Writing Out a DOM as an XML File

After you have constructed a DOM--either by parsing an XML file or building it programmatically--you frequently want to save it as XML. This section shows you how to do that using the Xalan transform package.

Using that package, you'll create a transformer object to wire a DOMSource to a StreamResult. You'll then invoke the transformer's transform() method to write out the DOM as XML data.

Reading the XML

The first step is to create a DOM in memory by parsing an XML file. By now, you should be getting comfortable with the process.

The following code provides a basic template to start from. (It should be familiar. It's basically the same code you wrote at the start of Chapter 6. If you saved it then, that version should be essentially equivalent to what you see here.)

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

import java.io.*;

public class TransformationApp 
{
  static Document document; 

  public static void main(String argv[])
  {
    if (argv.length != 1) {
      System.err.println (
        "Usage: java TransformationApp filename");
      System.exit (1);
    }

    DocumentBuilderFactory factory =
      DocumentBuilderFactory.newInstance();
    //factory.setNamespaceAware(true);
    //factory.setValidating(true); 

    try {
      File f = new File(argv[0]);
      DocumentBuilder builder =
        factory.newDocumentBuilder();
      document = builder.parse(f);
  
    } catch (SAXParseException spe) {
      // Error generated by the parser
      System.out.println("\n** Parsing error"
        + ", line " + spe.getLineNumber()
        + ", uri " + spe.getSystemId());
      System.out.println("  " + spe.getMessage() );
  
      // Use the contained exception, if any
      Exception x = spe;
      if (spe.getException() != null)
        x = spe.getException();
      x.printStackTrace();

    } catch (SAXException sxe) {
      // Error generated by this application
      // (or a parser-initialization error)
      Exception x = sxe;
      if (sxe.getException() != null)
        x = sxe.getException();
      x.printStackTrace();

    } catch (ParserConfigurationException pce) {
      // Parser with specified options can't be built
      pce.printStackTrace();

    } catch (IOException ioe) {
      // I/O error
      ioe.printStackTrace();
    }
  } // main 
}

Creating a Transformer

The next step is to create a transformer you can use to transmit the XML to System.out.

import javax.xml.transform.Transformer;
import javax.xml.transform.TransformerFactory;
import javax.xml.transform.TransformerException;
import javax.xml.transform.TransformerConfigurationException;

import javax.xml.transform.dom.DOMSource; 

import javax.xml.transform.stream.StreamResult; 

import java.io.*;

Here, you add a series of classes that should now be forming a standard pattern: an entity (Transformer), the factory to create it (TransformerFactory), and the exceptions that can be generated by each. Because a transformation always has a source and a result, you then import the classes necessary to use a DOM as a source (DOMSource) and an output stream for the result (StreamResult).

try {
  File f = new File(argv[0]);
  DocumentBuilder builder = factory.newDocumentBuilder();
  document = builder.parse(f);

   // Use a Transformer for output
  TransformerFactory tFactory =
    TransformerFactory.newInstance();
  Transformer transformer = tFactory.newTransformer();

  DOMSource source = new DOMSource(document);
  StreamResult result = new StreamResult(System.out);
  transformer.transform(source, result);

Here, you create a transformer object, use the DOM to construct a source object, and use System.out to construct a result object. You then tell the transformer to operate on the source object and output to the result object.

In this case, the "transformer" isn't actually changing anything. In XSLT terminology, you are using the identity transform, which means that the "transformation" generates a copy of the source, unchanged.

Note: You can specify a variety of output properties for transformer objects, as defined in the W3C specification at http://www.w3.org/TR/xslt#output. For example, to get indented output, you can invoke
transformer.setOutputProperty(OutputKeys.INDENT, "yes");

Finally, add the following highlighted code to catch the new errors that can be generated:

} catch (TransformerConfigurationException tce) {
  // Error generated by the parser
  System.out.println ("* Transformer Factory error");
  System.out.println("  " + tce.getMessage() );

   // Use the contained exception, if any
  Throwable x = tce;
  if (tce.getException() != null)
    x = tce.getException();
  x.printStackTrace(); 
} catch (TransformerException te) {
  // Error generated by the parser
  System.out.println ("* Transformation error");
  System.out.println("  " + te.getMessage() );

  // Use the contained exception, if any
  Throwable x = te;
  if (te.getException() != null)
    x = te.getException();
  x.printStackTrace();

} catch (SAXParseException spe) {
  ...

TransformerExceptions are thrown by the transformer object.

TransformerConfigurationExceptions are thrown by the factory.

To preserve the XML document's DOCTYPE setting, it is also necessary to add the following code:

import javax.xml.transform.OutputKeys; ... if (document.getDoctype() != null){ String systemValue = (new File(document.getDoctype().getSystemId())).getName(); transformer.setOutputProperty( OutputKeys.DOCTYPE_SYSTEM, systemValue ); }

Writing the XML

For instructions on how to compile and run the program, see Compiling and Running the Program from the SAX tutorial, Chapter 5. (If you're working along, substitute TransformationApp for Echo as the name of the program. If you are compiling the sample code, use TransformationApp02.) When you run the program on slideSample01.xml, this is the output you see:

<?xml version="1.0" encoding="UTF-8"?>
<!-- A SAMPLE set of slides -->
<slideshow author="Yours Truly" date="Date of publication"
title="Sample Slide Show">

  <!-- TITLE SLIDE -->
  <slide type="all">
    <title>Wake up to WonderWidgets!</title>
  </slide>

  <!-- OVERVIEW -->
  <slide type="all">
    <title>Overview</title>
    <item>Why <em>WonderWidgets</em> are great</item>
    <item/>
    <item>Who <em>buys</em> WonderWidgets</item>
  </slide>

</slideshow>

Note: The order of the attributes may vary, depending on which parser you are using.

Writing Out a Subtree of the DOM

It is also possible to operate on a subtree of a DOM. In this section, you'll experiment with that option.

The only difference in the process is that now you will create a DOMSource using a node in the DOM, rather than the entire DOM. The first step is to import the classes you need to get the node you want. Add the following highlighted code to do that:

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

The next step is to find a good node for the experiment. Add the following highlighted code to select the first <slide> element:

try {
  File f = new File(argv[0]);
  DocumentBuilder builder = factory.newDocumentBuilder();
  document = builder.parse(f);

  // Get the first <slide> element in the DOM
  NodeList list = document.getElementsByTagName("slide");
  Node node = list.item(0);

Then make the following changes to construct a source object that consists of the subtree rooted at that node:

DOMSource source = new DOMSource(document);
DOMSource source = new DOMSource(node);
StreamResult result = new StreamResult(System.out);
transformer.transform(source, result);

<?xml version="1.0" encoding="UTF-8"?>
<slide type="all">
    <title>Wake up to WonderWidgets!</title>
  </slide>

Cleaning Up

Because it will be easiest to do now, make the following changes to back out the additions you made in this section. (TransformationApp04.java contains these changes.)

Import org.w3c.dom.DOMException;
import org.w3c.dom.Node;
import org.w3c.dom.NodeList; 
...
  try {
    ...
    // Get the first <slide> element in the DOM
    NodeList list = document.getElementsByTagName("slide");
    Node node = list.item(0);
    ...
    DOMSource source = new DOMSource(node);
    StreamResult result = new StreamResult(System.out);
    transformer.transform(source, result);

Summary

At this point, you've seen how to use a transformer to write out a DOM and how to use a subtree of a DOM as the source object in a transformation. In the next section, you'll see how to use a transformer to create XML from any data structure you are capable of parsing.