一、学习目标
1、Listener监听器
2、Listener监听器作用
3、Listener监听器的创建与销毁
二、重点知识
为了获取Filter程序在web.xml文件中的配置信息,Servlet API提供了一个FilterConfig接口,该接口封装了Filter程序在web.xml中的所有注册信息,并且提供了一系列获取这些配置信息的方法 。
1、Listener监听器
Filter和Listener是Servlet规范中的两个高级特性,不同于Servlet,他们不用于处理客户端请求。Filter是用于对request、response对象进行修改,Listener用于对context、session、request事件进行监听。善用Servlet规范中的这两个高级特性能够轻松地解决一些特殊问题。
2、Listener监听器作用
是监听应用启动和关闭,需要实现ServletContextListener接口;
是监听session的创建与销毁;属性的新增、移除和更改,需要实现HttpSessionListener和HttpSessionAttributeListener接口。
2.1 是HttpSessionBindingListener接口
第一种:实现ServletContextListener接口,接口有两个方法:
(1)public void contextInitialized(ServletContextEvent sce); //应用启动事件
(2)public void contextDestroyed(ServletContextEvent sce); //应用停止事件
第二种:实现HttpSessionListener和HttpSessionAttributeListener接口
1)HttpSessionListener接口有两个方法:
(1)public void sessionCreated(HttpSessionEvent httpsessionevent);
(2)public void sessionDestroyed(HttpSessionEvent httpsessionevent);
2)HttpSessionAttributeListener接口有三个方法:
(1)public void attributeAdded(HttpSessionBindingEvent httpsessionbindingevent);
(2)public void attributeRemoved(HttpSessionBindingEvent httpsessionbindingevent);
(3)public void attributeReplaced(HttpSessionBindingEvent httpsessionbindingevent);
3、 Listener监听三个域对象创建与销毁
3.1 监听ServletContext域对象的创建与销毁:实现ServletContextListener接口
ServletContext域对象的生命周期:
创建:启动服务器时创建
销毁:关闭服务器或者从服务器移除项目
@WebListener
public class MyServletContextListener implements ServletContextListener{
@Override
public void contextInitialized(ServletContextEvent arg0) {
System.out.println("初始化");
}
@Override
public void contextDestroyed(ServletContextEvent arg0) {
System.out.println("销毁了");
}
}
作用:
利用ServletContextListener监听器在创建ServletContext域对象时完成一些想要初始化的工作或者执行自定义任务调度。
3.2 监听ServletRequest域对象的创建与销毁:实现ServletRequestListener接口。
ServletRequest域对象的生命周期:
创建:访问服务器任何资源都会发送请求(ServletRequest)出现,访问.html和.jsp和.servlet都会创建请求。
销毁:服务器已经对该次请求做出了响应。
@WebListener
public class MyServletRequestListener implements ServletRequestListener{
@Override
public void requestDestroyed(ServletRequestEvent arg0) {
System.out.println("ServletRequest销毁了");
}
@Override
public void requestInitialized(ServletRequestEvent arg0) {
System.out.println("ServletRequest创建了");
}
}
3.3 监听HttpSession域对象的创建与销毁:实现HttpSessionListener接口:
HttpSession域对象的生命周期:
创建:只要调用了getSession()方法就会创建,一次会话只会创建一次,
销毁:1.超时(默认为30分钟)2.非正常关闭,销毁3.正常关闭服务器(序列化)
@WebListener
public class MyHttpSessionListener implements HttpSessionListener{
@Override
public void sessionCreated(HttpSessionEvent arg0) {
System.out.println("HttpSession创建了");
}
@Override
public void sessionDestroyed(HttpSessionEvent arg0) {
System.out.println("HTTPSession销毁了");
}
}
作用:每位用户登录网站时都会创建一个HTTPSession对象,利用这个统计在线人数。
- Listener监听三个域对象属性状态的改变
监听HttpSession属性的改变:实现HttpSessionAttributeListener接口。
监听ServletContext属性的改变:实现ServletContextAttribute接口。
监听ServletRequest属性的改变:实现ServletRequestAttribute接口。
三、实际运用
1.创建监听器:
在chepter11创建一个chapter11.listener包,编写一个mylistener类,用来实现若干个监听器接口。
代码示例:
mylistener.java
package chapter11.listener;
import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
import javax.servlet.ServletRequestEvent;
import javax.servlet.ServletRequestListener;
import javax.servlet.annotation.WebListener;
import javax.servlet.http.HttpSessionEvent;
import javax.servlet.http.HttpSessionListener;
/**
* Application Lifecycle Listener implementation class mylistener
*
*/
@WebListener
public class mylistener implements ServletContextListener, HttpSessionListener, ServletRequestListener {
/**
* Default constructor.
