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This chapter is from the book

This chapter is from the book

Parsing with the Document Object Model (DOM)

There is another approach to parsing XML documents called the Document Object Model (DOM) approach. This API produces an in-memory treelike version of the XML document, as well as a number of methods for traversing the model and obtaining the data stored in it.

There are many nice things about the DOM approach from a processing point of view. The first is that it is more intuitive than the SAX approach. As object-oriented programmers, we like being given an object structure. In addition, we can make changes to this model and persist these changes back to the XML document as output. SAX 2.0 allows this as well. This opens a world of possibilities where you can use the XML document as a sort of database.

Following the same approach that we used with SAX, we will show an example and then explain the whys and what fors of the code.

We will solve the same problem with DOM as we did with SAX—translating an XML document into an object. The code for this example is shown in Listing 3.4. The DOM approach creates a treelike structure in memory representing the XML document.

Listing 3.4 The Document Object Model Approach

/*
 * TicketRequestDOMParser.java
 *
 * Created on January 19, 2002, 5:33 PM
 */

package unleashed.ch3;

import unleashed.TicketRequest2;
import javax.xml.parsers.DocumentBuilder;
import javax.xml.parsers.DocumentBuilderFactory;
import javax.xml.parsers.FactoryConfigurationError;
import javax.xml.parsers.ParserConfigurationException;

import org.xml.sax.SAXException;
import org.xml.sax.SAXParseException;

import java.io.*;

import org.w3c.dom.Document;
import org.w3c.dom.DOMException;
import org.w3c.dom.Node;
import org.w3c.dom.*;

/**
 *
 * @author Stephen Potts
 * @version
 */
public class TicketRequestDOMParser
{
  static Document document;
  TicketRequest2 tr2;
  String eleName;
  
   
   public TicketRequestDOMParser()
   {
     DocumentBuilderFactory factory =
        DocumentBuilderFactory.newInstance();
     try
     {
       tr2 = new TicketRequest2();
       DocumentBuilder builder = factory.newDocumentBuilder();
       document = builder.parse( new File(
       "C:/unleashed/ch3/ticketRequest.xml"));
       traverse(document);
     }catch (SAXException sxe)
     {
       Exception e = sxe;
       if (sxe.getException() != null)
         e = sxe.getException();
       e.printStackTrace();
     }
     catch (ParserConfigurationException pce)
     {
       pce.printStackTrace();
     }
     catch (IOException ioe)
     {
       ioe.printStackTrace();
     }
   }
   
   private void traverse(Node cNode)
   {
     switch (cNode.getNodeType() )
     {
       case Node.DOCUMENT_NODE:
         System.out.println("Element " + cNode.getNodeName());
         processChildren( cNode.getChildNodes());
         break;
         
       case Node.ELEMENT_NODE:
         eleName = cNode.getNodeName();
         System.out.println("Element " + eleName);
         NamedNodeMap attributeMap = cNode.getAttributes();
         int numAttrs = attributeMap.getLength();
         for (int i=0; i<attributeMap.getLength(); i++)
         {
           Attr attribute = (Attr)attributeMap.item(i);
           String attrName = attribute.getNodeName();
           String attrValue = attribute.getNodeValue();
           storeElementValue(attrName, attrValue);
         }
         if (eleName.equals("isCommissionable"))
         {
           storeElementValue("isCommissionable", "");
         }
         
         processChildren( cNode.getChildNodes());
         break;
       case Node.CDATA_SECTION_NODE:
       case Node.TEXT_NODE:
         
