package com.ykh.session;

import javax.servlet.ServletContext;

import javax.servlet.http.HttpSession;

import javax.servlet.http.HttpSessionEvent;

import javax.servlet.http.HttpSessionListener;

public class SeesionLister implements HttpSessionListener{

	 @Override

	    public void sessionCreated(HttpSessionEvent event) {

	        // System.out.println("创建了");

	        HttpSession session = event.getSession();// 获得Session对象

	        // 通过Session获得servletcontext对象

	        ServletContext servletContext = session.getServletContext();

	        /**

	         * 1.获取num值

	         * 2.加1

	         * 3.存入servletcontext

	         */

	        Object object = servletContext.getAttribute("num");

	        if (object == null) {

	            servletContext.setAttribute("num", 1);

	        } else {

	            Object num = servletContext.getAttribute("num");

	            int num1 = (int) num;

	            servletContext.setAttribute("num", num1 + 1);

	        }

	    }

	    @Override

	    public void sessionDestroyed(HttpSessionEvent event) {

	        // System.out.println("销毁了");

	        HttpSession session = event.getSession();// 获得Session对象

	        // 通过Session获得servletcontext对象

	        ServletContext servletContext = session.getServletContext();

	        /**

	         *  1.获取num值

	         *  2.减1

	         *  3.存入servletcontext

	         */

	        servletContext.setAttribute("num", (long) servletContext.getAttribute("num") - 1);

	    }

}

package com.ykh.session;

import java.io.IOException;

import java.io.PrintWriter;

import javax.servlet.ServletContext;

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;

/**

 * Servlet implementation class SeesionTest

 */

@WebServlet("/Test")

public class SeesionTest extends HttpServlet {

	private static final long serialVersionUID = 1L;

    /**

     * @see HttpServlet#HttpServlet()

     */

    public SeesionTest() {

        super();

        // TODO Auto-generated constructor stub

    }

	/**

	 * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)

	 */

	protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {

		// TODO Auto-generated method stub

		req.setCharacterEncoding("utf-8");

       doPost(req,resp);

	}

	/**

	 * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)

	 */

	protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {

		// TODO Auto-generated method stub

		//doGet(request, response);

		req.setCharacterEncoding("utf-8");

        resp.setContentType("text/html;charset=utf-8");

        HttpSession session = req.getSession();// 创建Session

        // session.setMaxInactiveInterval(10);//设置失效时间10秒 不是销毁

        PrintWriter writer = resp.getWriter();

        ServletContext servletContext = getServletContext();

        Object num = servletContext.getAttribute("num");

        writer.write("在线人数:" + num);

	}

}