什么是监听器?

类似于前端的事件绑定,java中的监听器用于监听web应用中某些对象、信息的创建、销毁、增加，修改，删除等动作的发生，然后作出相应的响应处理。当范围对象的状态发生变化的时候，服务器自动调用监听器对象中的方法。常用于统计在线人数和在线用户，系统加载时进行信息初始化，统计网站的访问量等等。

监听器怎么分类?

按监听的对象划分

• a.ServletContext对象监听器
• b.HttpSession对象监听器
• c.ServletRequest对象监听器

按监听的事件划分

• a.对象自身的创建和销毁的监听器
• b.对象中属性的创建和消除的监听器
• c.session中的某个对象的状态变化的监听器

一共有哪些监听器?分别处理的是什么事情?

java中一共给我们提供了八个监听器接口,分别用于监听三个域对象,每个监听器都有专门监听的事件

Request 

1. ServletRequestListener           (处理request对象创建和销毁)
2. ServleRequestAttributeListener   (处理域对象中的数据添加 替换 删除)

Session

1. HttpSessionListener              (处理session对象创建和销毁)
2. HttpSessionAttributeListener      (处理session域对象中的数据添加 修改 删除)
3. HttpSessionBindingListener       (处理session对象监听器绑定和解绑定接口)
4. HttpSessionActivationListener     (处理session对象钝化和活化状态接口)

Application 

1. ServletContextListener            (处理application对象创建和销毁)
2. ServletContextAttributeListener   (处理application域对象中的数据添加 修改 删除)

监听器如何使用?

两步走使用

• 1定义监听器,根据需求实现对应接口
• 2在web.xml中配置监听器,让监听器工作

接下来我们就挨个认识一下每个监听器及内部方法的作用

一 Requet域监听器

Requet域共有两个监听器接口,分别是

ServletRequestListener 

ServleRequestAttributeListener

接下来我们就认识一些每个接口和接口中每个方法的用处

定义监听器类

package com.msb.listener;
import javax.servlet.*;
/*** @Author: Ma HaiYang* @Description: MircoMessage:Mark_7001*/
public class MyRequestListener implements ServletRequestListener, ServletRequestAttributeListener {@Overridepublic void requestDestroyed(ServletRequestEvent sre) {// 监听HttpServletRequest对象的销毁  项目中任何一个Request对象的销毁都会触发该方法的执行ServletRequest servletRequest = sre.getServletRequest();System.out.println("request"+servletRequest.hashCode()+"对象销毁了");}@Overridepublic void requestInitialized(ServletRequestEvent sre) {// 监听HttpServletRequest对象的创建并初始化 项目中任何一个Request对象的创建并初始化都会触发该方法的执行ServletRequest servletRequest = sre.getServletRequest();System.out.println("request"+servletRequest.hashCode()+"对象初始化");}@Overridepublic void attributeAdded(ServletRequestAttributeEvent srae) {// 任何一个Request对象中调用 setAttribute方法增加了数据都会触发该方法ServletRequest servletRequest = srae.getServletRequest();String name = srae.getName();Object value = srae.getValue();System.out.println("request"+servletRequest.hashCode()+"对象增加了数据:"+name+"="+value);}@Overridepublic void attributeRemoved(ServletRequestAttributeEvent srae) {// 任何一个Request对象中调用 removeAttribute方法移除了数据都会触发该方法ServletRequest servletRequest = srae.getServletRequest();String name = srae.getName();Object value = srae.getValue();System.out.println("request"+servletRequest.hashCode()+"对象删除了数据:"+name+"="+value);}@Overridepublic void attributeReplaced(ServletRequestAttributeEvent srae) {// 任何一个Request对象中调用 setAttribute方法修改了数据都会触发该方法ServletRequest servletRequest = srae.getServletRequest();String name = srae.getName();Object value = srae.getValue();Object newValue=servletRequest.getAttribute(name);System.out.println("request"+servletRequest.hashCode()+"对象增修改了数据:"+name+"="+value+"设置为:"+newValue);}
}

