Java Servlets are the JEE way of doing anything web based. They are pretty easy to write. Here's short example to read a query parameter, and display it.

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

public class ExampleServlet extends HttpServlet {
  public void doGet(HttpServletRequest request, 
	 HttpServletResponse response)
        throws ServletException, IOException
  {
    PrintWriter out = response.getWriter();
    String DATA = request.getParameter("DATA");

    if(DATA != null){
      out.println(DATA);
    } else {
      out.println("No text entered.");
    }
    out.close();
  }
}

And don't forget to set it up in web.xml:

<web-app xmlns="http://java.sun.com/xml/ns/j2ee" version="2.4"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http:/java.sun.com/dtd/web-app_2_3.dtd">
  <servlet>
    <servlet-name>example</servlet-name>
    <servlet-class>ExampleServlet</servlet-class>
  </servlet>

  <servlet-mapping>
    <servlet-name>example</servlet-name>
    <url-pattern>/example</url-pattern>
  </servlet-mapping>
</web-app>

All pretty straight forward. What if we could harness this technique in the Ruby world? Rack is a minimal framework to make talking to a Ruby webserver nicer, and is pretty flexible and great to build frameworks on top of. Imagine if you would, an API like Java Servlets... but implemented on rack....

I present to you... racklets! Let's start by making a new ruby file, and putting it in a well known load path: WEB-INF/lib. So, we'll make WEB-INF/lib/example_racklet.rb :

require 'ragamuffin'
class ExampleRacklet < Ragamuffin::Racklet
  def do_get(request, response)
    data = request.params['DATA']
    if data
      response.write data
    else
      response.write 'No data'
    end
  end
end

Next we need a web.xml to tell racklets what to load, and what urls to map it. This is kept in WEB-INF/web.xml:

<web-app>
  <racklet>
    <racklet-name>example</racklet-name>
    <racklet-class>ParamRacklet</racklet-class>
  </racklet>

  <racklet-mapping>
    <racklet-name>example</racklet-name>
    <url-pattern>/example</url-pattern>
  </racklet-mapping>
</web-app>

Since we're on rack, you'll want a config.ru to run our ragamuffin web application.

require 'ragamuffin'
run Ragamuffin::WebApplication.new

This handles loading up the web.xml and updating $LOAD_PATH to include the WEB-INF/lib directory. So now we're in business to something like shotgun to run our server:

$ shotgun 
== Shotgun starting Rack::Handler::Mongrel on localhost:9393

Alternatively, you can use the included ragamuffin script to run it without a config.ru file.

$ bin/ragamuffin 
== Ragamuffin starting Rack::Handler::Mongrel on localhost:9393

Now go ahead and browse to http://localhost:9393/example and you'll see:

No Data

Give it the DATA parameter, and you'll see it displayed, ie http://localhost:9393/example?DATA=Hello%20World

Hello World