Java Servlets

What are Java servlets?

Java's solution to CGI programs.

Wrong way to use Java as CGI:

Write stand-alone Java program.
Invoke it via new CGI shell process.

The right way:
Use servlets and a servlet-compatible server.

How it works:

Browser makes a servlet request to webserver.
Webserver recognizes the URL as a servlet URL.
(e.g., http://www.obvious.com/servlets/blah)
Webserver loads servlet if not already loaded.
Servlet executes and delivers output to webserver (HTML output).
Webserver delivers output to Browser.

What you need to write servlets:

Either JDK 1.1 and JSDK (Java Servlet Development Kit).
JDK 1.2
A servlet-compatible webserver.

Example: HelloWorld servlet

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

      public class helloworld extends HttpServlet {

        public void doGet (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 ("");

          // The greeting.
          out.println ("Yo, Hello World!");

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

          // Screen I/O
          System.out.println ("Inside servlet ... servlet complete");
        }

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

      }

So, what are GET and POST?

When Form data is sent by the browser, it can be sent 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 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.
When you use GET, the doGet() method of your servlet is called, otherwise the doPost() method is called.
The standard practice is to use POST, unless you need to use GET.
You need to use GET when you want a link to invoke a CGI program (servlet).

Your First Servlet - HelloWorld

Now let's write Java - a simple servlet that writes "Hello World!" to the browser: (source)

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

      public class HelloWorld extends HttpServlet {

        public void doGet (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 ("");

          // The greeting.
          out.println ("Yo, Hello World!");

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

          // Screen I/O
          System.out.println ("Inside servlet ... servlet complete");
        }

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

      }

Note:

The servlet class libraries need to be imported, along with java.io.PrintWriter.
There are two methods, one for each type of request: GET or POST.
You can decide not to "really" implement one method and instead have it call the other.
There are two object parameters to each method, the "request" and the "respponse".
The HttpServletResponse instance has an OutputStream that is used to write directly to the requesting browser.

To make writing easier, we wrap a PrintWriter around the OutputStream instance:

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

We set the content-type (as required by the HTTP protocol):

          // Set the content type of the response.
          resp.setContentType ("text/html");

We write HTML to the output, e.g.,

          out.println ("");
          out.println (" Test ");

Don't forget to close the output stream:
```
          out.close();
  
```
Optionally, for debugging, we can also write to the local screen where the webserver is running:
```
          System.out.println ("Inside servlet ... servlet complete");
  
```

with your assigned port number.

Extracting Parameters in a Servlet

Next, let us create an HTML Form, and have a servlet pick up the Form data entered by the user.

First, an HTML with a form:

    <html>
    <head><title>Test Post</title>
    <body>
       <form action="http://localhost:8502/servlets/Testform" method="post">
       Enter a string: <input type="text" name="param1">
       And then press "Go": <input type="submit" value="Go">
       </form>
    </body>
    </html>

Next, the TestForm.java servlet: (source)

       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); 
        }

       }

Note:

We need to import java.util.Enumeration.
Whatever parameters were provided are all in the HttpServletRequest instance.
We can list these parameters by getting an Enumeration instance from the request by calling getParameterNames():
```
           Enumeration e = req.getParameterNames();
  
```
The "name" of a parameter is really the string in the name attribute of the tag for the particular Form element.

For example, the name string is param1 below.

       Enter a string: <input type="text" name="param1">

Now, if we want to retrieve the actual string typed in by the user, we use that name in getParameter():
```
             String whatTheUserTyped = req.getParameter ("param1");
  
```