 配置监听器  使用web.xml 或者通过@WebListener注解都可以

com.msb.listener.MyRequestListener

准备Servlet

@WebServlet(urlPatterns = "/myServlet3.do")
public class MyServlet3 extends HttpServlet {@Overrideprotected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {req.setAttribute("name", "zhangsan");req.setAttribute("name", "lisi");req.removeAttribute("name");}
}

二  Session域监听器

Session域共有四个监听器接口,分别是

HttpSessionListener
HttpSessionAttributeListener
HttpSessionBindingListener
HttpSessionActivationListener

接下来我们就认识一些每个接口和接口中每个方法的用处

监听器代码

HttpSessionListener
HttpSessionAttributeListener

package com.msb.listener;
import javax.servlet.annotation.WebListener;
import javax.servlet.http.HttpSessionAttributeListener;
import javax.servlet.http.HttpSessionBindingEvent;
import javax.servlet.http.HttpSessionEvent;
import javax.servlet.http.HttpSessionListener;
/*** @Author: Ma HaiYang* @Description: MircoMessage:Mark_7001*/
@WebListener
public class MySessionListener implements HttpSessionListener , HttpSessionAttributeListener {@Overridepublic void sessionCreated(HttpSessionEvent se) {System.out.println("任何一个Session对象创建");}@Overridepublic void sessionDestroyed(HttpSessionEvent se) {System.out.println("任何一个Session对象的销毁");}@Overridepublic void attributeAdded(HttpSessionBindingEvent se) {System.out.println("任何一个Session对象中添加了数据");}@Overridepublic void attributeRemoved(HttpSessionBindingEvent se) {System.out.println("任何一个Session对象中移除了数据");}@Overridepublic void attributeReplaced(HttpSessionBindingEvent se) {System.out.println("任何一个Session对象中修改了数据");}
}

HttpSessionBindingListener

package com.msb.listener;
import javax.servlet.http.HttpSessionBindingEvent;
import javax.servlet.http.HttpSessionBindingListener;
/*** @Author: Ma HaiYang* @Description: MircoMessage:Mark_7001*/
/*
* 可以监听具体的某个session对象的事件的
*
* HttpSessionListener 只要在web.xml中配置或者通过@WebListener注解就可以注册监听所有的Session对象
* HttpSessionBindingListener 必须要通过setAttribute方法和某个session对象绑定之后,监听单独的某个Session对象
* */
public class MySessionBindingListener implements HttpSessionBindingListener {// 绑定方法/*session.setAttribute("mySessionBindingListener",new MySessionBindingListener())*/@Overridepublic void valueBound(HttpSessionBindingEvent event) {System.out.println("监听器和某个session对象绑定了");}// 解除绑定方法/** 当发生如下情况,会触发该方法的运行* 1 session.invalidate(); 让session不可用* 2 session到达最大不活动时间,session对象回收 ;* 3 session.removeAttribute("mySessionBindingListener");手动解除绑定* */@Overridepublic void valueUnbound(HttpSessionBindingEvent event) {}
}

HttpSessionActivationListener

package com.msb.listener;
import javax.servlet.annotation.WebListener;
import javax.servlet.http.HttpSessionActivationListener;
import javax.servlet.http.HttpSessionEvent;
/*** @Author: Ma HaiYang* @Description: MircoMessage:Mark_7001*/public class MySessionActivationListener implements HttpSessionActivationListener {@Overridepublic void sessionWillPassivate(HttpSessionEvent se) {System.out.println("session即将钝化");}@Overridepublic void sessionDidActivate(HttpSessionEvent se) {System.out.println("session活化完毕");}
}