*/
/**
* @see HttpSessionListener#sessionCreated(HttpSessionEvent)
*/
public void sessionCreated(HttpSessionEvent arg0) {
// TODO Auto-generated method stub
System.out.println("servletsession被创建了");
}
/**
* @see ServletRequestListener#requestDestroyed(ServletRequestEvent)
*/
public void requestDestroyed(ServletRequestEvent arg0) {
// TODO Auto-generated method stub
System.out.println("servletrequest被销毁了");
}
/**
* @see ServletRequestListener#requestInitialized(ServletRequestEvent)
*/
public void requestInitialized(ServletRequestEvent arg0) {
// TODO Auto-generated method stub
System.out.println("servletrequest被创建了");
}
/**
* @see HttpSessionListener#sessionDestroyed(HttpSessionEvent)
*/
public void sessionDestroyed(HttpSessionEvent arg0) {
// TODO Auto-generated method stub
System.out.println("servletsession被销毁了");
}
/**
* @see ServletContextListener#contextDestroyed(ServletContextEvent)
*/
public void contextDestroyed(ServletContextEvent arg0) {
// TODO Auto-generated method stub
System.out.println("servletcontext被销毁了");
}
/**
* @see ServletContextListener#contextInitialized(ServletContextEvent)
*/
public void contextInitialized(ServletContextEvent arg0) {
// TODO Auto-generated method stub
System.out.println("servletcontext被创建了");
}
}
添加监听信息类,在web.xml文件中添加 mylistener事件监听器信息:
<listener>
<listener-class>
chapter11.listener.mylistener
</listener-class>
</listener>
运行展示:
设置监听超时信息;
在web.xml文件中设置session的超时时间为2min
<session-config>
<session-timeout>2</session-timeout>
</session-config>
运行展示:
2.创建测试页面:
在chepter11中新建一个testattribute.jsp用来观察属性事件监听器的作用。
代码示例:
<?xml version="1.0" encoding="UTF-8" ?>
<%@ page language="java" contentType="text/html; charset=UTF-8"
pageEncoding="UTF-8"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8" />
<title>Insert title here</title>
</head>
<body>
<h3>这是个测试对象属性信息监听器的页面</h3>
<%
getServletContext().setAttribute("username", "itcast");
getServletContext().setAttribute("username", "itheima");
getServletContext().removeAttribute("username");
session.setAttribute("username", "itcast");
session.setAttribute("username", "itheima");
session.removeAttribute("username");
request.setAttribute("username", "itcast");
request.setAttribute("username", "itheima");
request.removeAttribute("username");
%>
</body>
</html>
创建监听器,在chapter11.listener里创建myattributelistener.java的监听类。
代码示例:
package chapter11.listener;
import javax.servlet.ServletContextAttributeEvent;
import javax.servlet.ServletContextAttributeListener;
import javax.servlet.ServletRequestAttributeEvent;
import javax.servlet.ServletRequestAttributeListener;
import javax.servlet.annotation.WebListener;
import javax.servlet.http.HttpSessionAttributeListener;
import javax.servlet.http.HttpSessionBindingEvent;
/**
* Application Lifecycle Listener implementation class myattributelistener
*
*/
@WebListener
public class myattributelistener implements ServletContextAttributeListener, HttpSessionAttributeListener, ServletRequestAttributeListener {
/**
* Default constructor.
*/
public myattributelistener() {
// TODO Auto-generated constructor stub
}
/**
* @see ServletContextAttributeListener#attributeAdded(ServletContextAttributeEvent)
*/
public void attributeAdded(ServletContextAttributeEvent sae) {
// TODO Auto-generated method stub
String name = sae.getName();
System.out.println("servletcontext添加属性:"+ name + "="+sae.getServletContext().getAttribute(name));
}
/**
* @see ServletContextAttributeListener#attributeRemoved(ServletContextAttributeEvent)
*/
public void attributeRemoved(ServletContextAttributeEvent sae) {
// TODO Auto-generated method stub
String name = sae.getName();
System.out.println("servletcontext移除属性:"+name);
}
/**
* @see ServletRequestAttributeListener#attributeRemoved(ServletRequestAttributeEvent)
*/
public void attributeRemoved(ServletRequestAttributeEvent sra) {
// TODO Auto-generated method stub
String name = sra.getName();
System.out.println("servletrequest移除属性"+name);
}
/**
* @see ServletRequestAttributeListener#attributeAdded(ServletRequestAttributeEvent)
*/
public void attributeAdded(ServletRequestAttributeEvent sra) {
// TODO Auto-generated method stub
String name = sra.getName();
System.out.println("servletrequest添加属性:"+ name + "="+sra.getServletRequest().getAttribute(name));
}
/**
* @see ServletRequestAttributeListener#attributeReplaced(ServletRequestAttributeEvent)
*/
public void attributeReplaced(ServletRequestAttributeEvent sra) {
// TODO Auto-generated method stub
String name = sra.getName();
System.out.println("servletrequest替换属性:"+ name + "="+sra.getServletRequest().getAttribute(name));
}
/**
* @see HttpSessionAttributeListener#attributeAdded(HttpSessionBindingEvent)
*/
public void attributeAdded(HttpSessionBindingEvent hbe) {
// TODO Auto-generated method stub
String name = hbe.getName();
System.out.println("httpsession添加属性:"+ name + "="+hbe.getSession().getAttribute(name));
}
/**
* @see HttpSessionAttributeListener#attributeRemoved(HttpSessionBindingEvent)
*/
public void attributeRemoved(HttpSessionBindingEvent hbe) {
// TODO Auto-generated method stub
String name = hbe.getName();
System.out.println("httpsession移除属性:"+name);
}
/**
* @see HttpSessionAttributeListener#attributeReplaced(HttpSessionBindingEvent)
*/
public void attributeReplaced(HttpSessionBindingEvent hbe) {
// TODO Auto-generated method stub
String name = hbe.getName();
System.out.println("httpsession替换属性:"+ name + "="+hbe.getSession().getAttribute(name));
}
/**
* @see ServletContextAttributeListener#attributeReplaced(ServletContextAttributeEvent)
*/
public void attributeReplaced(ServletContextAttributeEvent sea) {
// TODO Auto-generated method stub
String name = sea.getName();
System.out.println("servletcontext替换属性:"+ name + "="+sea.getServletContext().getAttribute(name));
}
}
添加监听信息,在web.xml文件中输入一下内容:
<listener>
<listener-class>
chapter11.listener.myattributelistener
</listener-class>
</listener>
运行展示: