1.设计监听器类

OnlineListener.java

package com.listener;

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

public class OnlineListener implements HttpSessionListener {

    private static int onlineCount=0;
    public void sessionCreated(HttpSessionEvent sessionEvent) {onlineCount++;}
    public void sessionDestroyed(HttpSessionEvent sessionEvent) {
        if(onlineCount>0) onlineCount–;
    }
    public static int getOnlineCount() {return onlineCount;}
}

2.修改web.xml

<listener>
<listener-class>com.listener.OnlineListener</listener-class>
</listener>

3.显示在线人数的页面

online.jsp

<%@ page language="java" contentType="text/html; charset=utf-8"
pageEncoding="utf-8" import="com.listener.OnlineListener" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>在线人数显示页面</title>
</head>
<body>
<h2>当前在线人数：<%=OnlineListener.getOnlineCount() %></h2>
</body>
</html>