StAX and namespaces - stax

Question

I am trying to convert some code from using DOM (via jDOM) to use StAX instead. At the same time I am migrating from DTD-based validation to XSD_based validation. Oh, and just for good measure I am introducing JAXB into the equation :)
Anyway, as an interim migration step I would like to allow users to still provide legacy documents (aka, using DTD and therefore no namespace). I will still validate the document using XSD, so the DTD is ignored. This works except that StAX (nor JAXB) seems to not like the non-namespaced document. I tried disabling namespace support (using javax.xml.stream.isNamespaceAware), but that did not have any effect. Explicitly adding xmlns to the document root fixed the problem, so I am fairly confident it is a namespacing issue.
Is there a way using StAX XMLEventReader to "introduce" a default namespace? Something along the lines of this approach (which is SAX specific), but for StAX...
Or any other ideas on how to achieve that?
An example document looks like:
<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC
"-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">
<hibernate-mapping package="org.hibernate.test.abstractembeddedcomponents.cid">
...
</hibernate-mapping>
The code I am currently using to read these documents is:
public JaxbRoot unmarshal(InputStream stream, Origin origin) {
try {
XMLEventReader staxReader = staxFactory().createXMLEventReader( stream );
try {
return unmarshal( staxReader, origin );
}
finally {
try {
staxReader.close();
}
catch ( Exception ignore ) {
}
}
}
catch ( XMLStreamException e ) {
throw new MappingException( "Unable to create stax reader", e, origin );
}
}
private XMLInputFactory staxFactory;
private XMLInputFactory staxFactory() {
if ( staxFactory == null ) {
staxFactory = buildStaxFactory();
}
return staxFactory;
}
#SuppressWarnings( { "UnnecessaryLocalVariable" })
private XMLInputFactory buildStaxFactory() {
XMLInputFactory staxFactory = XMLInputFactory.newInstance();
// tried with and without, no effect
//staxFactory.setProperty( "javax.xml.stream.isNamespaceAware", false );
return staxFactory;
}
#SuppressWarnings( { "unchecked" })
private JaxbRoot unmarshal(XMLEventReader staxEventReader, final Origin origin) {
XMLEvent event;
try {
event = staxEventReader.peek();
while ( event != null && !event.isStartElement() ) {
staxEventReader.nextEvent();
event = staxEventReader.peek();
}
}
catch ( Exception e ) {
throw new MappingException( "Error accessing stax stream", e, origin );
}
if ( event == null ) {
throw new MappingException( "Could not locate root element", origin );
}
final Schema validationSchema;
final Class jaxbTarget;
final String elementName = event.asStartElement().getName().getLocalPart();
if ( "entity-mappings".equals( elementName ) ) {
final Attribute attribute = event.asStartElement().getAttributeByName( ORM_VERSION_ATTRIBUTE_QNAME );
final String explicitVersion = attribute == null ? null : attribute.getValue();
validationSchema = validateXml ? resolveSupportedOrmXsd( explicitVersion ) : null;
jaxbTarget = JaxbEntityMappings.class;
}
else {
validationSchema = validateXml ? hbmSchema() : null;
jaxbTarget = JaxbHibernateMapping.class;
}
final Object target;
final ContextProvidingValidationEventHandler handler = new ContextProvidingValidationEventHandler();
try {
JAXBContext jaxbContext = JAXBContext.newInstance( jaxbTarget );
Unmarshaller unmarshaller = jaxbContext.createUnmarshaller();
unmarshaller.setSchema( validationSchema );
unmarshaller.setEventHandler( handler );
target = unmarshaller.unmarshal( staxEventReader );
}
catch ( JAXBException e ) {
throw new MappingException( ... );
}
return new JaxbRoot( target, origin );
}
In my testing the DTD being there or not has no effect. And like I said before, simply changing
<hibernate-mapping package="org.hibernate.test.abstractembeddedcomponents.cid">
to
<hibernate-mapping xmlns="http://www.hibernate.org/xsd/hibernate-mapping" package="org.hibernate.test.abstractembeddedcomponents.cid">
fixes the failures I see, which are:
[org.xml.sax.SAXParseException: cvc-elt.1: Cannot find the declaration of element 'hibernate-mapping'.]
at ...
Caused by: org.xml.sax.SAXParseException: cvc-elt.1: Cannot find the declaration of element 'hibernate-mapping'.
at com.sun.org.apache.xerces.internal.util.ErrorHandlerWrapper.createSAXParseException(ErrorHandlerWrapper.java:195)
at com.sun.org.apache.xerces.internal.util.ErrorHandlerWrapper.error(ErrorHandlerWrapper.java:131)
at com.sun.org.apache.xerces.internal.impl.XMLErrorReporter.reportError(XMLErrorReporter.java:384)
at com.sun.org.apache.xerces.internal.impl.XMLErrorReporter.reportError(XMLErrorReporter.java:318)
at com.sun.org.apache.xerces.internal.impl.xs.XMLSchemaValidator.handleStartElement(XMLSchemaValidator.java:1916)
at com.sun.org.apache.xerces.internal.impl.xs.XMLSchemaValidator.startElement(XMLSchemaValidator.java:705)
at com.sun.org.apache.xerces.internal.jaxp.validation.ValidatorHandlerImpl.startElement(ValidatorHandlerImpl.java:550)
at com.sun.xml.internal.bind.v2.runtime.unmarshaller.ValidatingUnmarshaller.startElement(ValidatingUnmarshaller.java:78)
at com.sun.xml.internal.bind.v2.runtime.unmarshaller.InterningXmlVisitor.startElement(InterningXmlVisitor.java:60)
at com.sun.xml.internal.bind.v2.runtime.unmarshaller.StAXEventConnector.handleStartElement(StAXEventConnector.java:247)
at com.sun.xml.internal.bind.v2.runtime.unmarshaller.StAXEventConnector.bridge(StAXEventConnector.java:116)
at com.sun.xml.internal.bind.v2.runtime.unmarshaller.UnmarshallerImpl.unmarshal0(UnmarshallerImpl.java:394)
... 27 more

