Weird behaviour of Java XPath resolver for documents with namespaces

descendant-or-self, java, xml, xml-namespaces, xpath

Solution

I tried your example with a JDK 1.7.0.51 and can confirm your results. This seems a bit strange at first, but the `DocumentBuilderFactory`'s default behaviour is to be not namespace aware.

So you have to turn it on at first:

factory.setNamespaceAware(true);

Then for the second document there are no results for the XPath expressions as expected.

You have to change your expressions to: `/Document/file:FileFormat` and `//file:FileFormat`. At a last step you have to register a `NamespaceContext` implementation, which maps the namespace prefixes used in your XPath expressions to namespace URIs. Unfortunately, there is no default implementation.

public String resolveXPath(File xmlFile, String xpathExpr) throws XPathException, ParserConfigurationException, SAXException, IOException, XPathExpressionException
{
    DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();

    // Turn namespace aware on
    factory.setNamespaceAware(true);
    DocumentBuilder builder = factory.newDocumentBuilder();
    Document doc = builder.parse(xmlFile);

    XPathFactory xPathfactory = XPathFactory.newInstance();
    XPath xpath = xPathfactory.newXPath();

    // Set the NamespaceContext        
    xpath.setNamespaceContext(new MyNamespaceContext());


    XPathExpression expr = xpath.compile(xpathExpr);

    return (String) expr.evaluate(doc, XPathConstants.STRING);
}

class MyNamespaceContext implements NamespaceContext {

  private Map<String, String> ns;
  private Map<String, String> nsReverted;

  public MyNamespaceContext() {
    ns = new TreeMap<String, String>();

    // Default namespaces and prefixes according to the documentation
    ns.put(XMLConstants.DEFAULT_NS_PREFIX, XMLConstants.NULL_NS_URI);
    ns.put(XMLConstants.XML_NS_PREFIX, XMLConstants.XML_NS_URI);
    ns.put(XMLConstants.XMLNS_ATTRIBUTE, XMLConstants.XMLNS_ATTRIBUTE_NS_URI);

    // Now our self defined namespace
    ns.put("file", "http://www.example.com/xml/file");


    nsReverted = new TreeMap<String, String>();
    for(Entry<String, String> entry : ns.entrySet()) {
      nsReverted.put(entry.getValue(), entry.getValue());
    }
  }

  @Override
  public String getNamespaceURI(String prefix) {
    if(prefix == null) {
      throw new IllegalArgumentException();
    } 
    final String uri = ns.get(prefix);
    return uri == null ? XMLConstants.NULL_NS_URI : uri;
  }

  @Override
  public String getPrefix(String namespaceURI) {
    if(namespaceURI == null) {
      throw new IllegalArgumentException();
    } 
    return nsReverted.get(namespaceURI);
  }

  @Override
  public Iterator getPrefixes(String namespaceURI) {
    return ns.keySet().iterator();
  }

}

Problem

Original source