First Servlet Example Login Screen Validation

After going through the introduction and architecture of servlets, let us write a simple Login Validation program. As you have seen in the architecture, there will be two programs – one running on client machine and the other on server.

Servlet Example explained in 10 steps

First let us write the client-side program.

I. Client Program – UserPass.html

Login Validation



Enter User Name  

Enter Password   




 

<form method="get" action="http://localhost:8888/india/roses">

• localhost: It is the system where Tomcat server is running. Here, localhost is treated as the server. In realtime, actual name of server comes.
• 8888: It is the port number on which Tomcat is running.
• india: It is the name of the folder you created and placed in web-app folder.
• roses: It is the alias name of the Validation servlet given web.xml file.

Above steps are clearly explained in Step-by-Step Execution of Servlet in Tomcat.

II. Servlet program – Validation.java

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

public class Validation extends HttpServlet
{
  public void service(HttpServletRequest req, HttpServletResponse res) throws ServletException, IOException
  {
    res.setContentType("text/html");  
    PrintWriter out = res.getWriter();
                                            // extract the user name and password from req object sent by client
    String str1 = req.getParameter("t1");
    String str2 = req.getParameter("t2");
                                            // some validation code
    if(str1.equals("snrao") && str2.equals("java"))
    {
      out.println("VALID");
    }
    else
    {
      out.println("INVALID");
    }

    out.close();
  }
}

Being the first Servlet Example, let us have a detailed discussion on each line.

1. import javax.servlet.*;

The above package is imported for the sake of class ServletException.

2. import javax.servlet.http.*;

The above package is imported for the abstract class
HttpServlet and interfaces HttpServletRequest and HttpServletResponse.

3. import java.io.*;

java.io package is needed for PrintWriter and IOException.

4. public class Validation extends HttpServlet

To write a servlet, it is required to extend the abstract class HttpServlet, like Applet is required to extend to write an applet.

Let us see the hierarchy of HttpServlet.

From the above hierarchy, it can be understood that to develop a servlet there are three styles.

a) by implementing Servlet interface
b) by extending GenericServlet
c) by extending HttpServlet

The disadvantage of first style is, it is required to override all the abstract methods of the interface Servlet eventhough we are not interested (like the interface WindowListener to close a frame) in a few. The better approach is extending GenericServlet (like WindowAdapter to close the frame) which contains only one abstract method service(). It is enough to the programmer to override only this method. It is a callback method. GenericServlet was used by the programmers when HTTP protocol was not standardized for the Web. When the HTTP became a defacto standard for Web, HttpServlet was designed to suit more for HTTP protocol. HttpServlet is an abstact class but without any abstract methods (all are non-abstract methods). Even the service() method is non-abstract and infact with HttpServlet, the service() method can be replaced by the methods doGet() or doPost(). This we will see later.

The above code works fine even if GenericServlet is extended instead of HttpServlet.

5. public void service(HttpServletRequest req, HttpServletResponse res) throws ServletException, IOException

service() method is a callback method called automatically when the container loads the servlet for execution when the client request comes. The method takes two parameters – an object of interface HttpServletRequest and an object of interface HttpServletResponse and let us see what these interfaces meant to servlet.

a) HttpServletRequest interface

Following is the hierarchy.

The first parameter can be either of ServletRequest or its sub-interface HttpServletRequest. We use ServletRequest as parameter when we extend GenericServlet and HttpServletRequest when HttpServlet is extended. The data send by the client (through HTML) finally reaches HttpServletRequest object after travelling intermittent stages. That is, req object of HttpServletRequest receives data sent by client. From this object, the servlet programmer can extract the data and can use for validation or other purposes as hereunder.

b) HttpServletResponse interface

The second parameter can be either of ServletResponse or its sub-interface HttpServletResponse. We use ServletResponse as parameter when we extend GenericServlet and HttpServletResponse when HttpServlet is extended. The responsibility of the data to be sent (like valid or invalid etc.) to the client is taken care by HttpServletResponse. The programmer job is simply to pass the response messages (to be reached to client) to HttpServletResponse and actual sending is taken care by the container.

c) The service() method throws two checked exceptions of javax.servlet.ServletException and java.io.IOException.

6. res.setContentType("text/html");

setContentType(String) is a method of ServletResponse inherited by HttpServletResponse and its job is sending the information to client’s browser of how the response sent by the servlet is to be formatted. It is here is text or HTML. That is, the data sent to the client is either in text format (as in ("VALID")) or HTML format (as in (<b>INVALID</b>)). The difference is HTML <b> tag. This is more called as setting the MIME (Multipurpose Internet Mail Extension) type. The other types can be of image/gif etc. which we use later.

7. PrintWriter out = res.getWriter();
8. out.println("VALID");

We know earlier the HttpServletResponse is meant to send data (known as response) to client. Being an interface, it cannot be used directly. As a convenience, the getWriter() method of ServletResponse (inherited by HttpServletResponse) returns an object of PrintWriter. println() is a method of PrintWriter used to attach a message, here, as a string parameter. This string message (here, VALID OR INVALID) is ultimately sent to client by the HttpServletResponse.

9. String str1 = req.getParameter("t1");

We know the data sent by client (known as request) finally reaches HttpServletRequest. getParameter(String) method of ServletRequest (inherited by HttpServletRequest) takes a string parameter and returns a string. The parameter is nothing but the <FORM> field name (here, t1) and the returned string value is what user entered in field t1. Using getParameter() method, the programmer extracts all the data sent by the client. The other way is getParameterNames() where all the data is extracted in a single stretch in a for loop.

10. out.close();

When the job is over, it is customary to close the streams as you have done in file copying.

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