本文實例講述了Java基於servlet監聽器實現在線人數監控功能的方法。分享給大家供大家參考，具體如下：
1、分析：
做一個網站在線人數統計，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，然後通過HttpSessionAttributeListener監聽，當用戶登錄成功，把用戶名設置到Session中。同時將用戶名方法到ServletContext的List中，最後通過HttpSessionListener監聽，當用戶註銷會話時，講用戶名從應用上下文範圍中的List列表中刪除。
2、注意事項
測試時，需要啟動不同的瀏覽器來登陸不同的用戶，只有點擊註銷按鈕才能減少在線用戶，關閉瀏覽器不能減少在線用戶。
3、項目源代碼
（1）java代碼
OnlineListener類
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; //應用上下文初始時會回調的方法@Override public void contextInitialized(ServletContextEvent e) { //初始化一個application對象application = e.getServletContext(); //設置一個列表屬性，用於保存在線用戶名this.application.setAttribute("online", new LinkedList<String>()); } //往會話中添加屬性時的回調方法@Override public void attributeAdded(HttpSessionBindingEvent e) { //取得用戶名列表List<String> onlines = (List<String>) this.application.getAttribute("online"); if("username".equals(e.getName())){ onlines.add((String) e.getValue()); } //將添加後的列表重新設置列application屬性中. this.application.setAttribute("online", onlines); } //會話銷毀時會回調的方法@Override public void sessionDestroyed(HttpSessionEvent e) { //取得用戶名列表List<String> onlines = (List<String>) this.application.getAttribute("online"); //取得當前用戶名String username = (String) e.getSession().getAttribute("username"); //將此用戶從列表中刪除onlines.remove(username); //講刪除後的列表重新設置到application屬性中. 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類
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"); //設置響應內容類型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");System.out.println("LoginServlet" + online); response.setContentType("text/html;charset=utf-8"); PrintWriter out = response.getWriter(); out.println(""); out.println(" <title>用戶列表</title>"); out.println(" "); 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=/"" + response.encodeURL("logoutListener") + "/">註銷</a>"); out.println(" "); out.println(""); out.flush(); out.close(); }}LogoutServlet類
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"); //設置響應內容類型//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(""); out.println(" <title>用戶列表</title>"); out.println(" "); 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='/'index.html/''>主頁</a>"); out.println(" "); out.println(""); out.flush(); out.close(); }}（2）web.xml代碼
<?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）表現層代碼
<!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"> 用戶名：<input type="text" name="username"> <input type="submit" value="登錄"><br><br> </form> </body></html>
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