12、监听器

实现一个监听器的接口；（有N种）

1、编写一个监听器

实现监听器的接口

package com.jiang.listener;

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

//统计网站在线人数：统计session
public class OnlineCountListener implements HttpSessionListener {

    //创建session监听：看你的一举一动
    //一旦创建session就会触发一次这个事件！->观察者模式
    public void sessionCreated(HttpSessionEvent se) {
        ServletContext ctx = se.getSession().getServletContext();
        Integer onlineCount = (Integer) ctx.getAttribute("OnlineCount");
        if(onlineCount==null){
            onlineCount = new Integer(1);
        }else{
            int count = onlineCount.intValue();
            onlineCount = new Integer(count+1);
        }
        ctx.setAttribute("OnlineCount",onlineCount);
    }

    //销毁session监听
    //一旦销毁session就会触发一次这个事件
    public void sessionDestroyed(HttpSessionEvent se) {
        ServletContext ctx = se.getSession().getServletContext();
        Integer onlineCount = (Integer) ctx.getAttribute("OnlineCount");
        if(onlineCount==null){
            onlineCount = new Integer(0);
        }else{
            int count = onlineCount.intValue();
            onlineCount = new Integer(count-1);
        }
        ctx.setAttribute("OnlineCount",onlineCount);
    }

    /*
    session销毁
    1、手动销毁 getSession().invalidote();
    2、自动销毁（web.xml）中配置
    * */
}

2.web.xml中注册监听器

 <!--注册监听器-->
    <listener>
        <listener-class>com.jiang.listener.OnlineCountListener</listener-class>
    </listener>

3.看情况是否使用（几乎不用）

监听器：GUI编程中经常使用

package com.jiang.listener;

import java.awt.*;
import java.awt.event.WindowAdapter;
import java.awt.event.WindowEvent;

public class TestPanel {
    public static void main(String[] args) {
        Frame frame = new Frame("清明节快乐");
        Panel panel = new Panel();
        frame.setLayout(null);

        frame.setBounds(300,300,500,500);
        frame.setBackground(new Color(0,0,255));

        panel.setBounds(50,50,300,300);
        panel.setBackground(new Color(0,255,0));

        frame.add(panel);
        frame.setVisible(true);

        //监听事件
        frame.addWindowListener(new WindowAdapter() {
            @Override
            public void windowOpened(WindowEvent e) {
                System.out.println("打开");
            }

            @Override
            public void windowClosing(WindowEvent e) {
                System.out.println("正在关闭");
                System.exit(0);
            }

            @Override
            public void windowClosed(WindowEvent e) {
                System.out.println("关闭已经结束");
            }

            @Override
            public void windowIconified(WindowEvent e) {

            }

            @Override
            public void windowDeiconified(WindowEvent e) {
                super.windowDeiconified(e);
            }

            @Override
            public void windowActivated(WindowEvent e) {
                System.out.println("激活");
            }

            @Override
            public void windowDeactivated(WindowEvent e) {
                System.out.println("未激活");
            }
        });
    }

}

https://www.bilibili.com/video/BV12J411M7Sj?p=27&spm_id_from=pageDriver