本文主要介紹了java使用監聽器實現一個統計網站在線人數的示例，具有一定的參考價值，有需要的朋友可以了解一下。
（1）創建一個監聽器實現類
要大致統計一個網站的在線人數，首先，可以通過ServletContextListener監聽，當Web應用上下文啟動時，在ServletContext中添加一個List，用來準備存放在線的用戶名；然後，可以通過HttpSessionAttributeListener監聽，當用戶登錄成功把用戶名設置到Session中時同時將用戶名存放到ServletContext中的List列表中；最後通過HttpSessionListener監聽，當用戶註銷會話時將用戶名從應用上下文範圍中的List列表中刪除。
所以，編寫OnLineListener類實現ServletContextListener、HttpSessionAttributeListener、HttpSessionListener接口，具體代碼如下：
package com.web.servlet; 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; //在線人數統計監聽器實現類public class OnlineListener implements ServletContextListener, HttpSessionAttributeListener, HttpSessionListener { private ServletContext application = null; public void contextDestroyed(ServletContextEvent arg0) { // TODO Auto-generated method stub } public void contextInitialized(ServletContextEvent arg0) { //初始化一個application對象this.application = arg0.getServletContext(); //設置一個列表屬性，用於保存在想用戶名this.application.setAttribute("online", new LinkedList<String>()); } //往會話中添加屬性時會回調的方法public void attributeAdded(HttpSessionBindingEvent arg0) { //取得用戶名列表List<String> online = (List<String>) this.application .getAttribute("online"); if ("username".equals(arg0.getName())) { //將當前用戶名添加到列表中online.add((String) arg0.getValue()); } //將添加後的列表重新設置到application屬性中this.application.setAttribute("online", online); } public void attributeRemoved(HttpSessionBindingEvent arg0) { // TODO Auto-generated method stub } public void attributeReplaced(HttpSessionBindingEvent arg0) { // TODO Auto-generated method stub } public void sessionCreated(HttpSessionEvent arg0) { // TODO Auto-generated method stub } //會話銷毀時會回調的方法public void sessionDestroyed(HttpSessionEvent arg0) { //取得用戶名列表List<String> online = (List<String>) this.application .getAttribute("online"); //取得當前用戶名String username = (String) arg0.getSession().getAttribute("username"); //將此用戶名從列表中刪除online.remove(username); //將刪除後的列表重新設置到application屬性中this.application.setAttribute("online", online); } } （2）在web.xml中註冊監聽器
監聽器實現好後，還需要在web.xml文件中進行註冊才能起作用，只需要在web.xml中像如下添加元素即可
<!-- 註冊一個監聽器--> <listener> <!-- 指定監聽器實現類的全限定名--> <listener-class> com.web.servlet.OnlineListener </listener-class> </listener
最後，我們創建幾個Servlet來測試這個監聽器實現的功能。
處理用戶登錄的Servlet類代碼：
package com.web.servlet; 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; //處理用戶登錄的Servlet public class LoginServlet 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");//設置相應內容類型String username= request.getParameter("username");//獲取請求參數中的用戶名//往session中添加屬性,會觸發HttpSessionAttributeListener中的attributeAdded方法if(username != null && !username.equals("")) { request.getSession().setAttribute("username",username); } //從應用上下文中獲取在線用戶名列表List<String> online = (List<String>)getServletContext().getAttribute("online"); response.setContentType("text/html;charset=utf-8"); PrintWriter out = response.getWriter(); out.println("<HTML>"); out.println(" <HEAD><TITLE>用戶列表</TITLE></HEAD>"); out.println(" <BODY>"); out.println("當前用戶是：" + username); out.print(" <hr/><h3>在線用戶列表</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: 要對鏈接URL進行自動重寫處理out.println("<hr/><a href="/" mce_href="/""" + response.encodeURL("logout") + "/">註銷</a>"); out.println(" </BODY>"); out.println("</HTML>"); out.flush(); out.close(); } }處理用戶登錄Servlet的類代碼
package com.web.servlet; import java.io.*; import java.util.List; import javax.servlet.ServletException; import javax.servlet.http.*; //處理用戶註銷會話的Servlet 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"); //銷毀會話,會觸發SessionLinstener中的sessionDestroyed方法request.getSession().invalidate(); //從應用上下文中獲取在線用戶名列表List<String> online = (List<String>)getServletContext().getAttribute("online"); response.setContentType("text/html;charset=utf-8"); PrintWriter out = response.getWriter(); out.println("<HTML>"); out.println(" <HEAD><TITLE>用戶列表</TITLE></HEAD>"); out.println(" <BODY>"); out.print(" <h3>在線用戶列表</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="/" mce_href="/""index.html/">主頁</a>"); out.println(" </BODY>"); out.println("</HTML>"); out.flush(); out.close(); } }然後創建一個index.html文件，用來供用戶登錄，代碼如下：
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> <html> <head> <meta http-equiv="content-type" content="text/html; charset=UTF-8"> <title>index.html</title> </head> <body> <form action = "login" method = "post"> 用戶名：<input type ="text" name = "username"/> <input type = "submit" value = "登錄"/><br/><br/> </form> </body> </html>
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