java中的监听器

package listener;

import javax.servlet.ServletContextAttributeEvent;
import javax.servlet.ServletContextAttributeListener;
import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;


public class MyListener implements ServletContextListener, ServletContextAttributeListener {

    //构造函数
    public MyListener() {
           System.out.println("监听器的构造函数！");
    }

  //监听上下文对象的初始化
    public void contextInitialized(ServletContextEvent sce)  { 
        System.out.println("上下文对象已经初始化！");
    }
    
    //监听上下文对象的销毁
    public void contextDestroyed(ServletContextEvent sce)  { 
        System.out.println("上下文对象已经销毁！"); 
    }
  //监听上下文对象的属性是否添加
    public void attributeAdded(ServletContextAttributeEvent scae)  { 
        System.out.println("上下文对象添加了一个属性！");
    }

    //监听上下文对象的属性是否删除
    public void attributeRemoved(ServletContextAttributeEvent scae)  { 
        System.out.println("上下文对象删除了一个属性！");
    }
   

    //监听上下文对象的属性是否修改
    public void attributeReplaced(ServletContextAttributeEvent scae)  { 
        System.out.println("上下文对象修改了属性！");
    }

    
}

<?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_2_5.xsd" id="WebApp_ID" version="2.5">
  <display-name>ch03</display-name>
  <welcome-file-list>
    <welcome-file>index.html</welcome-file>
    <welcome-file>index.htm</welcome-file>
    <welcome-file>index.jsp</welcome-file>
    <welcome-file>default.html</welcome-file>
    <welcome-file>default.htm</welcome-file>
    <welcome-file>default.jsp</welcome-file>
  </welcome-file-list>
 
  <listener>
    <listener-class>listener.MyListener</listener-class>
  </listener>

</web-app>