本文实例讲述了Java基于servlet监听器实现在线人数监控功能的方法。分享给大家供大家参考，具体如下：
1、分析：
做一个网站在线人数统计，可以通过ServletContextListener监听，当Web应用上下文启动时，在ServletContext中添加一个List.用来准备存放在线的用户名，然后通过HttpSessionAttributeListener监听，当用户登录成功，把用户名设置到Session中。同时将用户名方法到ServletContext的List中，最后通过HttpSessionListener监听，当用户注销会话时，讲用户名从应用上下文范围中的List列表中删除。
2、注意事项
测试时，需要启动不同的浏览器来登陆不同的用户，只有点击注销按钮才能减少在线用户，关闭浏览器不能减少在线用户。
3、项目源代码
（1）java代码
OnlineListener类
package com.smalle.listener;import java.util.LinkedList;import java.util.List;import javax.servlet.ServletContext;import javax.servlet.ServletContextEvent;import javax.servlet.ServletContextListener;import javax.servlet.http.HttpSessionAttributeListener;import javax.servlet.http.HttpSessionBindingEvent;import javax.servlet.http.HttpSessionEvent;import javax.servlet.http.HttpSessionListener;public class OnlineListener implements ServletContextListener, HttpSessionAttributeListener, HttpSessionListener { private ServletContext application = null; //应用上下文初始时会回调的方法 @Override public void contextInitialized(ServletContextEvent e) { //初始化一个application对象 application = e.getServletContext(); //设置一个列表属性，用于保存在线用户名 this.application.setAttribute("online", new LinkedList<String>()); } //往会话中添加属性时的回调方法 @Override public void attributeAdded(HttpSessionBindingEvent e) { //取得用户名列表 List<String> onlines = (List<String>) this.application.getAttribute("online"); if("username".equals(e.getName())){ onlines.add((String) e.getValue()); } //将添加后的列表重新设置列application属性中. this.application.setAttribute("online", onlines); } //会话销毁时会回调的方法 @Override public void sessionDestroyed(HttpSessionEvent e) { //取得用户名列表 List<String> onlines = (List<String>) this.application.getAttribute("online"); //取得当前用户名 String username = (String) e.getSession().getAttribute("username"); //将此用户从列表中删除 onlines.remove(username); //讲删除后的列表重新设置到application属性中. this.application.setAttribute("online", onlines); } public void sessionCreated(HttpSessionEvent e) {} public void attributeRemoved(HttpSessionBindingEvent e) {} public void attributeReplaced(HttpSessionBindingEvent e) {} public void contextDestroyed(ServletContextEvent e) {}}LoginServlet类
package com.smalle.listener;import java.io.IOException;import java.io.PrintWriter;import java.util.List;import javax.servlet.ServletException;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;public class LoginServlet extends HttpServlet { private static final long serialVersionUID = 1L; public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { this.doPost(request, response); } public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { request.setCharacterEncoding("utf-8"); //设置响应内容类型 String username= request.getParameter("username"); //获取请求参数中的用户名 //往session中添加属性,会触发HttpSessionAttributeListener中的attributeAdded方法 if(username != null && !username.equals("")) { request.getSession().setAttribute("username",username); } //从应用上下文中获取在线用户名列表 List<String> online = (List<String>)getServletContext().getAttribute("online");System.out.println("LoginServlet" + online); response.setContentType("text/html;charset=utf-8"); PrintWriter out = response.getWriter(); out.println(""); out.println(" <title>用户列表</title>"); out.println(" "); out.println("当前用户是：" + username); out.print(" <hr><h3>在线用户列表</h3>"); int size = online == null ? 0 : online.size(); for (int i = 0; i < size; i++) { if(i > 0){ out.println("<br>"); } out.println(i + 1 + "." + online.get(i)); } //注意: 要对链接URL进行自动重写处理 out.println("<hr/><a href=/"" + response.encodeURL("logoutListener") + "/">注销</a>"); out.println(" "); out.println(""); out.flush(); out.close(); }}LogoutServlet类
package com.smalle.listener;import java.io.IOException;import java.io.PrintWriter;import java.util.List;import javax.servlet.ServletException;import javax.servlet.http.HttpServlet;import javax.servlet.http.HttpServletRequest;import javax.servlet.http.HttpServletResponse;public class LogoutServlet 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 { request.setCharacterEncoding("utf-8"); //设置响应内容类型 //销毁会话,会触发SessionLinstener中的sessionDestroyed方法 request.getSession().invalidate(); //从应用上下文中获取在线用户名列表 List<String> online = (List<String>)getServletContext().getAttribute("online"); response.setContentType("text/html;charset=utf-8"); PrintWriter out = response.getWriter(); out.println(""); out.println(" <title>用户列表</title>"); out.println(" "); out.print(" <h3>在线用户列表</h3>"); int size = online == null ? 0 : online.size(); for (int i = 0; i < size; i++) { if(i > 0){ out.println("<br>"); } out.println(i + 1 + "." + online.get(i)); } out.println("<hr><a href='/'index.html/''>主页</a>"); out.println(" "); out.println(""); out.flush(); out.close(); }}（2）web.xml代码
<?xml version="1.0" encoding="UTF-8"?><web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://java.sun.com/xml/ns/javaee" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd" id="WebApp_ID" version="3.0"> <display-name>testServlet</display-name> <listener> <listener-class>com.smalle.listener.OnlineListener</listener-class> </listener> <servlet> <servlet-name>LoginServlet</servlet-name> <servlet-class>com.smalle.listener.LoginServlet</servlet-class> </servlet> <servlet-mapping> <servlet-name>LoginServlet</servlet-name> <url-pattern>/loginListener</url-pattern> </servlet-mapping> <servlet> <servlet-name>LogoutServlet</servlet-name> <servlet-class>com.smalle.listener.LogoutServlet</servlet-class> </servlet> <servlet-mapping> <servlet-name>LogoutServlet</servlet-name> <url-pattern>/logoutListener</url-pattern> </servlet-mapping> <welcome-file-list> <welcome-file>index.jsp</welcome-file> </welcome-file-list></web-app>
（3）表现层代码
<!DOCTYPE html><html> <head> <title>index.html</title> <meta name="content-type" content="text/html; charset=UTF-8"> </head> <body> <form action="loginListener" method="post"> 用户名：<input type="text" name="username"> <input type="submit" value="登录"><br><br> </form> </body></html>
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