JavaWeb_初识监听器Listener

　　监听器(listener)：对项目起到监听的作用，它能感知到包括request(请求域)，session(会话域)和applicaiton(应用程序)的初始化和属性的变化

　　监听器是Servlet规范中定义的一种特殊类，用于监听ServletContext、HttpSession和ServletRequest等域对象的创建和销毁事件，它还可以监听域对象的属性发生修改的事件，可以在事件发生前或者发生后做一些必要的处理

　　这三个监听器接口分别是ServletContextAttributeListener, HttpSessionAttributeListener 和ServletRequestAttributeListener，这三个接口中都定义了三个方法来处理被监听对象中的属性的增加，删除和替换的事件，同一个事件在这三个接口中对应的方法名称完全相同，只是接受的参数类型不同

session和application监听器

　　application监听器

package com.Gary.listener;

import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
import javax.servlet.annotation.WebListener;

@WebListener
public class ApplicationListener implements ServletContextListener{

    @Override
    public void contextDestroyed(ServletContextEvent arg0) {
        // TODO Auto-generated method stub
        System.out.println("application被销毁出来");
    }

    @Override
    public void contextInitialized(ServletContextEvent arg0) {
        // TODO Auto-generated method stub
        System.out.println("application被创建出来，这里可以做一些初始化的事情");
    }

}

ApplicationListener.java

package com.Gary.listener;

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

@WebListener
public class SessionListener implements HttpSessionListener {

    public SessionListener() {
        // TODO Auto-generated constructor stub
    }

    public void sessionCreated(HttpSessionEvent arg0)  { 
         // TODO Auto-generated method stub
         System.out.println("sessionCreated被创建");
    }

    public void sessionDestroyed(HttpSessionEvent arg0)  { 
         // TODO Auto-generated method stub
        System.out.println("sessionCreated被销毁");
    }
    
}

SessionListener.java

package com.Gary.listener;

import javax.servlet.ServletRequestEvent;
import javax.servlet.ServletRequestListener;
import javax.servlet.annotation.WebListener;


@WebListener
public class RequestListener implements ServletRequestListener {

  
    public RequestListener() {
        // TODO Auto-generated constructor stub
    }


    public void requestDestroyed(ServletRequestEvent arg0)  { 
        System.out.println("requestDestroyed");
    }


    public void requestInitialized(ServletRequestEvent arg0)  { 
         // TODO Auto-generated method stub
         System.out.println("requestInitialized");
    }
    
}

RequestListener.java

配置监听器方式

　　监听器申明

@WebListener

　　web.xml中配置监听器

      <listener>
      <listener-class></listener-class> 
      </listener>

<!--   <listener> -->
<!--           <listener-class>com.Gary.listener.SessionListener</listener-class> -->
<!--   </listener> -->

<!-- Session超过1分钟后，服务器会自动放弃Session信息    默认三十分钟-->
    <session-config>
    <session-timeout>1</session-timeout>
    </session-config>

application的属性监听器

　　当整个项目启动时，系统会往application中塞一些东西调用一次attributeAdded()，进入attributelistener.jsp后若不存在application.setAttribute("number",100)时候会再一次调用attributeAdded()，当存在application.setAttribute("number",100)时系统会调用attributeReplaced()

　　ApplicationAttributeListener用来监听application.setAttribute()和application.removeAttribute()

　　同理session和requset

<%
application.setAttribute("number",100);
// application.removeAttribute("number");
%>

<%@ page language="java" contentType="text/html; charset=ISO-8859-1"
    pageEncoding="ISO-8859-1"%>
<!DOCTYPE html>
<html>
<head>
<meta charset="ISO-8859-1">
<title>Insert title here</title>
</head>
<body>
<%
application.setAttribute("number",100);
// application.removeAttribute("number");
%>
</body>
</html>

attributelistener.jsp

package com.Gary.listener;

import javax.servlet.ServletContextAttributeEvent;
import javax.servlet.ServletContextAttributeListener;
import javax.servlet.annotation.WebListener;

