How do I capture session creation and removal events?

By Wayan in Servlet 0 Comment

The Servlet specification define an HttpSessionListener interface that can be implemented if we want to listen to session creation and removal events. The interface has two methods that we can implement, the sessionCreated(HttpSessionEvent event) and sessionDestroyed(HttpSessionEvent event) methods. To activate the listener we need to register it in the servlet container. To register the listener we can use the @WebListener annotation. Let’s see the full code snippet below.

package org.kodejava.servlet;

import javax.servlet.annotation.WebListener;
import javax.servlet.http.HttpSession;
import javax.servlet.http.HttpSessionEvent;
import javax.servlet.http.HttpSessionListener;
import java.util.Date;

@WebListener
public class MySessionListener implements HttpSessionListener {

    // Notification that a new session was created
    @Override
    public void sessionCreated(HttpSessionEvent event) {
        HttpSession session = event.getSession();

        System.out.println("New session created  : " + session.getId());
        System.out.println("Session creation time: " + new Date(session.getCreationTime()));
    }

    // Notification that a session was invalidated
    @Override
    public void sessionDestroyed(HttpSessionEvent event) {
        HttpSession session = event.getSession();

        System.out.println("Session destroyed  : " + session.getId());
    }
}

Maven dependencies

<dependency>
    <groupId>javax.servlet</groupId>
    <artifactId>javax.servlet-api</artifactId>
    <version>4.0.1</version>
</dependency>

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How do I count number of online users?

By Wayan in Servlet 18 Comments

When you have a web application you might want to know how many users are currently online or connected to your website. If you have visited some web online forums you can see; usually on the first page; the list of their online users or maybe just the number of currently online users.

How do we know / count how many sessions or users are currently connected to our website. Do you care to know? Let’s see what Java Servlet API offers us on this matter.

Servlet API has an interface javax.servlet.http.HttpSessionListener, an implementation of this interface will have the ability to be notified by the servlet engine at anytime when a new session was created or destroyed.

This interface has two methods to be implemented; these methods are sessionCreated(HttpSessionEvent se) and sessionDestroyed(HttpSessionEvent se). These methods will be called as a notification that a new session was created, and the session was about to be destroyed respectively.

Now let’s create our session listener. The code below is what our class is going to be implemented.

package org.kodejava.servlet;

import javax.servlet.annotation.WebListener;
import javax.servlet.http.HttpSessionEvent;
import javax.servlet.http.HttpSessionListener;
import javax.servlet.http.HttpSession;
import java.util.List;
import java.util.ArrayList;

@WebListener
public class SessionCounter implements HttpSessionListener {
    public static final String COUNTER = "SESSION-COUNTER";
    private final List<String> sessions = new ArrayList<>();

    public void sessionCreated(HttpSessionEvent event) {
        System.out.println("SessionCounter.sessionCreated");
        HttpSession session = event.getSession();
        sessions.add(session.getId());
        session.setAttribute(SessionCounter.COUNTER, this);
    }

    public void sessionDestroyed(HttpSessionEvent event) {
        System.out.println("SessionCounter.sessionDestroyed");
        HttpSession session = event.getSession();
        sessions.remove(session.getId());
        session.setAttribute(SessionCounter.COUNTER, this);
    }

    public int getActiveSessionNumber() {
        return sessions.size();
    }
}

To display information of current online users we need to create a simple JSP page. This JSP file will get the number of online user from HttpSession attribute named counter that we set in our listener above.

<%@ page import="org.kodejava.servlet.SessionCounter" %>
<html>
<head>
    <title>Session Counter</title>
</head>
<body>
<%
    SessionCounter counter = (SessionCounter) session.getAttribute(
            SessionCounter.COUNTER); 
%>

Number of online user(s): <%= counter.getActiveSessionNumber() %>
</body>
</html>

The final step to make the listener working is to register it in the web.xml file. Below is the example how to register the listener in web.xml.

<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">
    <display-name>Servlet Examples</display-name>
    <session-config>
        <session-timeout>1</session-timeout>
    </session-config>
</web-app>

Maven dependencies

<dependency>
    <groupId>javax.servlet</groupId>
    <artifactId>javax.servlet-api</artifactId>
    <version>4.0.1</version>
</dependency>

Maven Central