Chapter 3: A Sample Servet

We saw what the purpose of a Servlet Container and a Web server is, in the previous chapter. In this chapter, we are going to look at how a Servlet code would look like.

So, lets get started!!!

Servlet Skeleton

If I ask you, what are the components of a Java class, you’ll happily tell me that, there are first package statements and then imports and then the class declaration. Within the class brackets, we have constructors, instance variables, methods etc. That was easy, wasn’t it?

The same way, every Servlet has a certain set of components that are mandatory for its well-being. (I just got carried away a bit) Or I must say, for its proper functioning.

A Servlets skeleton would look like below:

/*
* servlet name
*
* servlet description
* All other stuff that is part of a Standard Class comment section
*/

//package declarations

//import statements

public class ServletName extends HttpServlet {

// Instance Variables

/**
* Code to Initialize the Servlet
*/
public void init() throws ServletException
{
// Servlet Initialization Code goes here
}

/**
* The Service Method
* Gets invoked for every request submitted to the Servlet
* This method is optional. We mostly use doGet, doPost Methods
*/
protected void service(HttpServletRequest req,
HttpServletResponse resp)
throws ServletException, IOException
{
// Code for the Service Method goes here
}


/**
* Process a GET request
*
*/
protected void doGet(HttpServletRequest request,
HttpServletResponse response)
throws IOException, ServletException
{
// Code for the doGet() method goes here
}

/**
* Process a POST request
*
*/
protected void doPost(HttpServletRequest request,
HttpServletResponse response)
throws IOException, ServletException
{
// Code for the doPost() method goes here
}

/**
* Process a PUT request
*
*/
protected void doPut(HttpServletRequest req,
HttpServletResponse resp)
throws ServletException, IOException
{
//Code for the doPut() method goes here
}

/**
* You can have any number of methods for your processing
* here. There is no limit as to the number of methods or
* any restrictions on what you can name them.
* Since this is all java code, you need to keep them
* syntactically correct as per Java Coding Standards.
*/

/**
* Clean-Up
*/
public void destroy()
{
// clean up activities before the Servlet is put to death
}
}

The above is what a Servlets skeleton would look like. Now let us take a look at some sample code as to how a properly coded Servlet would look like:


import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.ServletException;
import java.io.PrintWriter;
import java.io.IOException;


public class OurFirstServlet extends HttpServlet
{

public void service(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException
{
response.setContentType("text/html");
PrintWriter out = response.getWriter();
out.println("< html >");
out.println("< head >< title >Servlet Example " +
" ");
out.println("< body >");
out.println("Not Much code, but this is enough for a Servlet.");
out.println("");
out.println("");
}
}

The above is a simple Servlet. It would display an almost blank HTML page that contains the message we put in “Not Much code, but this is enough for a Servlet.”

Note: A Servlet is not a simple java class, that you can run using a main() method. You have deploy this Servlet on a web server in order to view the output. Lets not get too ahead of ourselves here. We’ll be looking at all that later in detail. For now, this is how a Servlet would look like and that wraps up our current chapter.

Previous Chapter: Web Servers & Servlet Containers

Next Chapter: A sample JSP

Chapter 2: Web Servers and Servlet Containers

In the previous chapter we looked at the history of the Web (Internet) and how JSPs and Servlets came into being. As you might have already guessed by now, any J2EE application needs a server on which it runs. It is called a Web Server. Also, the Servlet needs something in which it would run (Think of the JVM in which all java applications run) and that is called the Servlet container.

In this chapter, we are going to take a look at these two entities.

So, lets get started!!!

Web Servers and Servlet Containers

A servlet is nothing but Java code that runs in a container. It generates HTML & other contents that get displayed in the web page that we see. It is purely coded in Java, so the benefits and restrictions of all regular Java classes apply here too. Servlets are compiled to form a platform neutral bytecode (All java code is platform neutral, isnt it? Serlvet is no different). Upon request, this bytecode file is loaded into a container. Some containers (servlet engines) are integrated with the Web server, while others are plug-ins or extensions to Web servers that run inside the JVM. Servlets look the same as static Web pages to the client, but they really are complete programs capable of complex operations.

The servlet container is an extension of a Web server in the same way CGI, ASP, and PHP are. A servlet functions just like them, but the language in which it is written is Java. That's the main difference. The servlet doesn't talk to the client directly. The Web server functions as the intermediary that does it for the servlet. In a chain of processes, the client sends a request to the Web server, which hands it to the container, which hands it to the servlet. The Servlet processes the request and generates a response. The response starts from the servlet and goes to the container and then to the Web server and back to the client. Of course there are several other steps that happen too but this is just the introduction so this much would suffice I believe.

The servlet architecture makes the container manage servlets through their lifecycle. The container invokes a servlet upon receiving an HTTP request, providing that servlet with request information and the container configurations. The servlet uses this to do its job which is processing the data it received as part of the request. When finished, it hands back HTML and objects that hold information about the response. The container then forms an HTTP response and returns it to the client which inturn displays the stuff sent by the Servlet on screen in the web browser.

Below is a simple illustration of the flow of control that starts and ends at the Web Browser. Control starts at the browser, goes to the server and then to the container which in turn invokes the Servlet. Our Servlet happily does the processing (which might involve hitting a database) and returns the data to the container which in turn passes it on to the web server which finally displays the contents on the web browser.

The Servlet container is often written in Java, since it eventually runs inside a JVM. However, some vendors implement their containers in different languages (they aren’t essentially bound to Java). The point here is the fact that, all we need is a servlet container that can read and execute our servlets. The language in which it is implemented is not necessarily important for us.

Previous Chapter: Chapter 1 - Servlet & JSP History

Next Chapter: A Sample Servlet