JAVA EE: Servlets : HttpSession Listener Demo [Count logged in Users]

Saturday, 21 June 2014

Servlets : HttpSession Listener Demo [Count logged in Users]

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Tracking Logged-In Users with HttpSessionListener

In Java web applications, tracking the number of active sessions or concurrent logged-in users is a common requirement for dashboards, system monitoring, and session management. The standard, cleanest way to achieve this is by implementing the HttpSessionListener interface.

The web container automatically triggers callback methods on HttpSessionListener whenever a new HTTP session is created or an existing session is invalidated or times out. By updating a counter stored in the application-wide ServletContext, you can maintain an accurate, real-time count of active users.

How the Listener Works

The HttpSessionListener interface defines two primary lifecycle methods:

sessionCreated(HttpSessionEvent se): Invoked automatically when a new user session is initiated. Here, you increment the active user counter.
sessionDestroyed(HttpSessionEvent se): Invoked when a user logs out (session invalidated) or when the session expires due to inactivity. Here, you decrement the counter.

@WebListener

    public class UserCounterListener implements HttpSessionListener {

        private static int activeUsers = 0;


        public void sessionCreated(HttpSessionEvent se) {

            activeUsers++;

            se.getSession().getServletContext().setAttribute("activeUsers", activeUsers);

        }


        public void sessionDestroyed(HttpSessionEvent se) {

            activeUsers--;

            se.getSession().getServletContext().setAttribute("activeUsers", activeUsers);

        }

    }

Key Advantages of Using HttpSessionListener

Automatic Session Lifecycle Management: No manual tracking or polling needed; the servlet container notifies the listener on lifecycle transitions.
Clean Separation: Decouples session analytics and user counting from individual business servlets.
Global Availability: Storing the count inside ServletContext makes it effortlessly accessible across all servlets, JSPs, and controllers.

Recommended Related Tutorials

Continue exploring servlet listeners and session handling with these tutorials from Ram N Java:

Click here to watch in Youtube : https://www.youtube.com/watch?v=fv8Idtevcg0

Click the below Image to Enlarge

HttpSessionListenerDemo Project Dir Structure

LoginServlet.java

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

import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

public class LoginServlet extends HttpServlet
{
    
    private static final long serialVersionUID = 1L;

    public void init() throws ServletException
    {
        System.out.println("-----------------------------------------");
        System.out.println(" Init method is called in "
                + this.getClass().getName());
        System.out.println("--------------------------------------");
    }
    
    public void doGet( HttpServletRequest request, HttpServletResponse response )
            throws ServletException, IOException
    {

        response.setContentType("text/html");
        PrintWriter out = response.getWriter();

        String userName = request.getParameter("username");
        out.print("Welcome " + userName);

        HttpSession session = request.getSession();
        session.setAttribute("uname", userName);

        ServletContext ctx = getServletContext();
        int totalUsers = (Integer) ctx.getAttribute("totalusers");
        int currentUsers = (Integer) ctx.getAttribute("currentusers");
        
        out.print("<br>total users= " + totalUsers);
        out.print("<br>current users= " + currentUsers);

        out.print("<br><a href='logout'>logout</a>");

        out.close();
    }
    
    public void destroy()
    {
        System.out.println("-----------------------------------------");
        System.out.println(" destroy method is called in "
                + this.getClass().getName());
        System.out.println("-----------------------------------------");
    }

}

LogoutServlet.java

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

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

public class LogoutServlet extends HttpServlet
{

    private static final long serialVersionUID = 1L;

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

        response.setContentType("text/html");
        PrintWriter out = response.getWriter();

        HttpSession session = request.getSession(false);
        session.invalidate();

        out.print("You are successfully logged out");

        out.close();
    }

}

CountUserListener.java

import javax.servlet.ServletContext;
import javax.servlet.http.HttpSessionEvent;
import javax.servlet.http.HttpSessionListener;

public class CountUserListener implements HttpSessionListener
{
    ServletContext ctx = null;
    static int     totalUserCount = 0, currentUserCount = 0;

    public void sessionCreated( HttpSessionEvent httpSessionEvent )
    {
        System.out.println("sessionCreated method has been called in "
                + this.getClass().getName());

        
        totalUserCount++;
        currentUserCount++;

        ctx = httpSessionEvent.getSession().getServletContext();
        
        ctx.setAttribute("totalusers", totalUserCount);
        ctx.setAttribute("currentusers", currentUserCount);

    }

    public void sessionDestroyed( HttpSessionEvent httpSessionEvent )
    {
        
        System.out.println("sessionDestroyed method has been called in "
                + this.getClass().getName());       
        currentUserCount--;
        ctx.setAttribute("currentusers", currentUserCount);
    }

}

web.xml

<?xml version="1.0" encoding="ISO-8859-1"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns="http://java.sun.com/xml/ns/javaee"
    xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd"
    metadata-complete="true" version="3.0">
    <display-name>HttpSessionListenerDemo</display-name>
    <description>
        This is a simple web application with a source code organization
        based on the recommendations of the Application Developer's Guide.
    </description>


    <servlet>
        <servlet-name>LoginServlet</servlet-name>
        <servlet-class>LoginServlet</servlet-class>
    </servlet>
    
    <servlet>
        <servlet-name>LogoutServlet</servlet-name>
        <servlet-class>LogoutServlet</servlet-class>
    </servlet>


    <servlet-mapping>
        <servlet-name>LoginServlet</servlet-name>
        <url-pattern>/login</url-pattern>
    </servlet-mapping>
    
    <servlet-mapping>
        <servlet-name>LogoutServlet</servlet-name>
        <url-pattern>/logout</url-pattern>
    </servlet-mapping>


    <listener>
        <listener-class>CountUserListener</listener-class>
    </listener>
    
</web-app>

index.html

<form action="login">
    Name:<input type="text" name="username"><br> 
    
    Password:<input type="password" name="password"><br> 
    
    <input type="submit" value="login" />
    
</form>

Environment Used

JDK version :1.7.0_51
Tomcat version : 7.0.50

To Download HttpSessionListenerCountUserDemoApp Project Click the below link

https://sites.google.com/site/javaee4321/servlets/HttpSessionListenerCountUserDemoApp.zip?attredirects=0&d=1

See also:

Servlets Tutorial

All Design Patterns Links