监听器Listener

package com.xxx.listener;


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

@WebListener
public class listen2 implements HttpSessionListener {
    /**
     * 统计在线人数 根据session数量
     */
    private int onlineNum;
    @Override
    public void sessionCreated(HttpSessionEvent httpSessionEvent) {

        onlineNum++;
        /**
         * 放在session中 浏览器刷新还是不变
         *  放在servletContext 范围扩大 都可以改变
         */
       // httpSessionEvent.getSession().setAttribute("onlineNum",onlineNum);
        httpSessionEvent.getSession().getServletContext().setAttribute("onlineNum",onlineNum);
    }

    @Override
    public void sessionDestroyed(HttpSessionEvent httpSessionEvent) {
        onlineNum--;
        httpSessionEvent.getSession().getServletContext().setAttribute("onlineNum",onlineNum);
    }
}

package com.xxx.servlet;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;
import java.io.IOException;

/**
 * 用来显示在线人数以及退出功能
 */
@WebServlet("/s03")
public class ser3 extends HttpServlet {
    @Override
    protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        HttpSession session=req.getSession();
        String key=req.getParameter("key");
        /**
         * 点击a标签时 发送key为out
         *      如果只登录 key为null
         */
        if(key!=null&&key.equals("out")){
           //销毁session
           session.invalidate();
           return;
       }
        int onlineNum=(int) session.getServletContext().getAttribute("onlineNum");
        resp.setContentType("text/html;charset=UTF-8");
        resp.getWriter().write("Num"+onlineNum);
        resp.getWriter().write("<a href='s03?key=out'>exit<a>");
    }
}