Servlets and XML: Made for Each Other

Listing 3 shows the section of the code that processes the HTTP parameters:

publicvoidservice(HttpServletRequestrequest,

                      HttpServletResponseresponse)

    throwsIOException, ServletException

  {

    response.setContentType("text/xml");

    PrintWriterout = response.getWriter();

    

    Enumerationkeys;

    Stringkey;

    StringrequestedSubtree = "";



    keys = request.getParameterNames();

    while (keys.hasMoreElements())

    {

      key = (String) keys.nextElement();

      if (key.equalsIgnoreCase("subtree"))

        requestedSubtree = request.getParameter(key);

    }

As in our earlier example, we set the content type to text/xml. Once that's done, we use the HttpServletRequest.getParameterNames method to retrieve all the parameters from the HTTP request.

After we've processed the parameters, we need to find the information the user requested. The information we're using builds a DOM tree from objects; that DOM tree contains the text of a Shakespearean sonnet, along with other information about the sonnet. We'll return a portion of the DOM tree based on the HTTP subtree parameter. Listing 4 shows some of the code that builds the DOM tree:

Documentdoc = null;

  Elementauthor = null;

  Elementlines = null;

  Elementtitle = null;

 

  publicvoidinitialize()

  {

      doc = (Document)Class.

        forName("org.apache.xerces.dom.DocumentImpl").

        newInstance();



    if (doc != null)

      {

        Elementroot = doc.createElement("sonnet");

        root.setAttribute("type", "Shakespearean");

  

        author = doc.createElement("author");

  

        ElementlastName = doc.createElement("last-name");

        lastName.appendChild(doc.createTextNode("Shakespeare"));

        author.appendChild(lastName);

We create an instance of the Java class that implements the DOM Document interface, then we ask that node to create various nodes for us. You could easily rewrite this application to generate the DOM tree by parsing an XML file. To simplify the example (and to lighten my workload) we've defined instance variables that hold the values of the nodes we intend to serve. These values are declared at the top of the class declaration, and are initialized in the initialize method.

Our last step is to deliver the requested part of the DOM tree back to the user. To do this, we use a recursive method, printDOMTree, that processes a node and all of its children. Because the method is recursive, it doesn't matter if we start at the root node of the document or anywhere else in the DOM tree. If the request is for one of the nodes we know about, we pass that node to the printDOMTree method. Otherwise, we pass the Document node. Listing 5 shows this step.

if (requestedSubtree.equalsIgnoreCase("author"))

      printDOMTree(author, out);

    elseif (requestedSubtree.equalsIgnoreCase("lines"))

      printDOMTree(lines, out);

    elseif (requestedSubtree.equalsIgnoreCase("title"))

      printDOMTree(title, out);

    else 

      printDOMTree(doc, out);

If the subtree parameter is author, the results are:

Shakespeare

William

British

1564

1616

If the subtree parameter is title, the results are:

You can see the HTML view of the complete listing or view the Java source file directly.

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