实现类：OnLineUser .java
package com.alpha.onLineUser;

/*
 * Info:统计当前在线人数
 * Date:2010-4-5 17:45:55
 * Auth:JavaAlpha
 */

import javax.servlet.ServletContext;
import javax.servlet.http.HttpSession;
import javax.servlet.http.HttpSessionEvent;
import javax.servlet.http.HttpSessionListener;

public class OnLineUser implements HttpSessionListener
{
  public void sessionCreated(HttpSessionEvent event)
  {
    ServletContext sct = event.getSession().getServletContext();
    
    Integer onLineUser = (Integer) sct.getAttribute("onLineUser");
    if (null == onLineUser) {
      onLineUser = new Integer(1);
    }else {
      int count = onLineUser.intValue();
      onLineUser = new Integer(count+1);
    }
    sct.setAttribute("onLineUser", onLineUser);
  }

  public void sessionDestroyed(HttpSessionEvent event)
  {
    ServletContext sct = event.getSession().getServletContext();
    Integer onLineUser = (Integer) sct.getAttribute("onLineUser");
    if (null == onLineUser) {
      onLineUser = new Integer(0);
    }else {
      int count = onLineUser.intValue();
      onLineUser = new Integer(count-1);
    }
    sct.setAttribute("onLineUser", onLineUser);
    
    //销毁session
    // HttpSession session = event.getSession();
    // session.invalidate();
  }
}
 
JSP页面：test.jsp
<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
  <body>
    当前在线人数：<%=application.getAttribute("onLineUser") %>
  </body>
</html>

web.xml 监听器配置：
<!-- 统计当前在线人数 -->
<listener>
  <listener-class>com.webapp.onLineUser.OnLineUser</listener-class>
</listener>