Today, I relearned the basic listener technology of java web, and used sessionListener to simply count the number of people online (one bug is that when the web page is closed, the session will not be closed and still exists, unless the server is closed). The code is as follows (the front-end web page code will not be displayed):

LoginServlet.java:

 1 import javax.servlet.ServletException;
 2 import javax.servlet.annotation.WebServlet;
 3 import javax.servlet.http.HttpServlet;
 4 import javax.servlet.http.HttpServletRequest;
 5 import javax.servlet.http.HttpServletResponse;
 6 import javax.servlet.http.HttpSession;
 7 import java.io.IOException;
 8 
 9 @WebServlet(name = "LoginServlet",urlPatterns = "/login")
10 public class LoginServlet extends HttpServlet {
11     @Override
12     protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
13        String username=request.getParameter("username");
14         HttpSession session=request.getSession();
15         session.setAttribute("username",username);
16         //Redirect to index.jsp
17         response.sendRedirect("index.jsp");
18     }
19 
20     @Override
21     protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
22 
23     }
24 }

ExitServlet.java:

 1 package servlet;
 2 
 3 import javax.servlet.ServletException;
 4 import javax.servlet.annotation.WebServlet;
 5 import javax.servlet.http.HttpServlet;
 6 import javax.servlet.http.HttpServletRequest;
 7 import javax.servlet.http.HttpServletResponse;
 8 import javax.servlet.http.HttpSession;
 9 import java.io.IOException;
10 
11 @WebServlet(name = "ExitServlet",urlPatterns = "/exit")
12 public class ExitServlet extends HttpServlet {
13     @Override
14     protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
15     //from session Remove from loginName parameter
16         response.getWriter().write("2222");
17         HttpSession session=request.getSession();
18         session.removeAttribute("username");
19         //Redirect to login.jsp
20         response.sendRedirect("login.jsp");
21     }
22 
23     @Override
24     protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
25     doPost(request, response);
26     }
27 }

OnlineListener.java:

 1 package listener;
 2 
 3 import bean.User;
 4 
 5 import javax.servlet.ServletContextEvent;
 6 import javax.servlet.ServletContextListener;
 7 import javax.servlet.http.HttpSessionAttributeListener;
 8 import javax.servlet.http.HttpSessionEvent;
 9 import javax.servlet.http.HttpSessionListener;
10 import javax.servlet.http.HttpSessionBindingEvent;
11 
12 @javax.servlet.annotation.WebListener()
13 public class OnlineListener implements ServletContextListener,
14         HttpSessionListener, HttpSessionAttributeListener {
15     public OnlineListener() {
16     }
17     @Override
18     public void contextInitialized(ServletContextEvent sce) {
19      
20     }
21     @Override
22     public void contextDestroyed(ServletContextEvent sce) {
23     
24     }
25     @Override
26     public void sessionCreated(HttpSessionEvent se) {
27         System.out.println("Session created");
28     }
29     @Override
30     public void sessionDestroyed(HttpSessionEvent se) {
31         System.out.println("Session destroyed");
32     }
33     
34     @Override
35     public void attributeAdded(HttpSessionBindingEvent sbe) {
36       //towards session Add a user to the scope
37         User.addUser(String.valueOf(sbe.getValue()));
38         System.out.println("session Add attribute as"+sbe.getName()+"The value is:"+sbe.getValue());
39     }
40     @Override
41     public void attributeRemoved(HttpSessionBindingEvent sbe) {
42      //take session User removal in session
43         User.removeUser(String.valueOf(sbe.getValue()));
44         System.out.println(sbe.getValue()+"Property removed");
45     }
46     @Override
47     public void attributeReplaced(HttpSessionBindingEvent sbe) {
48       //Get old property values
49         String oldValue=String.valueOf(sbe.getValue());
50         //Get new property value
51         String newValue=String.valueOf(sbe.getSession().getAttribute(sbe.getName()));
52         //Remove old properties and add new ones
53         User.removeUser(oldValue);
54         User.addUser(newValue);
55     }
56 }

The results of running with two different browsers are as follows:

The project catalog is as follows:

(keep learning! Come on! Encourage yourself)