         System.out.println("Text " + cNode.getNodeValue());
         if (! cNode.getNodeValue().trim().equals(""))
         {
           System.out.println("eleName " + eleName);
           System.out.println("Text " + cNode.getNodeValue());
           storeElementValue(eleName, cNode.getNodeValue());
         }
         break;
     }
   }
   
   private void processChildren(NodeList nList)
   {
     if(nList.getLength() != 0)
     {
       for (int i=0; i<nList.getLength(); i++)
         traverse(nList.item(i));
     }
   }
   
   
   private void storeElementValue(String elementName,
                   String elementValue)
   {
     if (elementName.equals("ticketRequest"))
     {
     }
     
     if (elementName.equals("custID"))
     {
       tr2.setCustID(Integer.parseInt(elementValue));
     }
     
     if (elementName.equals("lastName"))
     {
       tr2.setLastName(elementValue);
     }
     
     if (elementName.equals("firstName"))
     {
       tr2.setFirstName(elementValue);
     }
     
     if (elementName.equals("cruiseID"))
     {
       tr2.setCruiseID(Integer.parseInt(elementValue));
     }
     
     if (elementName.equals("destination"))
     {
       tr2.setDestination(elementValue);
     }
     
     if (elementName.equals("port"))
     {
       tr2.setPort(elementValue);
     }
     
     if (elementName.equals("sailing"))
     {
       tr2.setSailing(elementValue);
     }
     
     if (elementName.equals("numberOfTickets"))
     {
       String numberOfTicketsString = elementValue;
       int numberOfTickets = 
          Integer.parseInt(numberOfTicketsString);
       tr2.setNumberOfTickets(numberOfTickets);
     }
     
     if (elementName.equals("isCommissionable"))
     {
       tr2.setCommissionable(true);
     }
   }
   
   public String toString()
   {
     return tr2.toString();
   }
   
   
   public static void main(String args[])
   {
     TicketRequestDOMParser tp = new TicketRequestDOMParser();
     System.out.println(tp);
   }
   
}

This program contains quite a few lines that require some explanation.

There are three main variables in this program:

  static Document document;
  TicketRequest2 tr2;
  String eleName;

The stars of the show are the document object, which holds the DOM structure; the eleName, which contains the name of whatever element we are working on at the moment; and the TicketRequest2, which is the product of our efforts.

The good news about DOM is that tree building is automatic. All that you have to do is create a factory, which creates a builder. The builder's parse method is called, which translates the XML document into the DOM tree.

DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
tr2 = new TicketRequest2();
DocumentBuilder builder = factory.newDocumentBuilder();
document = builder.parse(
  new File("C:/projects/sampledir/unleashed/ch3/ticketRequest.xml"));

Once we have created a DOM tree, we call the traverse() method and pass the document into it:

traverse(document);

The traverse() method is a big switch statement that contains three main cases: document, element, and text. Whenever an element is encountered, its attributes are processed with the following code:

          Attr attribute = (Attr)attributeMap.item(i);
           String attrName = attribute.getNodeName();
           String attrValue = attribute.getNodeValue();
           storeElementValue(attrName, attrValue);

The isCommissionable switch requires special treatment because it has no value. Because it is a flag, its presence alone tells you that the boolean needs to be set to true.

         if (eleName.equals("isCommissionable"))
         {
           storeElementValue("isCommissionable", "");
         }

Whenever we find either an attribute or element text that corresponds to a field in our object, we make a call and pass in two strings:

           storeElementValue(eleName, cNode.getNodeValue());

Because some nodes contain other nodes, we have to process our tree recursively. The processChildren() method takes in the children of a node in a list. It tests to see whether any children really do exist. If so, it calls the traverse() method on that branch of the tree:

   private void processChildren(NodeList nList)
   {
     if(nList.getLength() != 0)
     {
       for (int i=0; i<nList.getLength(); i++)
         traverse(nList.item(i));
     }
   }

The main method of this program is very simple. It instantiates the object and prints the result:

  TicketRequestDOMParser tp = new TicketRequestDOMParser();
     System.out.println(tp);

The result of running this example is shown here:

------------------------------------------------
custID = 10003
lastName = Carter
firstName = Joseph
------------------------------------------------
cruiseID = 3004
destination = Hawaii
port = Honolulu
sailing = 7/7/2001
numberOfTickets = 5
isCommissionable = true
------------------------------------------------

Prior to seeing this output, you will see some tracing information to show you some of the program flow. (It is not shown here.) The TicketRequest object is shown via its toString() method.

You can see from this that it is a relatively straightforward process to transform an XML file into a Java object. Once you have a Java object in your program, you are back in familiar territory.

How do we move XML documents between servers to support distributed computing? That is the topic of the next section.

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