Servlet Tutorial

Servlet technology is used to create web application (resides at server side and generates dynamic web page).

Servlet technology is robust and scalable because of java language. Before Servlet, CGI (Common Gateway Interface) scripting language was popular as a server-side programming language. But there was many disadvantages of this technology. We have discussed these disadvantages below.

There are many interfaces and classes in the servlet API such as Servlet, GenericServlet, HttpServlet, ServletRequest, ServletResponse etc.

What is a Servlet?

Servlet can be described in many ways, depending on the context.

Servlet is a technology i.e. used to create web application.
Servlet is an API that provides many interfaces and classes including documentations.
Servlet is an interface that must be implemented for creating any servlet.
Servlet is a class that extend the capabilities of the servers and respond to the incoming request. It can respond to any type of requests.
Servlet is a web component that is deployed on the server to create dynamic web page.

Steps to Create Servlet Application using tomcat server

To create a Servlet application you need to follow the below mentioned steps. These steps are common for all the Web server. In our example we are using Apache Tomcat server. Apache Tomcat is an open source web server for testing servlets and JSP technology. Download latest version of Tomcat Server and install it on your machine.

After installing Tomcat Server on your machine follow the below mentioned steps :

Create directory structure for your application.
Create a Servlet
Compile the Servlet
Create Deployement Descriptor for your application
Start the server and deploy the application

All these 5 steps are explained in details below, lets create our first Servlet Application.

1. Creating the Directory Structure

Sun Microsystem defines a unique directory structure that must be followed to create a servlet application.

Create the above directory structure inside Apache-Tomcat\webapps directory. All HTML, static files(images, css etc) are kept directly under Web application folder. While all the Servlet classes are kept inside classesfolder.

The web.xml (deployement descriptor) file is kept under WEB-INF folder.

Creating a Servlet

There are three different ways to create a servlet.

By implementing Servlet interface
By extending GenericServlet class
By extending HttpServlet class

But mostly a servlet is created by extending HttpServlet abstract class. As discussed earlier HttpServlet gives the definition of service() method of the Servlet interface. The servlet class that we will create should not override service() method. Our servlet class will override only doGet() or doPost() method.

When a request comes in for the servlet, the Web Container calls the servlet's service() method and depending on the type of request the service() method calls either the doGet() or doPost() method.

NOTE: By default a request is Get request.

import javax.servlet.*;
import javax.servlet.http.*;
import java.io.*;

public MyServlet extends HttpServlet
{
 public void doGet(HttpServletRequest request,HttpServletResposne response) 
                     throws ServletException
 {
   response.setContentType("text/html");
   PrintWriter out = response.getWriter();
   out.println("<html><body>");
   out.println("<h1>Hello Readers</h1>");
   out.println("</body></html>");
 }
}

Write above code in a notepad file and save it as MyServlet.java anywhere on your PC. Compile it(explained in next step) from there and paste the class file into WEB-INF/classes/ directory that you have to create inside Tomcat/webapps directory.

Compiling a Servlet

To compile a Servlet a JAR file is required. Different servers require different JAR files. In Apache Tomcat server servlet-api.jar file is required to compile a servlet class.

Steps to compile a Servlet

Set the Class Path.

Download servlet-api.jar file.
Paste the servlet-api.jar file inside Java\jdk\jre\lib\ext directory.

Compile the Servlet class.

NOTE: After compiling your Servlet class you will have to paste the class file into WEB-INF/classes/directory.

Create Deployment Descriptor

Deployment Descriptor(DD) is an XML document that is used by Web Container to run Servlets and JSP pages. DD is used for several important purposes such as:

Mapping URL to Servlet class.
Initializing parameters.
Defining Error page.
Security roles.
Declaring tag libraries.

We will discuss about all these in details later. Now we will see how to create a simple web.xml file for our web application.

Start the Server

Double click on the startup.bat file to start your Apache Tomcat Server.

Or, execute the following command on your windows machine using RUN prompt.

C:\apache-tomcat-7.0.14\bin\startup.bat

Starting Tomcat Server for the first time

If you are starting Tomcat Server for the first time you need to set JAVA_HOME in the Enviroment variable. The following steps will show you how to set it.