一开发步骤

1 编写监听器

2 注册监听器

二实战

1 监听器编写

import javax.servlet.ServletContext;
import javax.servlet.http.HttpSessionEvent;
import javax.servlet.http.HttpSessionListener;
/**
* @ClassName: OnlineCount
* @Description: 在线人数监听
* @Date: 2020/6/15
* @Author: cakin
*/
public class OnlineCount implements HttpSessionListener {
    /**
     * 功能描述：会话创建时刻进行监听
     *
     * @param se 会话事件
     * @author cakin
     * @date 2020/6/15
     * @description: 创建一个会话时，在线人数加1
     */
    public void sessionCreated(HttpSessionEvent se) {
        ServletContext servletContext = se.getSession().getServletContext();
        Integer onlineCount = (Integer) servletContext.getAttribute("onlineCount");
        System.out.println("增加一个session");
        if (onlineCount == null) {
            onlineCount = 1;
        } else {
            int count = onlineCount.intValue();
            onlineCount = ++count;
        }
        servletContext.setAttribute("onlineCount", onlineCount);
    }
    /**
     * 功能描述：会话销毁时刻进行监听
     *
     * @param se 会话事件
     * @author cakin
     * @date 2020/6/15
     * @description: 销毁会话时，在线人数建1
     */
    public void sessionDestroyed(HttpSessionEvent se) {
        ServletContext servletContext = se.getSession().getServletContext();
        System.out.println("减少一个session");
        Integer onlineCount = (Integer) servletContext.getAttribute("onlineCount");
        if (onlineCount == null) {
            onlineCount = 0;
        } else {
            int count = onlineCount.intValue();
            onlineCount = --count;
        }
        servletContext.setAttribute("onlineCount", onlineCount);
    }
}

2 注册监听器

<?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">
    <!--监听器所在的类-->
    <listener>
        <listener-class>OnlineCount</listener-class>
    </listener>
    <!--超时时间为1分钟-->
    <session-config>
        <session-timeout>1</session-timeout>
    </session-config>
</web-app>

3 JSP页面测试

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
  <head>
    <title>$Title$</title>
  </head>
  <body>
  <h1>当前在线人数为<%=this.getServletConfig().getServletContext().getAttribute("onlineCount")%></h1>
  </body>
</html>