Servlets and Forms

Here we will learn how to extract HTML form parameters from a servlet.

First, let's explain how an HTML form passes stuff back to the server:

What is an HTML form? It's a form that presents text fields, buttons, drop-down lists etc to the user. When a user clicks on buttons, enters text and clicks on, typically, a submit button, all the users choices (and text) need to get back to the servlet.
The information is passed by the browser to the webserver, which then passes the stuff to the appropriate servlet. This information is sometimes called the "form parameters".
Stuff is sent back by the browser in one of two ways:
1. using the GET method and
2. using the POST method.
In the GET method, the form data (parameters) is appended to the URL, as in:
```
      http://www.yahoo.com/search?music
  
```
Here, the text field contains the string "music".
In the POST method, the browser simply sends the form data directly.
When you create an HTML form, you decide whether you want to use GET or POST. The standard practice is to use POST, unless you need to use GET.
When you use GET, the doGet() method of your servlet is called, otherwise the doPost() method is called.

First, the HTML. Create a test HTML form with the following (assuming your assigned port number is 8019 and that the Servlet URL is ../servlets/...)

    <html>
    <head><title>Test Post</title>
    <body>
       <form action="http://localhost:8019/servlets/Testform" method="post">
       Enter a string: <input type="text" name="param1">
       And then press "Go": <input type="submit" value="Go">
       </form>
    </body>
    </html>

NOTE: we will not cover html forms here. For details, see my tutorial on HTML.

Now, create the servlet TestForm.java:

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


       public class TestForm extends HttpServlet {

         public void doPost (HttpServletRequest req, HttpServletResponse resp)
            throws ServletException, IOException
         {
           // Set the content type of the response.
           resp.setContentType ("text/html");

           // Create a PrintWriter to write the response.
           java.io.PrintWriter out = new PrintWriter (resp.getOutputStream());

           // The first part of the response.
           out.println ("");
           out.println (" Test ");
           out.println ("");

           // Now get the parameters and output them back.
           out.println ("Request parameters: ");
           Enumeration e = req.getParameterNames();
           while (e.hasMoreElements()) {
             String name = (String) e.nextElement();
             String value = req.getParameter (name);
             if (value != null)
               out.println (" name=[" + name + "] value=[" + value + "]");
             else
               out.println ("
 name=[" + name + "] did not have a value");
           }

           // Last part.
           out.println ("");
           out.println ("");
           out.close();
         }

        public void doGet (HttpServletRequest req, HttpServletResponse resp)
          throws ServletException, IOException
        {
          doPost (req, resp); 
        }

       }

Compile, copy the class file to the servlets directory and test.