三  Application监听器

Application域共有两个监听器接口,分别是

ServletContextListener
ServletContextAttributeListener

接下来我们就认识一些每个接口和接口中每个方法的用处

监听器代码

package com.msb.listener;
import javax.servlet.ServletContextAttributeEvent;
import javax.servlet.ServletContextAttributeListener;
import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
/*** @Author: Ma HaiYang* @Description: MircoMessage:Mark_7001*/
public class MyApplicationListener implements ServletContextListener , ServletContextAttributeListener {@Overridepublic void contextInitialized(ServletContextEvent sce) {System.out.println("ServletContext创建并初始化了");}@Overridepublic void contextDestroyed(ServletContextEvent sce) {System.out.println("ServletContext销毁了");}@Overridepublic void attributeAdded(ServletContextAttributeEvent scae) {System.out.println("ServletContext增加了数据");}@Overridepublic void attributeRemoved(ServletContextAttributeEvent scae) {System.out.println("ServletContext删除了数据");}@Overridepublic void attributeReplaced(ServletContextAttributeEvent scae) {System.out.println("ServletContext修改了数据");}
}

案例:

1.需求:记录每次请求中如下的信息并存储进入日志文件请求的来源

• 浏览器所在电脑IP
• 请求的资源 URL 
• 请求发生的时间 

监听器代码

package com.msb.listener;
import javax.servlet.ServletRequest;
import javax.servlet.ServletRequestEvent;
import javax.servlet.ServletRequestListener;
import javax.servlet.annotation.WebListener;
import javax.servlet.http.HttpServletRequest;
import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.PrintWriter;
import java.text.SimpleDateFormat;
import java.util.Date;
/*** @Author: Ma HaiYang* @Description: MircoMessage:Mark_7001*/
@WebListener
public class RequestLogListener implements ServletRequestListener {private SimpleDateFormat simpleDateFormat=new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");@Overridepublic void requestDestroyed(ServletRequestEvent sre) {}@Overridepublic void requestInitialized(ServletRequestEvent sre) {// 获得请求发出的IP// 获得请求的URL// 获得请求产生的时间HttpServletRequest request = (HttpServletRequest)sre.getServletRequest();String remoteHost = request.getRemoteHost();String requestURL = request.getRequestURL().toString();String reqquestDate = simpleDateFormat.format(new Date());// 准备输出流try {PrintWriter pw =new PrintWriter(new FileOutputStream(new File("d:/msb.txt"),true));pw.println(remoteHost+" "+requestURL+" "+reqquestDate );pw.close();} catch (FileNotFoundException e) {e.printStackTrace();}}
}

2.需求:

1当任何一个账户处于登录状态时,在线统计总数+1,离线时-1

2通过session监听器实现计数,但是在线人数要保存在Application域中

准备监听器

package com.msb.listener;
import javax.servlet.ServletContext;
import javax.servlet.annotation.WebListener;
import javax.servlet.http.HttpSession;
import javax.servlet.http.HttpSessionEvent;
import javax.servlet.http.HttpSessionListener;
/*** @Author: Ma HaiYang* @Description: MircoMessage:Mark_7001*/
@WebListener
public class OnLineNumberListener implements HttpSessionListener  {@Overridepublic void sessionCreated(HttpSessionEvent se) {// 向application域中 增加一个数字HttpSession session = se.getSession();ServletContext application = session.getServletContext();Object attribute = application.getAttribute("count");if(null == attribute){// 第一次放数据application.setAttribute("count", 1);}else{int count =(int)attribute;application.setAttribute("count", ++count);}}@Overridepublic void sessionDestroyed(HttpSessionEvent se) {// 向application域中 减少一个数字HttpSession session = se.getSession();ServletContext application = session.getServletContext();int count =(int)application.getAttribute("count");application.setAttribute("count", --count);}
}

 准备销毁监听的servlet

package com.msb.servlet;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;
import java.io.IOException;
/*** @Author: Ma HaiYang* @Description: MircoMessage:Mark_7001*/
@WebServlet(urlPatterns = "/logout.do")
public class Logout extends HttpServlet {@Overrideprotected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {HttpSession session = req.getSession();session.invalidate();}
}

index.jsp
 

<%@ page contentType="text/html;charset=UTF-8" language="java" %>