eclipselink/Moxy : inheritance and attribute name oveloading based on type

Question

I\'m facing a marshalling/unmarshalling problem involving inheritance and polymorphism using MOXy\'s JAXB implementation and external metadata bindings file.

I have

Answer 1

Below are the answers to your questions. The answer to question 2, is also an answer to question 1.

---

1st question : I understand that this is normal, Jaxb needs some way to determine the type of MyContaioner.myObject attribute. The problem is that I have no access to the incoming XML files, so I cant add xsi:type fields to them. Is there a way to determine a class based on the presence of a specific attribute in it ? regardless of it's value. If the source xml contains a @attrFromC attribute, I know the object should be of type C. If it contains attrFromB, it's B.

You can leverage the ClassExtractor extension in EclipseLink JAXB (MOXy) for this use case:

MyClassExtractor

A ClassExtractor is some code that you can implement to help MOXy determine which class it should instanitate. You are passed a Record and you can ask for the presence of the attributes at the current element by XPath to determine which class should be instantiated.

package com.test.example;

import org.eclipse.persistence.descriptors.ClassExtractor;
import org.eclipse.persistence.sessions.*;

public class MyClassExtractor extends ClassExtractor{

    @Override
    public Class<?> extractClassFromRow(Record record, Session session) {
        if(null != record.get("@attrFromB")) {
            return B.class;
        } else if(null != record.get("@attrFromC")) {
            return C.class;
        } else {
            return A.class;
        }
    }

}

Metadata (oxm.xml)

You can configure the ClassExtractor using the @XmlClassExtractor annotation. You can also do this via the external metadata file. I have adapted the one included in your question to include this:

<?xml version="1.0"?>
<xml-bindings 
    xmlns="http://www.eclipse.org/eclipselink/xsds/persistence/oxm"
    package-name="com.test.example"
    version="2.3">
    <java-types>
        <java-type name="A" xml-accessor-type="NONE">
           <xml-class-extractor class="com.test.example.MyClassExtractor"/>
           <xml-root-element name="MyObject" />
           <java-attributes>
              <xml-attribute java-attribute="name" name="nameA" />
           </java-attributes>
        </java-type>  
        <java-type name="B" xml-accessor-type="NONE">
           <xml-root-element name="MyObject" />
           <java-attributes>
              <xml-attribute java-attribute="name" name="nameB" />
              <xml-attribute java-attribute="attrFromB"/>
           </java-attributes>
        </java-type>
        <java-type name="C" xml-accessor-type="NONE">
           <xml-root-element name="MyObject" />
           <java-attributes>
              <xml-attribute java-attribute="name" name="nameC" />
              <xml-attribute java-attribute="attrFromC"/>
           </java-attributes>
        </java-type>
        <java-type name="MyContainerClass" xml-accessor-type="NONE">
           <xml-root-element name="MyContainer" />
           <java-attributes>
              <xml-element java-attribute="myObject" name="MyObject" />
           </java-attributes>
        </java-type>
    </java-types>
</xml-bindings>

Demo

The following demo code unmarshals each of the XML documents from your question, and outputs the type being held by the myObject property:

package com.test.example;

import java.io.StringReader;
import java.util.*;
import javax.xml.bind.*;
import org.eclipse.persistence.jaxb.JAXBContextFactory;

public class Demo {

    public static void main(String[] args) throws Exception {
        Map<String, Object> properties = new HashMap<String, Object>();
        properties.put(JAXBContextFactory.ECLIPSELINK_OXM_XML_KEY, "com/test/example/oxm.xml");
        JAXBContext jc = JAXBContext.newInstance(new Class[] {MyContainerClass.class}, properties);
        Unmarshaller unmarshaller = jc.createUnmarshaller();

        StringReader aXml = new StringReader("<MyContainer><MyObject nameA='foo'/></MyContainer>");
        MyContainerClass myContainerA = (MyContainerClass) unmarshaller.unmarshal(aXml);
        System.out.println(myContainerA.getMyObject().getClass());

        StringReader bXml = new StringReader("<MyContainer><MyObject nameB='foo' attrFromB='bar'/></MyContainer>");
        MyContainerClass myContainerB = (MyContainerClass) unmarshaller.unmarshal(bXml);
        System.out.println(myContainerB.getMyObject().getClass());

        StringReader cXml = new StringReader("<MyContainer><MyObject nameC='foo' attrFromC='bar'/></MyContainer>");
        MyContainerClass myContainerC = (MyContainerClass) unmarshaller.unmarshal(cXml);
        System.out.println(myContainerC.getMyObject().getClass());
    }

}

Output

[EL Warning]: 2012-01-20 10:36:41.828--Ignoring attribute [name] on class [com.test.example.B] as no Property was generated for it.
[EL Warning]: 2012-01-20 10:36:41.828--Ignoring attribute [name] on class [com.test.example.C] as no Property was generated for it.
class com.test.example.A
class com.test.example.B
class com.test.example.C

---

2nd question : The other problem is that I dont know if Jaxb is capable of overriding xml attribute names like it is expected inside the XML file (@nameA, @nameB and nameC all referring to A.name), is there a way to do it ?

