1. 监听器Listener

监听器就是监听某个对象的状态变化的组件。监听器的相关概念事件源:

被监听的对象(三个域对象request , session , servletContext(Application) )
监听器：监听事件源对象，事件源对象的状态的变化都会触发监听器。
注册监听器：将监听器与事件源进行绑定。
响应行为：监听器监听到事件源的状态变化时，所涉及的功能代码(程序员编写代码)

按照被监听的对象划分: ServletRequest(Request)域； HttpSession域(Session会话域)；ServletContext(Application应用程序域)域。按照监听的内容分：监听域对象的创建与销毁的；监听域对象的属性变化的。

image.png

三大域对象的创建与销毁的监听器

ServletContextListener(监听Application应用程序域)

监听ServletContext域的创建与销毁的监听器，Servlet域的生命周期：在服务器启动创建，服务器关闭时销毁；监听器的编写步骤：

编写一个监听器类去实现监听器接口
覆盖监听器的方法(重写)

ServletContextListener监听器的主要作用：
初始化的工作：初始化对象；初始化数据。

HttpSessionListener(监听Session会话域)

监听Httpsession域的创建与销毁的监听器。HttpSession对象的生命周期 :第一次调用request.getSession时创建；销毁有以下几种情况(服务器关闭、session过期、手动销毁)

ServletRequestListener(监听Requset请求域)

监听ServletRequest域创建与销毁的监听器。ServletRequest的生命周期 :每一次请求都会创建request ，请求结束则销毁。

监听三大域对象属性变化的域对象的通用方法

触发添加属性的监听器的方法
setAttribute(name,value)

触发修改属性的监听器的方法
removeAttribute(name)：触发删除属性的监听器的方法

2. 与session中的绑定的对象相关的监听器(对象感知监听器)

(1)即将要被绑定到session中的对象有几种状态
绑定状态：就是一个对象被放到session域中
解绑状态：就是这个对象从session域中移除了
钝化状态：是将session内存中的对象持久化(序列化)到磁盘
活化状态：就是将磁盘上的对象再次恢复到session内存中

绑定与解绑的监听器HttpSessionBindingListener(接口)

活化与钝化监听器(一定要是一个对象-java类)

META-INF与context文件名一定要一致
注：一个普通的Javabean类只要实现了Serializable(接口)，就可以实现活化和钝化；而如果既实现了Serializable(序列化，接口)又实现了HttpSessionActivationListener(接口)，那么这个java类不但可以实现活化和钝化，而且还可以知道什么时候被活化和钝化

image.png

context.xml

<?xml version="1.0" encoding="UTF-8"?>
<Context>
<!--    maxIdleSwap:session中的对象多长时间不使用就钝化,单位分钟-->
<!--    directory:钝化后的对象的文件写到磁盘的那个目录下 配置钝化的对象文件在 work/catalina/localhost/钝化文件-->
    <Manager className="org.apache.catalina.session.PersistentManager" maxIdleSwap="1">
        <Store className="org.apache.catalina.session.FileStore" directory="Javawebtest123"/>
    </Manager>
</Context>

钝化文件的路径

C:\Users\liutianyu.IntelliJIdea2019.3\system\tomcat\Tomcat_8_0_21_ListenerDemo\work\Catalina\localhost\ListenerDemo_war_exploded\自定义的文件名

3. 监听器应用举例 --- 统计网站在线人数

主页面 index.jsp文件

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
  <head>
    <title>$Title$</title>
  </head>
  <body>
  <a href="login.jsp">登录</a>
  <a href="ShowUser.do">显示在线用户</a>
  </body>
</html>

登录页面 login.jsp

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
    <title>Title</title>
</head>
<body>
<center>
    <h3>用户登录</h3>
</center>
<form action="${pageContext.request.contextPath}/LoginServlet" method="post">
    <table border="1" width="550px" cellpadding="0" cellspacing="0" align="center">
        <tr>
            <td height="35" align="center">用户名</td>
            <td>
                <input type="text" name="username">
            </td>
        </tr>
        <tr>
            <td height="35" align="center">密码</td>
            <td>
                <input type="password" name="password">
            </td>
        </tr>
        <tr>
            <td height="35" colspan="2" align="center">
                <input type="submit" value="登录">
            </td>
        </tr>
    </table>
</form>
</body>
</html>

查看当前在线用户界面 showuser.jsp

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<%@taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %>
<html>
<head>
    <title>Title</title>
</head>
<body>
<a href="logout.do">登出</a>
<hr>
<p>当前在线用户</p>

<ul>
    <c:forEach items="${users}" var="user">
        <li>${user.value}</li>
    </c:forEach>
</ul>
</body>
</html>

