets and JavaServer Pages
 *  from Prentice Hall and Sun Microsystems Press,
 *  http://www.moreservlets.com/.
 *  &copy; 2002 Marty Hall; may be freely used or adapted.
 */

public class ActiveSessionCounter
                             implements HttpSessionListener {
  private static int sessionCount = 0;
  private static int sessionLimit = 1000;
  private ServletContext context = null;

  /** Each time a session is created, increment the
   *  running count. If the count exceeds the limit,
   *  print a warning in the log file.
   */
  
  public void sessionCreated(HttpSessionEvent event) {
    sessionCount++;
    if (context == null) {
      recordServletContext(event);
    }
    String warning = getSessionCountWarning();
    if (warning != null) {
      context.log(warning);
    }
  }

  /** Each time a session is destroyed, decrement the
   *  running count. A session can be destroyed when a
   *  servlet makes an explicit call to invalidate, but it
   *  is more commonly destroyed by the system when the time
   *  since the last client access exceeds a limit.
   */
  
  public void sessionDestroyed(HttpSessionEvent event) {
    sessionCount--;
  }

  /** The number of sessions currently in memory. */
  
  public static int getSessionCount() {
    return(sessionCount);
  }

  /** The limit on the session count. If the number of
   *  sessions in memory exceeds this value, a warning
   *  should be issued.
   */
  
  public static int getSessionLimit() {
    return(sessionLimit);
  }

  /** If the number of active sessions is over the limit,
   *  this returns a warning string. Otherwise, it returns
   *  null.
   */
  
  public static String getSessionCountWarning() {
    String warning = null;
    if (sessionCount > sessionLimit) {
      warning = "WARNING: the number of sessions in memory " +
                "(" + sessionCount + ") exceeds the limit " +
                "(" + sessionLimit + "). Date/time: " +
                new Date();
    }
    return(warning);
  }

  private void recordServletContext(HttpSessionEvent event) {
    HttpSession session = event.getSession();
    context = session.getServletContext();
  }
}