session统计在线人数

import javax.servlet.ServletContext;
import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
import javax.servlet.http.*;

public class ServletListener implements ServletContextListener,
        HttpSessionListener, HttpSessionAttributeListener {

    // Public constructor is required by servlet spec
    public ServletListener() {
    }
    // -------------------------------------------------------
    // ServletContextListener implementation
    // -------------------------------------------------------
    //session创建时人数自增
    public void sessionCreated(HttpSessionEvent se) {
        /* Session is created. */
        HttpSession session = se.getSession();
        session.setMaxInactiveInterval(3*60);
        ServletContext servletContext = session.getServletContext();
        int count = (int) servletContext.getAttribute("count");
        servletContext.setAttribute("count",++count);
        System.out.println("调用了sessionCreate方法！！");
        System.out.println("当前在线人数："+count);
    }

    public void sessionDestroyed(HttpSessionEvent se) {
        /* Session is destroyed. */
        ServletContext servletContext = se.getSession().getServletContext();
        int count = (int) servletContext.getAttribute("count");
        servletContext.setAttribute("count",--count);
        System.out.println("调用了sessionDestroy方法！！");
        System.out.println("当前在线人数："+count);
    }

    public void contextInitialized(ServletContextEvent sce) {
      /* This method is called when the servlet context is
         initialized(when the Web application is deployed).
         You can initialize servlet context related data here.
      */
        ServletContext servletContext = sce.getServletContext();
        servletContext.setAttribute("count",0);
    }

    public void contextDestroyed(ServletContextEvent sce) {
      /* This method is invoked when the Servlet Context
         (the Web application) is undeployed or
         Application Server shuts down.
      */
    }

    // -------------------------------------------------------
    // HttpSessionListener implementation
    // -------------------------------------------------------

    // -------------------------------------------------------
    // HttpSessionAttributeListener implementation
    // -------------------------------------------------------

    public void attributeAdded(HttpSessionBindingEvent sbe) {
      /* This method is called when an attribute 
         is added to a session.
      */
    }

    public void attributeRemoved(HttpSessionBindingEvent sbe) {
      /* This method is called when an attribute
         is removed from a session.
      */
    }

    public void attributeReplaced(HttpSessionBindingEvent sbe) {
      /* This method is invoked when an attibute
         is replaced in a session.
      */
    }
}

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd"
         version="4.0">
    <listener>
        <listener-class>ServletListener</listener-class>
    </listener>
</web-app>