JAXB-Eclipselink: XmlRootElement and inheritance

Question

Using Eclipselink/MOXy 2.3 i have following usecase in marshalling to XML:

abstract class MyAbstract {
}

class MyImpl extends MyAbstract {
}

class A {

    private MyAbstract myAbstract;

    // MyImpl is behind this
    public MyAbstract getMyAbstract() {
        return myAbstract;
    }

}

I have following mapping defined in oxm.xml:

<java-type name="foo.MyAbstract" xml-accessor-type="NONE">
    <xml-see-also>
        foo.MyImpl
    </xml-see-also>
</java-type>

<java-type name="foo.MyImpl">
    <xml-root-element name="MyImpl" />
</java-type>

<java-type name="bar.A" xml-accessor-type="NONE">
    <xml-root-element name="A" />
    <java-attributes>
        <xml-element java-attribute="myAbstract" type="foo.MyAbstract" />
    </java-attributes>
</java-type>

Now this results in:

<A>
    <myAbstract xsi:type="myImpl">
        <!-- Mapped members of MyImpl + MyAbstract -->
    </myAbstract>
</A>

Since i didnt want the property-name in the exported xml I changed:

<java-type name="bar.A" xml-accessor-type="NONE">
    <xml-root-element name="A" />
    <java-attributes>
        <xml-element java-attribute="myAbstract" type="foo.MyAbstract" xml-path="."/>
    </java-attributes>
</java-type>

which resulted in:

<A>
    <!-- Members of MyImpl + MyAbstract marshalled without any wrapping element-->
</A>

What i want is:

 <A>
    <MyImpl>
        <!-- Members of MyImpl + MyAbstract -->
    </MyImpl>
 </A>

The question is: how do i achieve this? MOXy is just ignoring my XmlRootElement on MyImpl...

EDIT:

Trying what Blaise suggested gives me following exception:

Exception [EclipseLink-60] (Eclipse Persistence Services - 2.3.2.v20111125-r10461):   
org.eclipse.persistence.exceptions.DescriptorException
The method [] or [getMyAbstract] is not defined in the object [bar.A].

Now this needs further information which i had left out before because i thought it was not relevant:

Class A is an interface which defines: public X getMyAbstract(); MyAbstract implements X (this is why i added the type-attribute in the mapping for interface A).

So, using xml-element-ref MOXy does not "see" the getter anymore, using xml-element it does.

Answer 1

The mapping that you are looking for is @XmlElementRef. This corresponds to the concept of substitution groups in XML schema.

bar/oxm.xml

Below is the external mapping document for the bar package. Note how the myAbstract property is mapped with xml-element-ref which is the XML representation of @XmlElementRef

<?xml version="1.0" encoding="UTF-8"?>
<xml-bindings
    xmlns="http://www.eclipse.org/eclipselink/xsds/persistence/oxm"
    package-name="bar">
    <java-types>
        <java-type name="A" xml-accessor-type="NONE">
            <xml-root-element name="A" />
            <java-attributes>
                <xml-element-ref java-attribute="myAbstract"/>
            </java-attributes>
        </java-type>
    </java-types>
</xml-bindings>

foo/oxm.xml

Below is the external metadata file for the foo package:

<?xml version="1.0" encoding="UTF-8"?>
<xml-bindings
    xmlns="http://www.eclipse.org/eclipselink/xsds/persistence/oxm"
    package-name="foo">
    <java-types>
        <java-type name="MyAbstract" xml-accessor-type="NONE">
            <xml-see-also>
                foo.MyImpl
            </xml-see-also>
        </java-type>
        <java-type name="MyImpl">
            <xml-root-element name="MyImpl" />
        </java-type>
    </java-types>
</xml-bindings>

Demo

Below is demo code for this example:

package forum8853855;

import java.util.*;
import javax.xml.bind.*;    
import org.eclipse.persistence.jaxb.JAXBContextFactory;
import bar.A;
import foo.MyImpl;

public class Demo {

    public static void main(String[] args) throws Exception {
        List<String> oxm = new ArrayList<String>(2);
        oxm.add("foo/oxm.xml");
        oxm.add("bar/oxm.xml");

        Map<String, Object> properties = new HashMap<String, Object>(1);
        properties.put(JAXBContextFactory.ECLIPSELINK_OXM_XML_KEY, oxm);

        JAXBContext jc = JAXBContext.newInstance(new Class[] {A.class}, properties);
        Marshaller marshaller = jc.createMarshaller();
        marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);

        A a = new A();
        a.setMyAbstract(new MyImpl());
        marshaller.marshal(a, System.out);
    }

}

Output

<?xml version="1.0" encoding="UTF-8"?>
<A>
   <MyImpl/>
</A>

For More Information

• http://blog.bdoughan.com/2010/11/jaxb-and-inheritance-using-substitution.html
• http://blog.bdoughan.com/2011/05/specifying-eclipselink-moxy-as-your.html