User.java 类

package com.lty.domain;

public class User {

    private String username;
    private String id;

    public String getUsername() {
        return username;
    }

    public void setUsername(String username) {
        this.username = username;
    }

    public String getId() {
        return id;
    }

    public void setId(String id) {
        this.id = id;
    }
}

单列模式工具类 OnlineUserMap.java

package com.lty.db;

import com.lty.domain.User;

import java.util.HashMap;
import java.util.Map;

public class OnlineUserMap {
    // 单列模式 --- 唯一的一个对象,不允许外部在使用该类创建对象
    private static OnlineUserMap onlineUserMap = new OnlineUserMap();
    private Map<String ,String> stringMap = new HashMap<>();

    private OnlineUserMap(){

    }
    public static OnlineUserMap getInstance(){
        return onlineUserMap;
    }

    public Map<String, String> getStringMap() {
        return stringMap;
    }

    public void setStringMap(Map<String, String> stringMap) {
        this.stringMap = stringMap;
    }
}

监听器

package com.lty.listener;

import com.lty.db.OnlineUserMap;
import com.lty.domain.User;

import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
import javax.servlet.annotation.WebListener;
import javax.servlet.http.HttpSessionAttributeListener;
import javax.servlet.http.HttpSessionEvent;
import javax.servlet.http.HttpSessionListener;
import javax.servlet.http.HttpSessionBindingEvent;
import java.util.HashMap;
import java.util.Map;

@WebListener()
public class OnlineListener implements
        HttpSessionListener, HttpSessionAttributeListener {

    // Public constructor is required by servlet spec
    public OnlineListener() {
    }


    // -------------------------------------------------------
    // HttpSessionListener implementation
    // -------------------------------------------------------
    public void sessionCreated(HttpSessionEvent se) {
        /* Session is created. */
    }

    public void sessionDestroyed(HttpSessionEvent se) {
        /* Session is destroyed. */

        // 将当前用户从Map中删除
        OnlineUserMap onlineUserMap = OnlineUserMap.getInstance();
        System.out.println("sessionDestroyed:" + onlineUserMap.getStringMap().get(se.getSession().getId()));
        onlineUserMap.getStringMap().remove(se.getSession().getId());

    }

    // -------------------------------------------------------
    // HttpSessionAttributeListener implementation
    // -------------------------------------------------------

    public void attributeAdded(HttpSessionBindingEvent sbe) {
      /* This method is called when an attribute 
         is added to a session.
      */
        String attr_name = sbe.getName();
        String attr_value = (String)sbe.getValue();

        System.out.println(attr_name);
        System.out.println(attr_value);
        System.out.println(sbe.getSession().getId());

        OnlineUserMap onlineUserMap = OnlineUserMap.getInstance();
        onlineUserMap.getStringMap().put(sbe.getSession().getId(),attr_value);
    }

    public void attributeRemoved(HttpSessionBindingEvent sbe) {
      /* This method is called when an attribute
         is removed from a session.
      */
    }

    public void attributeReplaced(HttpSessionBindingEvent sbe) {
      /* This method is invoked when an attribute
         is replaced in a session.
      */
    }
}

servlet
登录servlet(session的不激活时间：10s自动销毁)

package com.lty.servlet;

import com.lty.db.OnlineUserMap;

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 java.io.IOException;

@WebServlet(name = "LoginServlet" , urlPatterns = "/LoginServlet")
public class LoginServlet extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        request.setCharacterEncoding("utf-8");
        String username = request.getParameter("username");
        request.getSession().setAttribute("username",username);
        request.getSession().setMaxInactiveInterval(10);

        request.setAttribute("users", OnlineUserMap.getInstance().getStringMap());
        request.getRequestDispatcher("showuser.jsp").forward(request,response);
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

    }
}

登出servlet

package com.lty.servlet;

import com.lty.db.OnlineUserMap;

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 java.io.IOException;

@WebServlet(name = "LogoutServlet" , urlPatterns = "/logout.do")
public class LogoutServlet extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        request.getSession().invalidate();
        request.setAttribute("users", OnlineUserMap.getInstance().getStringMap());
        request.getRequestDispatcher("showuser.jsp").forward(request,response);
    }
}

查看在线用户

package com.lty.servlet;

import com.lty.db.OnlineUserMap;

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 java.io.IOException;

@WebServlet(name = "ShowServlet" , urlPatterns = "/ShowUser.do")
public class ShowServlet extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

        request.setAttribute("users", OnlineUserMap.getInstance().getStringMap());
        request.getRequestDispatcher("showuser.jsp").forward(request,response);
    }
}

image.png

JavaWeb-监听器(Listener)

JavaWeb-监听器(Listener)

1. 监听器Listener

三大域对象的创建与销毁的监听器

ServletContextListener(监听Application应用程序域)

HttpSessionListener(监听Session会话域)

ServletRequestListener(监听Requset请求域)

监听三大域对象属性变化的域对象的通用方法

2. 与session中的绑定的对象相关的监听器(对象感知监听器)

活化与钝化监听器(一定要是一个对象-java类)

3. 监听器应用举例 --- 统计网站在线人数