Advanced SAX

Invoking the setFeature() and setProperty() methods can result in SAXNotSupportedException s and SAXNotRecognizedException s.

Both of these are also in the org.xml.sax package.

The first, SAXNotSupportedException , indicates that the parser “knows” about the feature or property but doesn’t support it. This is commonly used when a standard property or feature is not yet coded in (such as in alpha or beta versions of parsers). So invoking setFeature("http://xml.org/sax/features/namespaces") on a parser in development might result in a SAXNotSupportedException . The parser recognizes the feature (and probably plans to support it at some point), but doesn’t have the ability to perform the requested processing.

The second exception, SAXNotRecognizedException , commonly occurs when your code uses vendor-specific features and properties, and then you switch out your parser implementations. The new implementation won’t know anything about the other vendor’s features or properties, and will throw a SAXNotRecognizedException .

You should always explicitly catch these exceptions so you can report them, rather than treating them as just another generic SAXException . Otherwise, you end up losing valuable information about what happened in your code. This means you may have to write a bit of extra code, but thus is the price for good exception handling; here’s a slightly updated version of the buildTree() method (detailed originally in Chapter 3) that handles these problems gracefully:

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);

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

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

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

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

    // Parse
    InputSource inputSource = new InputSource(xmlURI);
    reader.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);
  }
}