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监听器可以使你的应用对某些事件作出反应。
package com.yyq.listener; 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; import java.util.LinkedList; import java.util.List; /** * Created by gao on 16-4-14. */ public class OnlineListener implements ServletContextListener, HttpSessionAttributeListener, HttpSessionListener { private ServletContext application = null; //往会话中添加属性时回调的方法 public void attributeAdded(HttpSessionBindingEvent httpSessionBindingEvent) { //取得用户名列表 List<String> online = (List<String>) this.application.getAttribute("online"); if ("username".equals(httpSessionBindingEvent.getName())) { //将当前用户名添加到列表中 online.add((String) httpSessionBindingEvent.getValue()); } //将添加后的列表重新设置到application属性中 this.application.setAttribute("online", online); } //应用上下文初始化会回调的方法 @Override public void contextInitialized(ServletContextEvent servletContextEvent) { //初始化一个application对象 this.application = servletContextEvent.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>()); } //会话销毁时会回调的方法 @Override public void sessionDestroyed(HttpSessionEvent httpSessionEvent) { //取得用户名列表 List<String> online = (List<String>) this.application.getAttribute("online"); //取得当前用户名 String username = (String) httpSessionEvent.getSession().getAttribute("username"); //将此用户名从列表中删除 online.remove(username); //将删除后的列表重新设置到application属性中 this.application.setAttribute("online", online); } //以下方法用空实现 @Override public void attributeRemoved(HttpSessionBindingEvent httpSessionBindingEvent) { } @Override public void attributeReplaced(HttpSessionBindingEvent httpSessionBindingEvent) { } @Override public void sessionCreated(HttpSessionEvent httpSessionEvent) { } @Override public void contextDestroyed(ServletContextEvent servletContextEvent) { } }
2、创建用户登录的Servlet类
package com.yyq.listener; import javax.servlet.ServletException; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; import java.io.IOException; import java.io.PrintWriter; import java.util.List; /** * Created by gao on 16-4-14. */ public class LoginServlet extends HttpServlet { @Override protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { //设置响应内容类型 req.setCharacterEncoding("utf-8"); //获取请求参数中的用户名 String username = req.getParameter("username"); //往Session中添加属性 //会触发HttpSessionAttributeListener中的attributeAdded方法 if (username != null && !username.equals("")) { req.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"); resp.setContentType("text/html;charset=utf-8"); PrintWriter out = resp.getWriter(); out.println("<html>"); out.println("<head><title>用户列表</title></head>"); out.println("<body>"); out.println("当前用户是：" + username); out.println("<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：要对连接URL进行自动重写处理 out.println("<hr/><a href=\"" + resp.encodeURL("logout") + "\">注销</a>"); out.println("</body>"); out.println("</html>"); out.flush(); out.close(); } @Override protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { this.doPost(req, resp); } @Override public void destroy() { //空 } @Override public void init() throws ServletException { //空 } }
package com.yyq.listener; import javax.servlet.ServletException; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse; import java.io.IOException; import java.io.PrintWriter; import java.util.List; /** * Created by gao on 16-4-14. */ public class LogoutServlet extends HttpServlet { @Override protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { this.doPost(req, resp); } @Override protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { //设置响应内容类型 req.setCharacterEncoding("utf-8"); //销毁会话，会触发SessionLinstener中的sessionDestroyed方法 req.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"); resp.setContentType("text/html;charset=utf-8"); PrintWriter out = resp.getWriter(); out.println("<html>"); out.println("<head><title>用户列表</title></head>"); out.println("<body>"); 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.jsp\">主页</hr>"); out.println("</body>"); out.println("</html>"); out.flush(); out.close(); } @Override public void destroy() { //空 } @Override public void init() throws ServletException { //空 } }
4、web.xml配置监听器、LoginServlet和LogoutServlet
<?xml version="1.0" encoding="UTF-8"?> <web-app xmlns="http://java.sun.com/xml/ns/javaee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd" version="3.0"> <!--注册一个监听器--> <listener> <!--指定监听器实现类的全限定名--> <listener-class>com.yyq.listener.OnlineListener</listener-class> </listener> <!--配置LoginServlet--> <servlet> <servlet-name>loginservlet</servlet-name> <servlet-class>com.yyq.listener.LoginServlet</servlet-class> </servlet> <servlet-mapping> <servlet-name>loginservlet</servlet-name> <url-pattern>/login</url-pattern> </servlet-mapping> <!--配置LogoutServlet--> <servlet> <servlet-name>logoutservlet</servlet-name> <servlet-class>com.yyq.listener.LogoutServlet</servlet-class> </servlet> <servlet-mapping> <servlet-name>logoutservlet</servlet-name> <url-pattern>/logout</url-pattern> </servlet-mapping> </web-app>
5、登录页面index.jsp
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
<title></title>
</head>
<body>
<form action="login" method="post">
用户名：<input type="text" name="username">
<input type="submit" value="登录">
<br />
<br />
</form>
</body>
</html>
6、启动Tomcat，输入：http://localhost:8080/index.jsp
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原文地址：http://www.cnblogs.com/yangyquin/p/5427260.html
