[java]HttpSessionListener实现统计在线人数
HttpSessionListener是个session监听器，它有两个方法：public void sessionCreated(HttpSessionEvent event){} 和 public void sessionDestroyed(HttpSessionEvent event){} , 前者是在session被创建的时候执行，后者是在session被销毁的时候执行，通过对当前session的监听，达到统计在线人数的效果。
代码如下：
首先建一个监听类 CountLineListener，实现 HttpSessionListener 接口，并添加未实现的方法 sessionCreated(){} 和 sessionDestroyed(){}：
package com.test.listener;
import javax.servlet.ServletContext;
import javax.servlet.http.HttpSessionEvent;
import javax.servlet.http.HttpSessionListener;
public class CountLineListener implements HttpSessionListener{
/***********
* 创建session时调用
*/
public void sessionCreated(HttpSessionEvent event) {
System.out.println("创建session......");
ServletContext context=event.getSession().getServletContext();
Integer count=(Integer)context.getAttribute("count");
if(count==null){
count=new Integer(1);
}else{
int co = count.intValue( );
count= new Integer(co+1);
}
System.out.println("当前用户人数："+count);
context.setAttribute("count", count);//保存人数
}
/************
* 销毁session时调用
*/
public void sessionDestroyed(HttpSessionEvent event) {
System.out.println("销毁session......");
ServletContext context=event.getSession().getServletContext();
Integer count=(Integer)context.getAttribute("count");
int co=count.intValue();
count=new Integer(co-1);
context.setAttribute("count", count);
System.out.println("当前用户人数："+count);
}
}
监听类写好了，接下来就要在web.xml里配置此监听类，添加代码：
<listener> <listener-class>com.test.listener.CountLineListener</listener-class> </listener>
针对以上可以结合 servlet 写个小例子：
(1) login.jsp
<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<title>My JSP 'index.jsp' starting page</title>
<meta http-equiv="pragma" content="no-cache">
<meta http-equiv="cache-control" content="no-cache">
<meta http-equiv="expires" content="0">
<meta http-equiv="keywords" content="keyword1,keyword2,keyword3">
<meta http-equiv="description" content="This is my page">
</head>
<body>
<form method="POST" action="<%=request.getContextPath()%>/MyServlet">
<input type="text" name="username"/>
<br/><input type="submit" value="登录"/>
</form>
</body>
</html>
点击登录---->MyServlet
(2) MyServlet.java
package com.test.servlet;
import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
public class MyServlet extends HttpServlet {
public void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
this.doPost(request, response);
}
public void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
String user= request.getParameter("username");
request.getSession().setAttribute("user", user);
request.getRequestDispatcher("/index.jsp").forward(request,response);
}
}
登录就跳到首页index.jsp，显示在线人数：
<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<title>My JSP 'index.jsp' starting page</title>
<meta http-equiv="pragma" content="no-cache">
<meta http-equiv="cache-control" content="no-cache">
<meta http-equiv="expires" content="0">
<meta http-equiv="keywords" content="keyword1,keyword2,keyword3">
<meta http-equiv="description" content="This is my page">
</head>
<body>
这里是首页，当前访问量：
<%
ServletContext context=session.getServletContext();
Integer count=(Integer)context.getAttribute("count");
%>
<%=count %>
<br/>
当前用户：${sessionScope.user }
</body>
</html>
这样就简单实现统计当前在线人数的效果了。如果在页面有一个"退出系统"的链接，可以调用session.invalidate()执行清除session，这样在线人数就会-1，那如果用户没有点击“退出系统”，而是直接关闭浏览器呢？我自己做了个测试，在关闭浏览器一会儿的话，服务器端会自动执行sessionDestroyed()方法进行销毁session，此时用户人数-1，但是有时候又不会执行，不知道为什么，自己还是慢慢研究吧......