JAXB and namespaces related issue

Question

I have the follow XML after marshall a JAXB class:

<?xml version="1.0" encoding="UTF-8"?>
<foo:Object xsi:schemaLocation="http://foo.com/ http://foo.com/foo.xsd" 
xmlns:ns0="http://lipsum.com/bar" xmlns:foo="http://foo.com/" 
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
   <foo:Some attr1="601"/>
   <foo:Others>
      <bar:Another xsi:schemaLocation="http://lipsum.com/bar 
      http://lipsum.com/bar.xsd" xmlns:bar="http://lipsum.com/bar">
         <bar:SomeOther attr2="01"/>
      </bar:Another>
  </foo:Others>
</foo:Object>

The <foo:Others/> element in the jaxb class are declared as:

    @XmlAnyElement(lax = true)
    @XmlElementRefs({
         @XmlElementRef(name="Another", type=Another.class, required=false)
    })
    protected List<Object> any;

It may contain an unknow number of other elements with their own namespaces.

The problem:

When i marshall the Some.class object, jaxb put the namespace of the Another.class into the root element as ns0, since i get the <bar:Another> marshalled as Element from another method with their namespace included i don't need it again into the root element.

That is a problem since <bar:Another> is not required, so if i marshall the Some.class object with an empty list i'am goin to have always the xmlns:ns0.

I need the @XmlElementRefs and @XmlElementRef annotations because i need to convert from json to XML and Jackson need to know the type that "any" list could have.

How can i tell to JAXB Oracle/Moxy implementation that:

1.- Ignore the namespaces from @XmlElementRef classes during the marshalling.
2.- Remove the not used namespaces aka NS[0-9] prefixes from the root element.

Any help would be apreciated.

Answer 1

Just in case some one needs this i got a way to remove the unwanted namespaces, the trick is provided from XMLStreamWriter

Step 1.- Create an WrapperXMLStreamWriter class that goin to wrap a XMLStreamWriter.

public class WrapperXMLStreamWriter implements XMLStreamWriter {
    private final XMLStreamWriter writer;

    public WrapperXMLStreamWriter(XMLStreamWriter writer) {
        this.writer = writer;
    }

    /* (non-Javadoc)
     * @see javax.xml.stream.XMLStreamWriter#writeStartElement(java.lang.String)
     */
    @Override
    public void writeStartElement(String localName) throws XMLStreamException {
        // for each overloaded method call to writer do the method
        writer.writeStartElement(localName);
    }

     /* (non-Javadoc)
     * @see javax.xml.stream.XMLStreamWriter#writeNamespace(java.lang.String, java.lang.String)
     */
    @Override
    public void writeNamespace(String prefix, String namespaceURI) throws XMLStreamException {
        // Here is the trick
        if(!prefix.startsWith("ns")){
            writer.writeNamespace(prefix, namespaceURI);
        }
    }

    ...
}

Step 2.- Crate a marshaller as usual

Map<String,String> prefixes = new HashMap<String, String>();

// 
prefixes.put("http://www.w3.org/2001/XMLSchema-instance","xsi");
prefixes.put("http://foo.com/", "foo");

Marshaller marshaller =  AppContextTool.getInstance().getContextForFoo().createMarshaller();

// 
marshaller.setProperty("com.sun.xml.bind.namespacePrefixMapper", new NamespacePrefixMapperImpl(prefixes));
marshaller.setProperty(Marshaller.JAXB_SCHEMA_LOCATION, "http://foo.com/ http://foo.com/foo.xsd");

3.- Marshall the Foo instance with the given XMLStreamWriter of the first step

Foo foo = new Foo();

ByteArrayOutputStream xml= new ByteArrayOutputStream();
try {
    XMLStreamWriter writer = 
                new WrapperXMLStreamWriter((XMLStreamWriter) XMLOutputFactory.newInstance().createXMLStreamWriter(xml, "UTF-8"));

    marshaller.marshal(foo, writer);
} catch (XMLStreamException | FactoryConfigurationError e) {
    e.printStackTrace();
}

4.- The result:

<?xml version="1.0" encoding="UTF-8"?>
<foo:Object xsi:schemaLocation="http://foo.com/ http://foo.com/foo.xsd" 
 xmlns:foo="http://foo.com/" 
 xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
    <foo:Some attr1="601"/>
        <foo:Others>
            <bar:Another xsi:schemaLocation="http://lipsum.com/bar 
  http://lipsum.com/bar.xsd" xmlns:bar="http://lipsum.com/bar">
          <bar:SomeOther attr2="01"/>
       </bar:Another>
  </foo:Others>
  </foo:Object>

No more NS namespaces, Hope this helps someone.