十三、监听器——网页在线人数

package com.listener;

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

public class MyListener implements HttpSessionListener,ServletContextListener{
    //监听Application对象
    @Override
    public void contextInitialized(ServletContextEvent sce) {
        int count=0;
        //获取Application对象
        ServletContext sc = sce.getServletContext();
        sc.setAttribute("count",count);
    }

    @Override
    public void contextDestroyed(ServletContextEvent sce) {
        // TODO Auto-generated method stub

    }
    //监听Sesion对象
    @Override
    public void sessionCreated(HttpSessionEvent se) {
        //获取Application对象中的计数器
        ServletContext sc = se.getSession().getServletContext();
        int count=(int) sc.getAttribute("count");
        //计数器自增
        ++count;
        //然后再将计数器存储到application中
        sc.setAttribute("count", count);
    }

    @Override
    public void sessionDestroyed(HttpSessionEvent se) {
        //获取Application对象中的计数器
        ServletContext sc = se.getSession().getServletContext();
        int count=(int) sc.getAttribute("count");
        //计数器自减
        --count;
        //然后再将计数器存储到application中
        sc.setAttribute("count", count);

    }

}

    <listener>
        <listener-class>com.listener.MyListener</listener-class>
    </listener>