统计用户在线人数

在统计用户在人数的时候，我们用到了监听器，监听器大致分为以下三种：

1. ServletRequestListener：用于监听请求的监听接口
2. HttpSessionListener：用于监听会话的监听接口
3. ServletContextListener：用于监听应用的回话接口

错误的统计办法

监听Request域

这种统计办法是错误的认为每次刷新页面后进行进行一次的count++运算

import javax.servlet.*;
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;

@WebListener()
public class MyRequestListener implements ServletRequestListener{
    private ServletContext sc;
    private Integer count;
    @Override
    //请求被初始化 Request
    public void requestInitialized(ServletRequestEvent sre) {
        //获取全局域
        sc = sre.getServletContext();
        //将count从全局域中获取出来
        count = (Integer) sc.getAttribute("count");
        System.out.println(count);
        count++;
        System.out.println(count);
        sc.setAttribute("count",count);
    }
}

import javax.servlet.ServletContext;
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;

@WebListener()
public class MyServletContextListener implements ServletContextListener{
    private ServletContext sc;
    @Override
    //Application被初始化的时候创建
    public void contextInitialized(ServletContextEvent sce) {
        Integer count = 0;
        //获取全局域
        sc = sce.getServletContext();
        //将count放入到全局域中
        sc.setAttribute("count",count);
    }
}

<%@ page contentType="text/html;charset=UTF-8" language="java" %>