一、Listener
- Listener是Servlet的监听器
- 监听客户端的请求和服务器端的操作
- 通过实现Listener接口的类可以在特定事件（Event）发生时，自动激发一些操作
二、HttpSessionBindingListener
当一个实现了该接口的对象被捆绑到session中或者从session中被解放的时候启用此监听。
关键点：
- 创建类实现HttpSessionBindingListener接口
- valueBound()
- valueUnbound()
- 不需要在web.xml中配置监听器
- 监听范围：一对一
比如User这个类实现了该接口，则当这个对象被捆绑到session或者从session中被解除捆绑的时候启用此监听器。当被捆绑的时候，会自动进入valueBound()方法；当解除绑定的时候，会自动进入valueUnbound()方法。
三、实例
需求：统计在线人数；
实体类User：
package cn.kgc.pojo;
import javax.servlet.http.HttpSessionBindingEvent;
import javax.servlet.http.HttpSessionBindingListener;
import cn.kgc.util.Constants;
public class User implements HttpSessionBindingListener{
private int id;
private String userName;
private String password;
private String email;
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getUserName() {
return userName;
}
public void setUserName(String userName) {
this.userName = userName;
}
public String getPassword() {
return password;
}
public void setPassword(String password) {
this.password = password;
}
public String getEmail() {
return email;
}
public void setEmail(String email) {
this.email = email;
}
// 当User对象放进session时（一个用户登录），会自动进入该方法
public void valueBound(HttpSessionBindingEvent event) {
Constants.USER_COUNT ++;
}
// session失效（某个用户注销）、超时、手动地在session中把某个属性移除
public void valueUnbound(HttpSessionBindingEvent event) {
Constants.USER_COUNT --;
}
}
统计人数工具类，在该类中只有一个静态变量来统计人数：
package cn.kgc.util;
// 在线用户人数
public class Constants {
public static int USER_COUNT = 0;
}
登录页面：
<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>
<%
String path = request.getContextPath();
String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";
%>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<base href="<%=basePath%>">
<title>My JSP 'login.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>
<form action="doLogin.jsp" method="post">
用户名：<input type="text" name="userName"/>
<input type="submit" value="登录"/>
</form>
</body>
</html>
登录数据处理页面：
<%@page import="cn.kgc.pojo.User"%>
<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>
<%
String path = request.getContextPath();
String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";
%>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<base href="<%=basePath%>">
<title>My JSP 'doLogin.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>
<%
String userName = request.getParameter("userName");
if (userName == null || userName.equals("")) {
response.sendRedirect("login.jsp");
} else {
User user = new User();
user.setUserName(userName);
// 走到这一步的时候监听器就会启动了
session.setAttribute("user", user);
response.sendRedirect("online.jsp");
}
%>
</body>
</html>
登录进来页面：
<%@page import="cn.kgc.pojo.User"%>
<%@page import="cn.kgc.util.Constants"%>
<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>
<%
String path = request.getContextPath();
String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";
%>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<base href="<%=basePath%>">
<title>My JSP 'online.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>
<%
User user = null;
if (session.getAttribute("user") == null) {
response.sendRedirect("login.jsp");
} else {
user = (User)session.getAttribute("user");
%>
欢迎你，<%=user.getUserName() %>
当前在线人数为：<%=Constants.USER_COUNT %>
<a href="loginout.jsp">离开</a>
<%} %>
</body>
</html>
离开页面：
<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>
<%
String path = request.getContextPath();
String basePath = request.getScheme()+"://"+request.getServerName()+":"+request.getServerPort()+path+"/";
%>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<base href="<%=basePath%>">
<title>My JSP 'loginout.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>
<%
// 离开、注销
session.invalidate();
%>
</body>
</html>
运行结果：
当登录一个人之后，会在online页面显示当前在线人数，当换一个浏览器登录之后，登录人数改变。
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