监听器

一监听器作用
主要是用来监听特定对象的创建对象或销毁，属性的变化的！是一个实现特定接口的普通java类。
Servlet中哪些对象需要监听？
request/session/servletContext
分别对应的request监听器，session相关监听器，servletContext监听器
二监听器（listener）概述
监听器接口

监听器对象创建/销毁的监听器接口
Interface ServletRequestListener 监听request对象的创建或销毁
Interface HttpSessionListener 监听session对象的创建或销毁
Interface ServletContextListener 监听servletContext对象的创建或销毁
监听对象属性的变化
Interface ServletRequestAttributeListener 监听request对象属性变化: 添加、移除、修改
Interface HttpSessionAttributeListener 监听session对象属性变化: 添加、移除、修改
Interface ServletContextAttributeListener 监听servletContext对象属性变化
session相关监听器
Interface HttpSessionBindingListener 监听对象绑定到session上的事件
Interface HttpSessionActivationListener(了解) 监听session序列化及反序列化的事件
三程序实现
1 生命周期监听
index.jsp

<%--
  Created by IntelliJ IDEA.
  User: pc
  Date: 2017/9/6
  Time: 17:48
  To change this template use File | Settings | File Templates.
--%>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>路径问题</title>
</head>
<body>
<h2>欢迎访问index页面</h2>
<%
    request.setAttribute("name","Yang");
%>
</body>
</html>

RequestListener.java

package entity;

import javax.servlet.ServletRequestEvent;
import javax.servlet.ServletRequestListener;

/**
 * Created by pc on 2017/9/12.
 */
public class RequestListener implements ServletRequestListener {
    public void requestDestroyed(ServletRequestEvent servletRequestEvent) {
       Object object = servletRequestEvent.getServletRequest().getAttribute("name");
        System.out.println("2.对象销毁"+object);
    }

    public void requestInitialized(ServletRequestEvent servletRequestEvent) {
        Object object1 = servletRequestEvent.getServletRequest().getAttribute("name");
        System.out.println("1.对象创建"+object1);
    }
}

.xml配置

<!DOCTYPE web-app PUBLIC
 "-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN"
 "http://java.sun.com/dtd/web-app_2_3.dtd" >
<web-app>
  <display-name>Archetype Created Web Application</display-name>
<listener>
  <listener-class>entity.RequestListener</listener-class>
</listener>
</web-app>

测试结果

2 属性监听（监听session变化）
index.jsp

<%--
  Created by IntelliJ IDEA.
  User: pc
  Date: 2017/9/6
  Time: 17:48
  To change this template use File | Settings | File Templates.
--%>
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>路径问题</title>
</head>
<body>
<h2>欢迎访问index页面</h2>
<%
    session.setAttribute("name","YY");
    session.removeAttribute("name");
%>
</body>
</html>

SessionAttributeListener.java

package entity;
import javax.servlet.http.*;

/**
 * Created by pc on 2017/9/12.
 */
public class SessionAttributeListener implements HttpSessionAttributeListener {

    public void attributeAdded(HttpSessionBindingEvent httpSessionBindingEvent) {
        System.out.println("创建session");
    }

    public void attributeRemoved(HttpSessionBindingEvent httpSessionBindingEvent) {
        System.out.println("移除session");
    }

    public void attributeReplaced(HttpSessionBindingEvent httpSessionBindingEvent) {
        System.out.println("修改session");
    }
}

.xml配置

<!DOCTYPE web-app PUBLIC
 "-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN"
 "http://java.sun.com/dtd/web-app_2_3.dtd" >

<web-app>
  <display-name>Archetype Created Web Application</display-name>
  <listener>
    <listener-class>entity.SessionAttributeListener</listener-class>
  </listener>
</web-app>

运行结果

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