Servlet - Handling HTTP request

In the previous few chapters, we learnt what a Servlet is, what a JSP is, how they fit together in the bigger picture in a J2EE Application. In this chapter, we are going to see how Servlets handle HTTP Requests.

So, lets get started!!!

Servlets & Web Pages:

JSP and servlets have greatly enhanced the way in which we can create and manage Web pages. The difficulty level of coding JSP is between that of coding HTML and pure Java. Servlets are pure Java. The idea behind having both is providing a way for non-programmers to contribute functionality through JSP. You can “program” a JSP page almost as easily as you can write an HTML page. For simple tasks like displaying the current date, you write a normal HTML page and add only a small amount of Java as a scriptlet. For big tasks like processing a shopping cart, you use JSP as the mediator between the Web form and a component(s) (bean or servlet) that has all the processing logic. Most of the code in a Web application will go into servlets. The JSP portion is a just front end to the application (with little logic of course) that a user can use/navigate comfortably.

We all know that the web (Internet) lives over the Http protocol. The servlet as expected is handle the http requests. Well, what use would a servlet be if it cannot handle the most commonly used protocol in the internet.

A lot of things happen when a servlet is invoked because of some user action in a web page. The sequence of events starts with a browser sending a request to a Web server. The server hands the request to a Servlet Container. The container loads the servlet (if it isn't already loaded), instantiates a request and response objects, and then hands these objects to the servlet by calling first its init() method, then its service() method, and lastly the destroy() method. The service() method will typically call one of the doXXX() methods such as doGet(). The response based on the output of the doXXX() methods will be sent back to the browser and will be displayed on screen.

Below is a diagrammatic representation of how data flows from a browser to the servlet and then back to the browser.

  Let’s retrace the sequence of steps:

1. Web Server receives request from the Browser (Http Request)
2. Web Server checks the request and invokes the appropriate Servlet/JSP in the Servlet Container
3. If this is the first time the servlet is invoked its init() method gets called
4. If not, the service() gets called.
5. The service() method in turn delegates the processing to one of the doXXX() methods based on the request received
6. The output of the doXXX() methods is sent back to the servlet container
7. The container analyzes the response and formats it appropriately
8. If the response is normal a Http Response is sent back to the client
9. Else, an error response is sent back to the client


Now that we know how the requests makes its way to the servlet and then comes back to the client, lets look at a sample Servlet that will actually compile and produce some output on the browser.

Servlet Source Code:

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

public class OurFirstServlet 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 >");
out.println("< title > A simple servlet. ");
out.println("");
out.println("< body >");
out.println("< h1 >Simple Servlet");
out.println("This is a Anand’s First Servlet");
out.println("< / body >");
out.println("< / html >");
}
}

The above is a simple servlet that doesn't do anything great but still displays something on the browser when invoked.

On invocation, the servlet code runs and the doGet() method will generate a response that would be equivalent to the below HTML code. This response will be sent to the browser which in turn will display the contents on screen.

< html >
< head >
< title > A simple servlet. < / title >
< / head >
< body >
< h1 >Simple Servlet< / h1 >
This is a Kinshuk’s First Servlet
< / body >
< / html >