Servlets

Instead of running a tool to transform an XML document into HTML, you could get a webserver to run a program that does the transformation.
Use a CGI script, a PHP script, a servlet using SAX, or a JavaServer page.
Later, we will do this using a tag library in a JavaServer page, but first it is useful to have a detour introducing the ideas of servlets, JavaServer pages and JavaBeans.
A servlet is a WWW page that is written in Java.
The author of the servlet has to compile the servlet.
When a browser accesses the URL of the servlet, the bytecodes of the servlet will be run.
This will done either by the webserver itself or the webserver will get some other program to arrange for the servlet to be run.
This program just starts a new thread to run the bytecodes of the servlet. So, no new process needs to be created to run the servlet.
The servlet does some work and generates some HTML.
The webserver gets the HTML from the servlet and passes that to the visitor's browser.

Suppose we want a WWW page that uses a form to get data from a visitor, and some Java source code that gets the data from the form and generates some HTML.

Here are the HTML instructions of a WWW form:

<HTML>
   <BODY>
      <FORM METHOD="POST" ACTION=
         "http://altair.dur.ac.uk:8080/barry.cornelius/servlet/choice">
         Please enter your choice<BR>
         <INPUT TYPE="text" NAME="choice">
         <BR>
         <INPUT TYPE="submit" VALUE="Submit choice">
         <INPUT TYPE="reset">
      </FORM>
   </BODY>
</HTML>

Here is the Java source code of a servlet:

import javax.servlet.http. HttpServlet;
import javax.servlet.http. HttpServletRequest;
import javax.servlet.http. HttpServletResponse;
import java.io.            IOException;
import java.io.            PrintWriter;
import javax.servlet.      ServletException;
public class choice extends HttpServlet
{
   public void doPost(final HttpServletRequest request,
                      final HttpServletResponse response)
         throws ServletException, IOException
   {      
      final String tChoiceString = request.getParameter("choice");
      response.setContentType("text/html");
      final PrintWriter tResponsePrintWriter = response.getWriter();
      StringBuffer tStringBuffer = new StringBuffer();
      tStringBuffer.append("<html>\n" );
      tStringBuffer.append("<title>Reply</title>\n" );
      tStringBuffer.append("Your choice was: " + tChoiceString + "\n" );
      tStringBuffer.append("</html>\n" );
      tResponsePrintWriter.println(tStringBuffer);
      tResponsePrintWriter.close();
   }
}

Here the class is derived from javax.servlet.http.HttpServlet and it overrides the doPost method of that class.
When doPost is called, it is passed two arguments: request enables access to the values on the form, and response provides a means of passing back HTML to the webserver.
In order for this to work, you need a webserver that understands servlets.
One possiblity is to use tomcat, a program provided by the Apache Software Foundation, or to use Apache's webserver augmented by tomcat.
Although I have successfully configured Apache's webserver to use tomcat, I have chosen not to put this into active use.
This is because a badly written servlet could be cpu-bound leading to poor performance of the computer running the webserver.
Instead, at the University of Durham, we use tomcat on its own (i.e., without Apache's webserver) on a different computer, a multi-processor machine called altair.dur.ac.uk.
The above form can be accessed by going to: http://www.dur.ac.uk/barry.cornelius/tomcat/choice/choice.html.
We could instead provide source code that uses the SAX API to generate HTML from an XML document.
Could use code like that given earlier in ConvertConsumables.java and Handler.java.
In this way, we could provide a servlet that automatically generates HTML from XML.