1. 监听器

1.  什么是监听器

• 监听器也叫Listener，是Servlet的监听器，它可以监听客户端的请求、服务端的操作等。通过监听器，可以自动激发一些操作，比如监听在线的用户的数量。
• 所谓的监听器，是指对整个web环境的监听，当被见识的对象发生变化时，立即调用相应的方法进行处理。

2.监听器最常见的应用场景：

• Java SE GUI编程；
• Android 手机开发编程。

3. 监听器术语

• 事件源：被监听对象。(目标对象)；
• 监听器对象：用于监听"事件源"的对象；
• 注册(绑定)：将"监听器对象"注册给"事件源"。当"事件源"发生某些行为时，监听对象将被执行；
• 事件：事件源行为的称呼；
• 事件对象：在“监听器对象”中获得“事件源”。

4. 监听器的编写流程

• 编写监听器实现类，需要实现指定的接口；
• 在web.xml文件配置监听器(部分监听不需要配置);

5. JavaEE规范规定了8个监听器接口用于监听不同的web对象

• 监听域对象创建与销毁
• 监听域对象属性变更(添加、替换、删除)
• 监听session作用域特殊Javabean

6. 域对象监听器

• ServletRequestListener
• HttpSessionListener
• ServletContext

7. 域对象属性监听器（

4大域对象：ServletContext 、Request、Session、PageContext

7.1 作用域属性操作

　　setAttribute(k,v)　　　　getAttribute(k)　　　　removeAttribute(k)

7.2 域对象属性变更监听器

• ServletRequestAttributeListener
• HttpSessionAttributeListener
• ServletContextAttributeListener

　　7.3 特殊的Javabean监听器(Session作用域）

• HttpSessionBindingListener
• HttpSessionActivationListener（钝化和活化)

前面六个监听器需要在web.xml中配置，最后的两个监听器不需要再web.xml中配置

8. 实例

　　8.1 ServletRequestListener

　　监听request对象创建或销毁，其中request对象的特点如下：

• 类型：Javax.servlet.http.HttpServletRequest
• 描述：来自客户端的请求经Servlet容器处理后，由request对象进行封装。注：客户端和服务器的一次通信就是一次请求（发送请求或得到相应）。
• 作用域：request。说明，这次请求结束后，它的生命周期 就结束了。
• 举例：
  • 创建ServletRequestListener实现类MyServletRequestListener；

package com.eagle.listener;

import javax.servlet.ServletRequestEvent;
import javax.servlet.ServletRequestListener;

public class MyServletRequestListener implements ServletRequestListener {

    public MyServletRequestListener() {
        super();
        System.out.println("request对象的构造方法");
    }

    public void requestInitialized(ServletRequestEvent sre) {
        System.out.println("request创建了！");
    }

    public void requestDestroyed(ServletRequestEvent sre) {
        System.out.println("request对象销毁了！");
    }

}

• • 在web.xml文件配置监听器。

  <listener>
    <listener-class>com.eagle.listener.MyServletRequestListener</listener-class>
  </listener>

服务器启动就先构造了request对象，然后每个页面请求都会触发

MyServletRequestListener.requestInitialized(ServletRequestEvent sre)方法和

MyServletRequestListener.requestDestroyed(ServletRequestEvent sre)方法

　　8.2 HttpSessionListener

• 类型：Javax.servlet.http.HttpSession
• 描述：表示一个会话，用来保存用户信息，以便跟踪每个用户的状态。（不要用来保存业务数据，request）
• 定义：是指在一段时间内客户端和服务器之间的一连串的相关的交互过程。
• 作用域：session。
• 举例
  • 创建MyHttpSessionListener

package com.eagle.listener;

import javax.servlet.http.HttpSessionEvent;
import javax.servlet.http.HttpSessionListener;

public class MyHttpSessionListener implements HttpSessionListener {

    public MyHttpSessionListener() {

        super();
        System.out.println("session对象构造方法");
    }

    public void sessionCreated(HttpSessionEvent arg0) {
        System.out.println("创建session对象");
    }

    public void sessionDestroyed(HttpSessionEvent arg0) {
        System.out.println("销毁session对象");
    }

