十二、listener监听器

1、创建监听器，实现监听器接口

package com.lyh.listener;
​
import javax.servlet.ServletContext;
import javax.servlet.http.HttpSessionEvent;
import javax.servlet.http.HttpSessionListener;
//统计网站在线人数，统计看session
public class OnlineCountListener implements HttpSessionListener {
    //创建session监听，看你的一举一动
    //一旦创建session就会触发一次事件
    @Override
    public void sessionCreated(HttpSessionEvent httpSessionEvent) {
        ServletContext context = httpSessionEvent.getSession().getServletContext();
        System.out.println(httpSessionEvent.getSession().getId());
        Integer onlineCount = (Integer) context.getAttribute("OnlineCount");
        if (onlineCount==null){
            onlineCount=new Integer(1);
        }else {
            int count = onlineCount.intValue();
            onlineCount=new Integer(count+1);
        }
        context.setAttribute("OnlineCount",onlineCount);
    }
    //销毁session监听
    @Override
    public void sessionDestroyed(HttpSessionEvent httpSessionEvent) {
        ServletContext context = httpSessionEvent.getSession().getServletContext();
        System.out.println(httpSessionEvent.getSession().getId());
        Integer onlineCount = (Integer) context.getAttribute("OnlineCount");
        if (onlineCount==null){
            onlineCount=new Integer(0);
        }else {
            int count = onlineCount.intValue();
            onlineCount=new Integer(count-1);
        }
        context.setAttribute("OnlineCount",onlineCount);
    }
}

2、在web.xml中注册监听器

<!--    监听器配置-->
<listener>
    <listener-class>com.lyh.listener.OnlineCountListener</listener-class>
</listener>

3、看着使用