In this article, we are going to understand how to create a listener class which listens to an event when a session is created or destroyed in a web application.
For this, we are going to create a webpage which asks the user to enter his name and city in a form and click the submit button.
On submitting the form, the user request goes to the requested Servlet class, which creates a new http session. This generates an event, which leads to
the calling of the Listener class
waiting for such event.
To do this, we are going to create a Listener class by implementing HttpSessionListener interface
and by providing implementation of its methods.
HttpSessionListener is a part of the java.util.* package.
HttpSessionListener methods
Let's take a look at the methods of HttpSessionListener interface.
Methods
Description
voidsessionCreated(HttpSessionEvent hse)
This method receives notification when an session is created.
voidsessionDestroyed(HttpSessionEvent hse)
This method receives notification when a session is invalidated or destroyed.
Creating a webpage which calls the Servlet
First we execute the webpage containing the form, asking the user to enter his/her name and city and press submit button.
On submitting the form, the user request goes to the Servlet class, where a new http session will be created which
will generate an event. This will eventually call the listener class, listening to such event.
Webpage1.htm
<html>
<head>
<title> ServletRequest Demo </title>
</head>
<body>
Please enter your details :
<form action = "MyServlet">
Name : <input type = "text" name = "username" />
City : <input type = "text" name = "cityname" /> <br/>
<input type = "submit" value = "submit" />
</form>
</body>
</html>
Creating a listener class to listen to an event a new Http Session
We are creating a Servlet class by extending GenericServlet abstract class.
In this Servlet class, we are just sending a message to the user, asking to open at the Tomcat Web Server window for the information
about the events of creation of a new Http Session within the web application.
MyServlet1.java
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
import java.util.*;
public class MyServlet1 extends HttpServlet
{
public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException
{
response.setContentType("text/html");
PrintWriter out = response.getWriter();
//Creates a variable to count the number of visits of the user
int counter = 0;
//Creates a session object
HttpSession session = request.getSession(true);
out.println("<b> Welcome, </b> " );
out.println("<br/> <br/> <br/> ");
boolean sessionInfo = session.isNew();
out.println("Is this a new session = " + sessionInfo);
out.println("<br/> ");
out.println("Session ID = " + session.getId());
out.println("<br/> ");
out.println("This session was created = " + new Date(session.getCreationTime()));
out.println("<br/> ");
out.println("Maximum time for which this session can be inactive = " + session.getMaxInactiveInterval() + " seconds");
out.println("<br/> <br/> <br/> ");
}
}
Next, we are creating a Listener class, listening to the an event when a session is creating or destroyed.
The information about request is displayed
on the Tomcat Web Server window, as we are using System.out.println() method.
SessionListener1.java
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
import java.util.EventListener;
import java.util.*;
public class SessionListener1 implements HttpSessionListener
{
public void sessionCreated(HttpSessionEvent se)
{
System.out.println("A new Session is created");
}
public void sessionDestroyed(HttpSessionEvent se)
{
System.out.println("A new Session is created");
}
}
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Directory Structure of Servlet files
The diagram above depicts how to arrange the Servlet files in a specific directory structure, as per Java Servlet Specification-
Project Folder - We have created a project folder named HttpSessionListenerEx within
the webapps folder of Tomcat installation folder. This folder contains the Servlet class(.java) file and WEB-INF folder.
WEB-INF - Within your project folder, you create another folder named WEB-INF and this folder should contain the deployment descriptor file(web.xml) of your Java Servlet program.
classes - Within the WEB-INF folder you should create a folder named classes. This folder should contain the compiled(.class) form of your Java Servlet class.
Creating the Deployment Descriptor file
As per the Java Servlet specifications, every web application based on Servlet must have a Deployment Descriptor file(an XML file) named web.xml.
So, let's create one -
In deployment descriptor file, <listener> tag has a child tag <listener class> :
<listener-class> tag is used to specify the full qualified name of the Listener class and in our example this class is named SessionListener1.
<servlet> tag has two child tags <servlet-name> and <servlet-class> :
<servlet-name> tag is used to specify a unique name for our Servlet class, we have given it a unique name MyServlet.
<servlet-class> tag is used to specify the full qualified name of the Servlet class and in our example our Servlet class is named MyServlet1.
Note
The child tag <servlet-name> of <servlet> tag is matched with the <servlet-name> child tag of
<servlet-mapping>. The
<url-pattern> child tag is used to specify the URL to access Servlet and we have named this URL MyServlet.
Setting the classpath
Much of the support for developing the web applications based on the Java Servlet technology does not come with the core Java.
Hence, in order to compile the Servlet programs, we have to set the classpath to a jar file named
servlet-api.jar.
This jar file provides all the classes that are required for the Servlet programming and it comes within the lib Folder of
Tomcat installation folder.
For example, in our case we have installed Tomcat Web Server within the C: Drive, hence the path to our lib folder containing the servlet-api.jar is - C:\apache-tomcat-9.0.2\lib
There are two ways to set the classpath -
You could set the classpath by entering this command at the Command Prompt, but this only temporarily sets the classpath to the path of servlet-api.jar file. If you restart your system, you will have to set the classpath again.
set classpath=C:\apache-tomcat-9.0.2\lib\servlet-api.jar
In order to set the classpath permanently, you need to set your classpath system variables. A simple tutorial on how to permanently set the classpath system variables, you may read
How to permanently set the classpath system variable
Compiling the Servlet class
After setting the classpath, you need to compile the Servlet class and the Listener class by entering the command at the folder where you've stored
the Servlet and the Listener class file.
javac -d WEB-INF/classes MyServlet1.java
javac -d WEB-INF/classes SessionListener1.java
Executing the Servlet
First we execute the webpage containing the form, asking the user to enter his/her name and city and press submit button.
As user enters the asked details and submits the form, the request not only goes to the Servlet and also to the listener class SessionListener1.java, listening
to the creation of a new Http Session.
Hence, you will see the Servlet class in execution and displaying the information about the newly
created session.
And, if you check the Tomcat Web Server window, you will also see a message notifying you
about the event of creation of a new session within the web application. This message is
displayed by the listener class.
