JavaWeb学习-监听器

ゞ浴缸里的玫瑰 2023-09-24 14:51 113阅读 0赞

**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 {
        @Override
        public 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");
        }
        @Override
        public 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");
        }
        @Override
        public 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);
        }
        @Override
        public 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);
        }
        @Override
        public 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

<?xml version="1.0" encoding="UTF-8"?>
    <web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
             xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
             xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd"
             version="4.0">
        
        <listener>
            <listener-class>com.msb.listener.MyRequestListener</listener-class>
        </listener>
    </web-app>

准备Servlet

@WebServlet(urlPatterns = "/myServlet3.do")
    public class MyServlet3 extends HttpServlet {
        @Override
        protected 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 {
        @Override
        public void sessionCreated(HttpSessionEvent se) {
            System.out.println("任何一个Session对象创建");
        }
        @Override
        public void sessionDestroyed(HttpSessionEvent se) {
            System.out.println("任何一个Session对象的销毁");
        }
        @Override
        public void attributeAdded(HttpSessionBindingEvent se) {
            System.out.println("任何一个Session对象中添加了数据");
        }
        @Override
        public void attributeRemoved(HttpSessionBindingEvent se) {
            System.out.println("任何一个Session对象中移除了数据");
        }
        @Override
        public 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())
         */
        @Override
        public 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
        * */
        @Override
        public 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 {
        @Override
        public void sessionWillPassivate(HttpSessionEvent se) {
            System.out.println("session即将钝化");
        }
        @Override
        public 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 {
        @Override
        public void contextInitialized(ServletContextEvent sce) {
            System.out.println("ServletContext创建并初始化了");
        }
        @Override
        public void contextDestroyed(ServletContextEvent sce) {
            System.out.println("ServletContext销毁了");
        }
        @Override
        public void attributeAdded(ServletContextAttributeEvent scae) {
            System.out.println("ServletContext增加了数据");
        }
        @Override
        public void attributeRemoved(ServletContextAttributeEvent scae) {
            System.out.println("ServletContext删除了数据");
        }
        @Override
        public 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");
        @Override
        public void requestDestroyed(ServletRequestEvent sre) {
        }
        @Override
        public 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  {
        @Override
        public 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);
            }
        }
        @Override
        public 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 {
        @Override
        protected 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" %>
    <html>
      <head>
        <title>$Title%sSourceCode%lt;/title>
      </head>
      <body>
      当前在线人数为:${applicationScope.count}
      </body>
    </html>

**3.项目重启免登录**

Session序列化和反序列化

> 1、序列化与反序列
> 
> 把对象转化为字节序列的过程称为序列化（保存到硬盘，持久化）
> 
> 把字节序列转化为对象的过程称为反序列化（存放于内存）
> 
> 2、序列化的用途
> 
> 把对象的字节序列永久保存到硬盘上，通常放到一个文件中。
> 
> 把网络传输的对象通过字节序列化，方便传输本节作业

3、实现步骤

要想实现序列化和反序列化需要手动配置

A、新建文件如图所示：

![d401617cad9549a0a62bc5e49718ec99.png][]

B、 Context.xml中文件如下

<?xml version="1.0" encoding="UTF-8"?>
    
      <Context>
    
           <Manager className="org.apache.catalina.session.PersistentManager">
    
             <Store className="org.apache.catalina.session.FileStore" directory="d:/session"/>
    
          </Manager>
    
      </Context>

C、注意实体类必须实现serializable 接口

**开发过程**

1 准备实体类

package com.msb.pojo;
    import java.io.Serializable;
    /**
     * @Author: Ma HaiYang
     * @Description: MircoMessage:Mark_7001
     */
    public class User  implements Serializable {
        private String username;
        private String pwd;

2.开发登录信息输入页面

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
    <html>
      <head>
        <title>$Title%sSourceCode%lt;/title>
      </head>
      <body>
      <form action="loginController.do" method="post">
        用户名:<input type="text" name="user"> <br/>
        密码:<input type="password" name="pwd"><br/>
        <input type="submit" value="提交">
      </form>
      </body>
    </html>

3开发登录信息验证Servlet

package com.msb.controller;
    import com.msb.listener.MySessionActivationListener;
    import com.msb.pojo.User;
    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("/loginController.do")
    public class LoginController extends HttpServlet {
        @Override
        protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
            String username = req.getParameter("user");
            String pwd = req.getParameter("pwd");
            // user
            User user =new User(username,pwd);
            // session
            HttpSession session = req.getSession();
            session.setAttribute("user", user);
        }
    }

4 开发校验当前是否已经登录的Controller

package com.msb.controller;
    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 = "/loginCheckController.do")
    public class LoginCheckController extends HttpServlet {
        @Override
        protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
            // 判断是否登录
            HttpSession session = req.getSession();
            Object user = session.getAttribute("user");
            Object listener = session.getAttribute("listener");// 获得对应的监听器
            String message ="";
            if(null != user){
                message="您已经登录过";
            }else{
                message="您还未登录";
            }
            resp.setCharacterEncoding("UTF-8");
            resp.setContentType("text/html;charset=UTF-8");
            resp.getWriter().println(message);
        }
    }

5测试, 先登录,然后请求loginCheckController.do 校验是否登录过,然后重启项目,再起请求loginCheckController.do 校验是否登录过,发现重启后,仍然是登录过的

6监听钝化和活化

准备监听器

package com.msb.listener;
    import javax.servlet.http.HttpSessionActivationListener;
    import javax.servlet.http.HttpSessionEvent;
    import java.io.Serializable;
    /**
     * @Author: Ma HaiYang
     * @Description: MircoMessage:Mark_7001
     */
    public class MySessionActivationListener implements HttpSessionActivationListener, Serializable {
        @Override
        public void sessionWillPassivate(HttpSessionEvent se) {
            System.out.println(se.getSession().hashCode()+"即将钝化");
        }
        @Override
        public void sessionDidActivate(HttpSessionEvent se) {
            System.out.println(se.getSession().hashCode()+"已经活化");
        }
    }

登录时绑定监听器

@WebServlet("/loginController.do")
    public class LoginController extends HttpServlet {
        @Override
        protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
            String username = req.getParameter("user");
            String pwd = req.getParameter("pwd");
            // user
            User user =new User(username,pwd);
            // session
            HttpSession session = req.getSession();
            session.setAttribute("user", user);
            // 绑定监听器
            session.setAttribute("listener", new MySessionActivationListener());
        }
    }

重启项目 重复测试即可

[d401617cad9549a0a62bc5e49718ec99.png]: https://img-blog.csdnimg.cn/d401617cad9549a0a62bc5e49718ec99.png