Chapter 12: Retrieving HTTP Request Header Information

In the Previous Chapter we saw how to get a Servlets Initialization parameters. In this chapter, we are going to take a look at how to get the Header information from the Http Request.

So, lets get started!!!

Retrieving HTTP Request Header Information

The Http Request header is the area where all the details of the request are bundled. This is where the browser specifies the file wanted, date, image file support, and a lot more. Let us take a look at a sample Servlet that is going to read through the Http Request Header.

Servlet code:

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

/**
* Displaying request headers
*
* @author Anand V
*/

public class ReadHttpRequestHeaderServlet extends HttpServlet {

public void doGet(HttpServletRequest request, HttpServletResponse response)
throws IOException, ServletException
{
response.setContentType("text/html");

PrintWriter out = response.getWriter();
out.println("< html >");
out.println("< head >");

String title = "Read Request Header Example";
out.println("< title >" + title + "");
out.println("< / head >");
out.println("< body >");

out.println("< h3 >" + title + "< / h3 >");
out.println("< table >");
Enumeration e = request.getHeaderNames();
while (e.hasMoreElements())
{
String headerName = (String)e.nextElement();
String headerValue = request.getHeader(headerName);
out.println("< tr >< td bgcolor=\"#CCCCCC\" > " + headerName);
out.println("< / td >< td > " + headerValue + " < / td > < / tr >");
}
out.println("< / table >");
out.println("< / body >");
out.println("< / html >");
}
}

When you deploy this Servlet on your local tomcat server you will see something like below:

Previous Chapter: Chapter 11 - Obtaining a Servlets Initialization Parameters

Next Chapter: Chapter 13 - Web Application Scope

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