原文地址
Listener採用了觀察者模式（24種模式之一），Listener是servlet的監聽器，他可以監聽客戶端的請求、服務器端的操作等， 通過監聽器，可以自動激發一些操作。比如：監聽在線用戶數量
當增加一個HttpSession時，就會激發sessinCreated(HttpSessionEvent sce)方法，這樣就可以給在線人數+1了。
常見的監聽器接口：
ServletContextAttributeListener 監聽對servletContext屬性的操作，比如刪除、增加、修改屬性等
ServletContextListener 監聽ServletContext，當創建ServletContext時，激發contextInitialized(ServletContextEvent sce)方法，當銷燬ServletContext時，激發ContextDestory(ServletContextEvent sce)方法
實例：
首先配置web.xml
<!--servlet 監聽器 start-->
<listener>
<listener-class>com.listener.MyServletContextListener</listener-class>
</listener>
<listener>
<listener-class>com.listener.MyServletContextAttributeListener</listener-class>
</listener>
<!-- servlet 監聽器 end -->
web.xml2.ServletContextListener接口的調用
/**
* 在xml中配置監聽器
* ServletContextListener 堅挺servletContext，當創建servletContext時，激發ContextInitialized(ServletContextEvent sce)方法
* 當銷燬servletContext時，激發contextDestoryed(ServletContextEvent sce)方法
*/
package com.listener;
import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
public class MyServletContextListener implements ServletContextListener
{
@Override
public void contextDestroyed(ServletContextEvent arg0)
{
System.out.println(arg0.toString());
}
@Override
public void contextInitialized(ServletContextEvent arg0)
{
System.out.println(arg0.toString());
}
}
MyServletContextListener3.ServletContextAttributeListener接口的調用
package com.listener;
import javax.servlet.ServletContextAttributeEvent;
import javax.servlet.ServletContextAttributeListener;
public class MyServletContextAttributeListener implements ServletContextAttributeListener
{
@Override
public void attributeAdded(ServletContextAttributeEvent arg0)
{
System.out.println("attributeAdd ");
System.out.println(arg0.getName() + ":" + arg0.getValue());
}
@Override
public void attributeRemoved(ServletContextAttributeEvent arg0)
{
System.out.println("attributeRemoved");
System.out.println(arg0.getName() + ":" + arg0.getValue());//value值將顯示之前的 值
}
@Override
public void attributeReplaced(ServletContextAttributeEvent arg0)
{
System.out.println("attributeRepaced");
System.out.println(arg0.getName() + ":" + arg0.getValue());//value值將顯示之前的 值
}
}
MyServletContextAttributeListener爲了方便，沒有用servlet舉例，直接寫的jsp
<%@ page language="java" contentType="text/html; charset=UTF-8"
pageEncoding="UTF-8"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>Insert title here</title>
</head>
<body>
<%
application.setAttribute("name1","value1");
application.setAttribute("name2","value2");
application.setAttribute("name1","value3");
application.setAttribute("name2","value4");
%>
<a href="listener2.jsp">next</a>
</body>
</html>
listener1.jsp<%@ page language="java" contentType="text/html; charset=UTF-8"
pageEncoding="UTF-8"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
<title>Insert title here</title>
</head>
<body>
<%
application.removeAttribute("name1");
%>
</body>
</html>
listener2.jsp