[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......