The Document Object Model, DOM, is the second major standard API for XML parsers, and the first tree-based API I’ll consider. Most major parsers implement both SAX and DOM. DOM programs start off similarly to SAX programs, by having a parser object read an XML document from an input stream or other source. However, where the SAX parser returns the document broken up into a series of small pieces, the equivalent DOM method returns an entire Document object that contains everything in the original XML document. You read information from the document by invoking methods on this Document object or on the other objects it contains. This makes DOM much more convenient when random access to widely separated parts of the original document is required. However, it is quite memory intensive compared to SAX, and not nearly as well suited to streaming applications.

A second advantage to DOM is that it is a read-write API. Whereas SAX can only parse existing XML documents, DOM can also create them. Documents created in this fashion are automatically well-formed. Attempting to create a malformed document throws an exception. Example 5.5 is a DOM-based program for connecting to the Fibonacci XML-RPC servlet. The request is formed as a new DOM document. The response is read as a parsed DOM document.

import java.net.*;
import java.io.*;
import org.w3c.dom.*;
import org.apache.xerces.dom.*;
import org.apache.xerces.parsers.*;
import org.apache.xml.serialize.*;
import org.xml.sax.InputSource; 


public class FibonacciDOMClient {

  public final static String DEFAULT_SERVER 
   = "http://www.elharo.com/fibonacci/XML-RPC";  
  
  public static void main(String[] args) {
      
    if (args.length <= 0) {
      System.out.println(
       "Usage: java FibonacciDOMClient number url"
      );
      return;
    }
    
    String server = DEFAULT_SERVER;
    if (args.length >= 2) server = args[1];
      
    try {
        
      // Build the request document
      DOMImplementation impl 
       = DOMImplementationImpl.getDOMImplementation();

      Document request 
       = impl.createDocument(null, "methodCall", null);

      Element methodCall = request.getDocumentElement();
       
      Element methodName = request.createElement("methodName");
      Text text = request.createTextNode("calculateFibonacci");
      methodName.appendChild(text);
      methodCall.appendChild(methodName);
      
      Element params = request.createElement("params");
      methodCall.appendChild(params);
      
      Element param = request.createElement("param");
      params.appendChild(param);
      
      Element value = request.createElement("value");
      param.appendChild(value);
      
      // Had to break the naming convention here because of a 
      // conflict with the Java keyword int
      Element intElement = request.createElement("int");
      Text index = request.createTextNode(args[0]);
      intElement.appendChild(index);
      value.appendChild(intElement);  
      
      // Transmit the request document
      URL u = new URL(server);
      URLConnection uc = u.openConnection();
      HttpURLConnection connection = (HttpURLConnection) uc;
      connection.setDoOutput(true);
      connection.setDoInput(true); 
      connection.setRequestMethod("POST");
      OutputStream out = connection.getOutputStream();
      
      OutputFormat fmt = new OutputFormat(request);
      XMLSerializer serializer = new XMLSerializer(out, fmt);
      serializer.serialize(request);
      
      out.flush();
      out.close();        

      // Read the response
      DOMParser parser = new DOMParser();
      InputStream in = connection.getInputStream();
      InputSource source = new InputSource(in);
      parser.parse(source);
      in.close();
      connection.disconnect();
      
      Document doc = parser.getDocument();
      NodeList doubles = doc.getElementsByTagName("double");
      Node datum = doubles.item(0);
      Text result = (Text) datum.getFirstChild();
      System.out.println(result.getNodeValue());
    }
    catch (Exception e) {
      System.err.println(e); 
    }
  
  } 

}

In DOM the request document is built as a tree. Everything in the document is a node in this tree including not only elements but also text nodes, comments, processing instructions and more. The document serves as a factory for creating the various kinds of node objects. Each node in this tree belongs to exactly one document. After being created the node object is appended to the child list of its parent node.

Once the Document object has been created and populated, it needs to be serialized onto the URLConnection’s output stream. Unfortunately, there is no standard parser independent way to do this in DOM Level 2. This will be added in DOM Level 3. In the meantime, you need to resort to parser-specific classes and methods. Here I’ve used Xerces’s org.apache.xml.serialize package. The basic design is that an XMLSerializer object is connected to an output stream. Options for serialization such as where to place line breaks and what character encoding to use are specified by an OutputFormat object. Here I just used the default OutputFormat. The document is written onto the stream using the XMLSerializer’s serialize() method.

Once the server receives and parses the request, it calculates and transmits its response as an XML document. This document must be parsed to extract the single string we actually want. DOM includes a number of methods and classes to extract particular parts of a document without necessarily walking down the entire tree. The one I use here is the Document class’s getElementsByTagName() method. This returns a NodeList containing one Node object for each element in the input document that has the name double. In this case there’s exactly one of those so it’s extracted from the list. We then get the first child of that node, which happens to be a Text node that contains the value we want. This value is retrieved by the getNodeValue() method.

The first problem with DOM should now be apparent. It’s more than a little complex, even for very simple problems like this one. However, DOM does have an internal logic; and once you become accustomed to it, you’ll find it’s actually not that hard to use. Still, the learning curve is quite steep; and frequent reference to the documentation is a necessity.

The second downside to DOM is that it does not expose as much of the information in an XML document as SAX does. Although the basic content of elements, text, and attributes is well supported by both DOM and SAX, there are many more esoteric aspects of XML documents that SAX provides but DOM does not. These include unparsed entities, notations, attribute types, and declarations in the DTD. Some of this will be provided in DOM Level 3.

The third downside to DOM is that it’s not as complete as SAX. Much of the code in Example 5.5 is actually part of the Xerces parser rather than standard DOM. Such parser specific code is virtually impossible to avoid when programming in DOM because DOM doesn’t give you any way to create a new XML document, create a new parser, or write a Document onto an output stream. All of these have to be provided by the parser. If I were to port Example 5.5 to Crimson or GNU JAXP, I’d have to rewrite about half of it. DOM3 is going to fill in a lot of these holes. However, since DOM3 is still just a working draft with little parser support, I chose to stick to DOM2 for the time being. A few of these holes can also be plugged by JAXP.