How to determine active users / sessions in a Java Web Application ?.

A simple tutorial to demonstrate how to determine active users / sessions in a Java Web Application.

package com.hubberspot.javaee.listener;

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

@WebListener
public class OnlineUsersCounter implements HttpSessionListener {

 private static int numberOfUsersOnline;
 
 public OnlineUsersCounter() {
  numberOfUsersOnline = 0;
 }
 
 public static int getNumberOfUsersOnline() { 
  return numberOfUsersOnline;
 }
 
    public void sessionCreated(HttpSessionEvent event) {

     System.out.println("Session created by Id : " + event.getSession().getId());
     synchronized (this) {
   numberOfUsersOnline++;
  }
     
    }

    public void sessionDestroyed(HttpSessionEvent event) {
     
     System.out.println("Session destroyed by Id : " + event.getSession().getId());
     synchronized (this) {
   numberOfUsersOnline--;
  }

    }
 
}

Running the below servlet on three different browsers will provide output as : (see fig below)

package com.hubberspot.javaee;

import java.io.IOException;
import java.io.PrintWriter;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebInitParam;
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 com.hubberspot.javaee.listener.OnlineUsersCounter;

// @WebServlet annotation has a initParams field which takes
// in initialization parameters for a servlet.
// @WebInitParam annotation takes in a name and value for the
// initialization parameters for the current Servlet.

@WebServlet(name = "HelloWorldServlet" , urlPatterns = { "/HelloWorldServlet" }
    , initParams = { @WebInitParam(name = "user" , value = "Jonty") })
public class HelloWorldServlet extends HttpServlet {

 protected void doGet(
   HttpServletRequest request, 
   HttpServletResponse response
   ) throws ServletException, IOException {

  response.setContentType("text/html");

  PrintWriter out = response.getWriter();
  
  // sessionCreated method gets executed
  HttpSession session = request.getSession();
  
  session.setMaxInactiveInterval(60);
  
  try {
   out.println("<html>");
   out.println("<body>");
   out.println("<h2>Number of Users Online : " 
      + OnlineUsersCounter.getNumberOfUsersOnline() 
      + "</h2>");
   out.println("</body>");
   out.println("</html>");
  } finally {
   out.close();
  }
        
 }

}

Output of the program :

1. Eclipse Browser