You can leverage an XmlAdapter for this question. This approach can also be used to answer your first question:

AAdapter

package com.test.example;

import javax.xml.bind.annotation.XmlAttribute;
import javax.xml.bind.annotation.adapters.XmlAdapter;

public class AAdapter extends XmlAdapter<AAdapter.AdaptedA, A> {

    @Override
    public AdaptedA marshal(A a) throws Exception {
        if(null == a) {
            return null;
        }
        AdaptedA adaptedA = new AdaptedA();
        if(a instanceof C) {
            C c = (C) a;
            adaptedA.nameC = c.getName();
            adaptedA.attrFromC = c.getAttrFromC();
        } else if(a instanceof B) {
            B b = (B) a;
            adaptedA.nameB = b.getName();
            adaptedA.attrFromB = b.getAttrFromB();
        } else if(a instanceof A) {
            adaptedA.nameA = a.getName();
        }
        return adaptedA;
    }

    @Override
    public A unmarshal(AdaptedA adaptedA) throws Exception {
        if(null == adaptedA) {
            return null;
        }
        if(null != adaptedA.attrFromC) {
            C c = new C();
            c.setName(adaptedA.nameC);
            c.setAttrFromC(adaptedA.attrFromC);
            return c;
        } else if(null != adaptedA.attrFromB) {
            B b = new B();
            b.setName(adaptedA.nameB);
            b.setAttrFromB(adaptedA.attrFromB);
            return b;
        } 
        A a = new A();
        a.setName(adaptedA.nameA);
        return a;
    }

    public static class AdaptedA {
        @XmlAttribute public String nameA;
        @XmlAttribute public String nameB;
        @XmlAttribute public String nameC;
        @XmlAttribute public String attrFromB;
        @XmlAttribute public String attrFromC;
    }

}

Metadata (oxm-2.xml)

<?xml version="1.0"?>
<xml-bindings 
    xmlns="http://www.eclipse.org/eclipselink/xsds/persistence/oxm"
    package-name="com.test.example"
    version="2.3">
    <java-types>
        <java-type name="MyContainerClass" xml-accessor-type="NONE">
           <xml-root-element name="MyContainer" />
           <java-attributes>
              <xml-element java-attribute="myObject" name="MyObject">
                <xml-java-type-adapter value="com.test.example.AAdapter"/>
              </xml-element>
           </java-attributes>
        </java-type>
    </java-types>
</xml-bindings>

Demo2

package com.test.example;

import java.io.StringReader;
import java.util.*;
import javax.xml.bind.*;
import org.eclipse.persistence.jaxb.JAXBContextFactory;

public class Demo2 {

    public static void main(String[] args) throws Exception {
        Map<String, Object> properties = new HashMap<String, Object>();
        properties.put(JAXBContextFactory.ECLIPSELINK_OXM_XML_KEY, "com/test/example/oxm-2.xml");
        JAXBContext jc = JAXBContext.newInstance(new Class[] {MyContainerClass.class}, properties);
        Unmarshaller unmarshaller = jc.createUnmarshaller();
        Marshaller marshaller = jc.createMarshaller();
        marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);

        StringReader aXml = new StringReader("<MyContainer><MyObject nameA='foo'/></MyContainer>");
        MyContainerClass myContainerA = (MyContainerClass) unmarshaller.unmarshal(aXml);
        System.out.println(myContainerA.getMyObject().getClass());
        marshaller.marshal(myContainerA, System.out);

        StringReader bXml = new StringReader("<MyContainer><MyObject nameB='foo' attrFromB='bar'/></MyContainer>");
        MyContainerClass myContainerB = (MyContainerClass) unmarshaller.unmarshal(bXml);
        System.out.println(myContainerB.getMyObject().getClass());
        marshaller.marshal(myContainerB, System.out);

        StringReader cXml = new StringReader("<MyContainer><MyObject nameC='foo' attrFromC='bar'/></MyContainer>");
        MyContainerClass myContainerC = (MyContainerClass) unmarshaller.unmarshal(cXml);
        System.out.println(myContainerC.getMyObject().getClass());
        marshaller.marshal(myContainerC, System.out);
    }

}

Output

class com.test.example.A
<?xml version="1.0" encoding="UTF-8"?>
<MyContainer>
   <MyObject nameA="foo"/>
</MyContainer>
class com.test.example.B
<?xml version="1.0" encoding="UTF-8"?>
<MyContainer>
   <MyObject nameB="foo" attrFromB="bar"/>
</MyContainer>
class com.test.example.C
<?xml version="1.0" encoding="UTF-8"?>
<MyContainer>
   <MyObject nameC="foo" attrFromC="bar"/>
</MyContainer>