}

• • 创建servlet CreateSession

package com.eagle.web;

import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

public class CreateSession extends HttpServlet {

    private static final long serialVersionUID = 1L;

    public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        request.setCharacterEncoding("utf-8");
        response.setContentType("text/html;charset=utf-8"); 
        HttpSession httpSession = request.getSession();
        httpSession.setMaxInactiveInterval(10);
        String id = httpSession.getId();
        if(httpSession.isNew()) {
            httpSession.setAttribute("name", "狼");        
            String name = (String) httpSession.getAttribute("name");
            response.getWriter().print("恭喜session创建成功，sessionid="+id+";name="+name);
        }else {
            String name = (String) httpSession.getAttribute("name");
            response.getWriter().print("服务器已经存在session了，sessionid="+id+";name="+name);
        }
    }

    public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request, response);
    }
}

• • web.xml文件中配置监听器

  <listener>
    <listener-class>com.eagle.listener.MyHttpSessionListener</listener-class>
  </listener>

 

　　8.3 servletContextListener

• 类型：javax.servlet.ServletContext
• 描述：从servlet配置对象获得的servlet上下文
• 作用域：application
• 这个对象的生命周期是最长的。服务器启动的时候就会创建application对象。从服务器存在到服务器终止，都一直存在，且只保留一个对象，所有用户共享一个application。不是很常用。
• 举例：
  • 创建MyServletContextListener监听器

package com.eagle.listener;

import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;

public class MyServletContextListener implements ServletContextListener {

    public MyServletContextListener() {
        System.out.println("ServletContext对象构建！");
    }

    public void contextInitialized(ServletContextEvent sce) {
        System.out.println("ServletContext对象初始化！");
    }
    
    public void contextDestroyed(ServletContextEvent sce) {
        System.out.println("ServletContext对象销毁！");
    }
}

• • 在web.xml文件中配置监听器

 <listener>
      <listener-class>com.eagle.listener.MyServletContextListener</listener-class>
  </listener>

 ./shutdown.sh 　　//停止Apache服务

　　

　　8.4 ServletRequestAttributeListener

• 编写servlet  　　ServletRequestAttribute；

package com.eagle.web;

import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class ServletRequestAttribute extends HttpServlet {

    private static final long serialVersionUID = 1L;

    public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // 编码
        request.setCharacterEncoding("utf-8");
        response.setContentType("text/html;charset=utf-8");
        request.setAttribute("name", "first");
        String name = (String) request.getAttribute("name");
        response.getWriter().print("name="+name);
        request.removeAttribute("name");
    }

    public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request, response);
    }
}

• 编写ServletRequestAttributeListener；

package com.eagle.listener;

import javax.servlet.ServletRequestAttributeEvent;
import javax.servlet.ServletRequestAttributeListener;

public class MyRequestAttribute implements ServletRequestAttributeListener {

    public MyRequestAttribute() {
        super();
        System.out.println("requestattribute初始化！");
    }

    public void attributeAdded(ServletRequestAttributeEvent sre) {
        System.out.println("request对象添加了新属性！");
    }

    public void attributeRemoved(ServletRequestAttributeEvent sre) {
        System.out.println("reqeust对象移除了属性！");
    }

    public void attributeReplaced(ServletRequestAttributeEvent sre) {
        System.out.println("request修改了属性！");
    }

}

• 配置web.xml。

<listener>
    <listener-class>com.eagle.listener.MyRequestAttribute</listener-class>
  </listener>

　　8.5 HttpSessionAttributeListener

• 编写servlet

package com.eagle.web;

import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

public class SessionAttribute extends HttpServlet {

    private static final long serialVersionUID = 1L;

    public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        HttpSession session = request.getSession();
        session.setAttribute("name", "first");
        session.setAttribute("name", "second");
        session.getAttribute("name");
        session.removeAttribute("name");
        response.getWriter().write("hello haohao...");
    }

    public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request, response);
    }
}