Answer 1

This can be done by implementing a filter which adds a default namespace declaration to the first (i.e. root) StartELement event. StAX already provides the EventReaderDelegate utility class, where the peek() and nextEvent() methods need to be overridden.
Here's the code:
import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
import javax.xml.namespace.QName;
import javax.xml.stream.XMLEventFactory;
import javax.xml.stream.XMLEventReader;
import javax.xml.stream.XMLStreamException;
import javax.xml.stream.events.StartElement;
import javax.xml.stream.events.XMLEvent;
import javax.xml.stream.util.EventReaderDelegate;
/**
* Filter adding default namespace declaration to root element.
*/
public class NamespaceAddingEventReader extends EventReaderDelegate {
private final XMLEventFactory factory = XMLEventFactory.newInstance();
private final String namespaceURI;
private int startElementCount = 0;
public NamespaceAddingEventReader(XMLEventReader reader, String namespaceURI) {
super(reader);
this.namespaceURI = namespaceURI;
}
/**
* Duplicate event with additional namespace declaration.
* #param startElement
* #return event with namespace
*/
private StartElement withNamespace(StartElement startElement) {
List<Object> namespaces = new ArrayList<Object>();
namespaces.add(factory.createNamespace(namespaceURI));
Iterator<?> originalNamespaces = startElement.getNamespaces();
while (originalNamespaces.hasNext()) {
namespaces.add(originalNamespaces.next());
}
return factory.createStartElement(
new QName(namespaceURI, startElement.getName().getLocalPart()),
startElement.getAttributes(),
namespaces.iterator());
}
#Override
public XMLEvent nextEvent() throws XMLStreamException {
XMLEvent event = super.nextEvent();
if (event.isStartElement()) {
if (++startElementCount == 1) {
return withNamespace(event.asStartElement());
}
}
return event;
}
#Override
public XMLEvent peek() throws XMLStreamException {
XMLEvent event = super.peek();
if (startElementCount == 0 && event.isStartElement()) {
return withNamespace(event.asStartElement());
} else {
return event;
}
}
}
To see how this is used, let's copy some XML without namespace declaration to System.out using the event API:
StringReader xml = new StringReader("<?xml version='1.0'?><alice>bob</alice>");
XMLEventReader reader = XMLInputFactory.newInstance().createXMLEventReader(xml);
reader = new NamespaceAddingEventReader(reader, "http://foo");
XMLEventWriter writer = XMLOutputFactory.newInstance().createXMLEventWriter(System.out);
writer.add(reader);
writer.flush();
Running the code will print
<?xml version='1.0' encoding='UTF-8'?><alice xmlns="http://foo">bob</alice>