@WebListener
public class ApplicationAttributeListener implements ServletContextAttributeListener{

    @Override
    public void attributeAdded(ServletContextAttributeEvent arg0) {
        // TODO Auto-generated method stub
        System.out.println("attributeAdded");
    }

    @Override
    public void attributeRemoved(ServletContextAttributeEvent arg0) {
        // TODO Auto-generated method stub
        System.out.println("attributeRemoved");
    }

    @Override
    public void attributeReplaced(ServletContextAttributeEvent arg0) {
        // TODO Auto-generated method stub
//        arg0.getName();
        System.out.println("attributeReplaced");
    }

}

ApplicationAttributeListener.java

Session实现单点登录(备份)

　　同一用户登录同一网址时会把前一个用户挤下来

public class SessionManager {
    public static Map<String,HttpSession> sessionMap = new HashMap<String,HttpSession >();
}

    public void attributeAdded(HttpSessionBindingEvent arg0)  {
        if(arg0.getName().equals("user")) {
            User u = (User) arg0.getValue();
            //判断是否有其他客户端已经登录
            if(SessionManager.sessionMap.containsKey(u.getUsername())) {
                //获得另外一个客户端的session
                HttpSession session = SessionManager.sessionMap.get(u.getUsername());
                //把登录信息从这个客户端抹除
                session.removeAttribute("user");
                //从session集合里面移除对应的用户名
                SessionManager.sessionMap.remove(u.getUsername());
            }
            
            SessionManager.sessionMap.put(u.getUsername(), arg0.getSession());
        }
    }

package com.sikiedu.util;

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

import javax.servlet.http.HttpSession;

public class SessionManager {
    public static Map<String,HttpSession> sessionMap = new HashMap<String,HttpSession >();
}

SessionManager.java

package com.sikiedu.listener;

import javax.servlet.annotation.WebListener;
import javax.servlet.http.HttpSession;
import javax.servlet.http.HttpSessionAttributeListener;
import javax.servlet.http.HttpSessionBindingEvent;

import com.sikiedu.model.User;
import com.sikiedu.util.SessionManager;

/**
 * Application Lifecycle Listener implementation class SessionAttributeListener
 *
 */
@WebListener
public class SessionAttributeListener implements HttpSessionAttributeListener {

    /**
     * Default constructor. 
     */
    public SessionAttributeListener() {
        // TODO Auto-generated constructor stub
    }

    /**
     * @see HttpSessionAttributeListener#attributeAdded(HttpSessionBindingEvent)
     */
    public void attributeAdded(HttpSessionBindingEvent arg0)  {
        if(arg0.getName().equals("user")) {
            User u = (User) arg0.getValue();
            //判断是否有其他客户端已经登录
            if(SessionManager.sessionMap.containsKey(u.getUsername())) {
                //获得另外一个客户端的session
                HttpSession session = SessionManager.sessionMap.get(u.getUsername());
                //把登录信息从这个客户端抹除
                session.removeAttribute("user");
                //从session集合里面移除对应的用户名
                SessionManager.sessionMap.remove(u.getUsername());
            }
            
            SessionManager.sessionMap.put(u.getUsername(), arg0.getSession());
        }
    }

    /**
     * @see HttpSessionAttributeListener#attributeRemoved(HttpSessionBindingEvent)
     */
    public void attributeRemoved(HttpSessionBindingEvent arg0)  { 
         // TODO Auto-generated method stub
        System.out.println("attributeRemoved");
    }

    /**
     * @see HttpSessionAttributeListener#attributeReplaced(HttpSessionBindingEvent)
     */
    public void attributeReplaced(HttpSessionBindingEvent arg0)  { 
         // TODO Auto-generated method stub
        System.out.println("sessionattributeReplaced");
    }
    
}

SessionAttributeListener.java

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原文地址：https://www.cnblogs.com/1138720556Gary/p/10323312.html