• 编写HttpSessionAttributeListener

package com.eagle.listener;

import javax.servlet.http.HttpSessionAttributeListener;
import javax.servlet.http.HttpSessionBindingEvent;

public class MySessionAttributeListener implements HttpSessionAttributeListener {

    public MySessionAttributeListener() {
        super();
        System.out.println("sessionAttributeListener初始化！");
    }

    public void attributeAdded(HttpSessionBindingEvent hbe) {
        System.out.println("sessionAttributeListener 添加了新属性！");
    }

    public void attributeRemoved(HttpSessionBindingEvent hbe) {
        System.out.println("sessionAttributeListener 移除了属性！");
    }

    public void attributeReplaced(HttpSessionBindingEvent hbe) {
        System.out.println("sessionAttributeListener 修改了属性！");
    }

}

• 配置web.xml

  <listener>
    <listener-class>com.eagle.listener.MySessionAttributeListener</listener-class>
  </listener>

注：设置session引发request属性修改监听器

　　8.6 ServletContextAttributeListener

• 编写servlet

package com.eagle.web;

import java.io.IOException;

import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class ServletContextAttribute extends HttpServlet {

    private static final long serialVersionUID = 1L;

    public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        ServletContext servletContext = request.getServletContext();
        servletContext.setAttribute("name", "first");
        servletContext.setAttribute("name", "second");
        servletContext.removeAttribute("name");
        response.getWriter().write("hello haohao...");
    }

    public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request, response);
    }
}

• 编写ServletContextAttributeListener

package com.eagle.listener;

import javax.servlet.ServletContextAttributeEvent;
import javax.servlet.ServletContextAttributeListener;

public class MyServletContextAttributeListener implements ServletContextAttributeListener {

    public MyServletContextAttributeListener() {
        super();
        System.out.println("ServletContextAttributeListener对象构造！");
    }

    public void attributeAdded(ServletContextAttributeEvent sce) {
        System.out.println("ServletContextAttributeListener对象添加属性！");
    }

    public void attributeReplaced(ServletContextAttributeEvent sce) {
        System.out.println("ServletContextAttributeListener对象修改属性！");
    }

    public void attributeRemoved(ServletContextAttributeEvent sce) {
        System.out.println("ServletContextAttributeListener对象移除属性！");
    }
}

• 配置web.xml

  <listener>
    <listener-class>com.eagle.listener.MyServletContextAttributeListener</listener-class>
  </listener>

　　

　　8.7 HttpSessionBindingListener

• 编写servlet

package com.eagle.web;

import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

import com.eagle.listener.MyHttpSessionBindingListener;

public class ServletHttpSessionBindingListener extends HttpServlet {

    private static final long serialVersionUID = 1L;

    public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        MyHttpSessionBindingListener ml = new MyHttpSessionBindingListener();
        HttpSession hs = request.getSession();
        hs.setAttribute("ml", ml);
        hs.invalidate();
        response.getWriter().write("hello haohao...");
    }

    public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doGet(request, response);
    }
}

• 编写HttpSessionBindingListener

package com.eagle.listener;

import javax.servlet.http.HttpSessionBindingEvent;
import javax.servlet.http.HttpSessionBindingListener;

public class MyHttpSessionBindingListener implements HttpSessionBindingListener {

    public MyHttpSessionBindingListener() {
        super();
        System.out.println("httpSessionBindingListening对象初始化");
    }

    public void valueBound(HttpSessionBindingEvent hse) {
        System.out.println("httpSessionBindingListening对象绑定！");
    }

    public void valueUnbound(HttpSessionBindingEvent hse) {
        System.out.println("httpSessionBindingListening对象解绑！");
    }

}

　　8.8 HttpSessionActivationListener

 　　参照：https://www.cnblogs.com/eaglesour/p/9474540.html