显示在线用户
方法一:用application对象。 简单描述如下： 当用户登录时维护一个会话对象：UserInfo类的实例，同时UserInfo类实现javax.servlet.http.HttpSessionBindingListener接口: import java.util.Map; import java.util.HashMap; import javax.servlet.http.*; import javax.servlet.ServletContext; public class UserInfo implements HttpSessionBindingListener { // 用户信息 int userId; String userName; ...... /** * 当UserInfo类被绑定到session中时触发这个事件 * 具体信息请参见Servlet/JSPs的规范 */ public void valueBound(HttpSessionBindingEvent event) { HttpSession session = event.getSession (); ServletContext ctx = session.getServletContext (); Map map = (Map)ctx.getAttribute ("users"); if (map == null) { // 如果是服务器启动后的第一个登录用户，新建一个容器 map = new HashMap (); ctx.setAttribute ("users", map); } map.put (userName, this); // 把自己加入容器中 } /** * 当UserInfo类从session中删除时触发这个事件 * 具体信息请参见Servlet/JSPs的规范 */ public void valueUnbound(HttpSessionBindingEvent event) { HttpSession session = event.getSession (); ServletContext ctx = session.getServletContext (); Map map = (Map)ctx.getAttribute ("users"); map.remove (this); // 把自己从容器中删除 } ...... } 以后在JSP中就可以这样显示在线用户数了： <% int userCount = 0; java.util.Map map = (java.util.Map)application.getAttribute ("users"); if (map == 0) userCount = 0; else userCount = map.size (); %>; 当然你也可以不实现javax.servlet.http.HttpSessoinBindLisenter接口，而用自己的程序来实现这个功能，但这种方法最保险，也最简单。如果用户关掉浏览器，过一段时间后session就会超时，这时发触发valueUnbound事件方法二：servlet：package SessionCount;import javax.servlet.*; import javax.servlet.http.*; public class SessionCounter implements HttpSessionListener { private static int activeSessions = 0; public void sessionCreated(HttpSessionEvent se) { activeSessions++; } public void sessionDestroyed(HttpSessionEvent se) { if(activeSessions > 0) activeSessions--; } public static int getActiveSessions() { return activeSessions; } }web.xml<?xml version="1.0" encoding="UTF-8"?><web-app version="2.4" xmlns="http://java.sun.com/xml/ns/j2ee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd"> <welcome-file-list> <welcome-file>index.jsp</welcome-file> </welcome-file-list> <listener> <listener-class> SessionCount.SessionCounter </listener-class> </listener> <session-config> <session-timeout>1</session-timeout></session-config></web-app>test.jsp<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%><%@ page import="SessionCount.SessionCounter" %><!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"><!--<link rel="stylesheet" type="text/css" href="styles.css">--> </head> <body> 在线人数:<%=SessionCounter.getActiveSessions()%> </body></html>?