Extension Interfaces and SAX

Extension Interfaces and SAX - XMLWriter
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Now that you understand how filters work in SAX, I want to introduce you to a specific filter, XMLWriter. This class, as well its subclass, DataWriter, can be downloaded from David Megginson’s site at http://www.megginson.com/Software.

David Megginson shepherded SAX through its early days and has now returned to the fold. David is a SAX guru, and even though he no longer actively works on XMLWriter (or DataWriter ), he has created some incredibly useful classes, and still hosts them on his personal web site.

XMLWriter extends XMLFilterImpl , and DataWriter extends XMLWriter . Both of these filter classes are used to output XML, which may seem a bit at odds with what you’ve learned so far about SAX. However, it’s not that unusual; you could easily insert statements into a startElement() or characters() callback that fires up a java.io.Writer and outputs to it. In fact, that’s awfully close to what XMLWriter and DataWriter do.

I’m not going to spend a lot of time on this class, because it’s not really the way you want to be outputting XML in the general sense; it’s much better to use DOM, JDOM, or another XML API if you want mutability. However, the XMLWriter class offers a valuable way to inspect what’s going on in a SAX pipeline. By inserting it between other filters and readers in your pipeline, it can be used to output a snapshot of your data.

For example, in the case where you’re changing namespace URIs, it might be that you want to actually store the XML document with the new namespace URI(be it a modified O’Reilly URI, an updated XSL URI, or whatever other use-case you come up with). This is a piece of cake with the XMLWriter class. Since you’ve already got SAXTreeViewer using the NamespaceFilter , I’ll use that as an example. First, add import statements for java.io.Writer (for output), and the com.megginson.sax.XMLWriter class. Once that’s in place, you’ll need to insert an instance of XMLWriter between the NamespaceFilter and the XMLReader instances; this means output will occur after namespaces have been changed but before the visual events occur:

  public void buildTree(DefaultTreeModel treeModel ,
                      DefaultMutableTreeNode base, String xmlURI)
    throws IOException, SAXException {

    String featureURI = "";
    XMLReader reader = null;

    try {
      // Create instances needed for parsing
      reader = XMLReaderFactory.createXMLReader();
      JTreeHandler jTreeHandler =
        new JTreeHandler(treeModel, base);

      XMLWriter writer = new XMLWriter(reader, new FileWriter("snapshot.xml"));
      NamespaceFilter filter = new NamespaceFilter(writer, 
              "http://www.oreilly.com",
              "http://safari.oreilly.com");

      // Register content handler
      filter.setContentHandler(jTreeHandler);

      // Register error handler
      filter.setErrorHandler(jTreeHandler);

      // Register entity resolver
      filter.setEntityResolver(new SimpleEntityResolver());

      // Register lexical handler
      filter.setProperty("http://xml.org/sax/properties/lexical-handler",
                         jTreeHandler);

      // Turn on validation
      featureURI = "http://xml.org/sax/features/validation";
      filter.setFeature(featureURI, true);

      // Turn on schema validation, as well
      featureURI = "http://apache.org/xml/features/validation/schema";
      filter.setFeature(featureURI, true);

      // Parse
      InputSource inputSource = new InputSource(xmlURI);
      filter.parse(inputSource);
    } catch (SAXNotRecognizedException e) {
      System.err.println("The parser class " + reader.getClass().getName() +
        " does not recognize the feature URI '" + featureURI + "'");
      System.exit(-1);
    } catch (SAXNotSupportedException e) {
      System.err.println("The parser class " + reader.getClass().getName() +
        " does not support the feature URI '" + featureURI + "'");
      System.exit(-1);
    }
  }

Be sure you set the parent of the NamespaceFilter instance to be the XMLWriter , not the XMLReader . Otherwise, no output will actually occur.

Once you’ve got these changes compiled in, run the example. You should get a snapshot.xml file created in the directory from which you’re running the example. Both XMLWriter and DataWriter offer a lot more in terms of methods to output XML, both in full and in part, and you should check out the Javadoc included with the down loaded package.

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