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In this tutorial, we are going to understand how to maintain a user session by using an HttpSession object. By using an object of HttpSession interface, the web container can locate if there an existing session which is associated with the user and if it doesn't find an existing session, it creates one.

This HttpSession object helps in storing a user information or a session data, to maintain its session and differentiate it from the other users. Such session exists for a certain interval and it survives even when different requests are made by the same user.

Methods of HttpServletRequest used to create a new session

Some useful methods available to the session object.

The methods in the table below are part of javax.servlet.http.HttpSession interface. Hence, these methods are available to the JSP implicit session object, which is a type of HttpSession.

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Maintaining a Session within a Servlet

In the upcoming example, we are creating a Servlet by extending the HttpServlet abstract class. The HttpServlet class extends the GenericServlet abstract class. Our servlet class is named MyServlet1.java, implements the doGet() method of the HttpServlet class to handle the default get request made by the client after executing this Servlet.


This Servlet does the following tasks -

• It creates a variable counter to count the number of visits of a user on this webpage.
• It calls the getSession(boolean) method to locate an existing session associated with the user.
• If the getSession(boolean) is unable to locate any existing session with the current user, it creates one and the information about this session is displayed, or, if it has located an existing session with the current user, it displays the session information and number of times user has visited this webpage.

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("Last time when this session was accessed = " + new Date(session.getLastAccessedTime()));
out.println("<br/> ");
out.println("Maximum time for which this session can be inactive = " + session.getMaxInactiveInterval() + " seconds");
out.println("<br/>  <br/>  <br/> ");




if(sessionInfo)
{
	
	session.setAttribute("visit", counter);
	counter = counter + 1;
	out.println("<b> This is your first visit to this website </b>");
}
else 
{
	
	counter = (int)session.getAttribute("visit");
	counter = counter + 1;
	out.println("<b> Number of times you have visited this website : <b> "+ counter);
}

session.setAttribute("visit", counter);


}
}

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 SessionEx 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 -


web.xml

<?xml version="1.0" encoding="UTF-8"?>
<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"
  metadata-complete="true">

  <display-name>Welcome tomcat</display-name>
  <description>
     Welcome tomcat
  </description>

  
<servlet>
 	<servlet-name>FirstServlet</servlet-name>
	<servlet-class>MyServlet1</servlet-class>
</servlet>



<servlet-mapping>
	<servlet-name>FirstServlet</servlet-name>
	<url-pattern>/Serv1</url-pattern>
</servlet-mapping>


</web-app>

In deployment descriptor file, <servlet> has two child tags <servlet-name> and <servlet-class> :

• <servlet-name> tag is used to specify a unique name for our Servlet class.


• <servlet-class> tag is used to specify the fully qualified name of the Servlet class .

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 the Servlet, Hence -

• We have named this URL as Serv1, to access the servlet, MyServlet1.java

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 both of the Servlet class by entering the command at the folder where you've stored the Servlet class file.

javac -d WEB-INF/classes MyServlet1.java

Executing the Servlet

Executing the webpage containing the Servlet for the first time gives us the information about the new session created. This page also gives us a personalised welcome message on our first visit. This Servlet also maintains a counter which displays the number of times a user has visited this webpage.




On refreshing this webpage the Servlet gives us current value of the counter i.e. the number of times we have visited this webpage and the information about the current session. This counter is maintained by creating an HttpSession object

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