JAXB throwing InstantiationException trying to marshal xsi:type based on abstract class

Question

I am having an issue with using inheritance and JAXB unmarshalling. I have read the number of examples (specifically a much-reference blog at http://blog.bdoughan.com/2010/11/jaxb-and-inheritance-using-xsitype.html and a very similar SO question here: JAXB xsi:type subclass unmarshalling not working) and am still having difficulties.

Like many of these other questions, I am trying to create an XML representation of an object whose fields rely on subclasses for information. I do not know at compile time what the concrete subclass implementation will be, so things like XmlSeeAlso is not really available.

In my test case I have a Root class with an abstract class (A) that has a concrete subtype (B):

@XmlRootElement
@ToString
    public class Root {

    private A theClass;

    public A getTheClass() {
        return theClass;
    }

    public void setTheClass(A theClass) {
        this.theClass = theClass;
    }
}

@ToString
public abstract class A {

}

@XmlRootElement
@ToString
public class B extends A {
    private  String b = "from B-" + System.currentTimeMillis()
    public String getB() {
       return b;
    }

    public void setB(String b) {
         this.b = b;
    }
}

Where @ToString are annotations from project Lombok.

I can marshall with no problem:

@Test
public void marshallTest() {

    try {
        Root r = new Root();
        B b = new B();

        r.setTheClass(b);
        Class<?>[] classArray = new Class<?> [] { Root.class, B.class };

        JAXBContext context = JAXBContext.newInstance(classArray);
        Marshaller marshaller = context.createMarshaller();

        try (FileWriter fw = new FileWriter(JAXB_TEST_XML)) {
            marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);
            marshaller.marshal(r, fw);
        }

        try(StringWriter sw = new StringWriter() ) {
            marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);
            marshaller.marshal(r, sw);
            log.debug("{}", sw.toString());
        }

    } catch (IOException | JAXBException e) {
        e.printStackTrace();
        fail(e.getMessage());
    }
}

Which produces the following xml:

<root>
   <theClass xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:type="b">
      <b>from B-1375211003868</b>
   </theClass>
</root>

When I try to unmarshall (using MOXY JAXB implemntation) I get:

This class does not define a public default constructor, or the constructor raised an exception.
Internal Exception: java.lang.InstantiationException
Descriptor: XMLDescriptor(xml.A --> [])

With the following code:

@Test
public void unmarshall() {

    try {
        JAXBContext context = JAXBContext.newInstance(new Class<?>[] {Root.class, A.class});
        DocumentBuilderFactory documentBuilderFactory = DocumentBuilderFactory.newInstance();

        documentBuilderFactory.setNamespaceAware(true);
        DocumentBuilder db = documentBuilderFactory.newDocumentBuilder();

        Root r = null;
        try(BufferedInputStream bis = new BufferedInputStream(new FileInputStream(JAXB_TEST_XML))) {
            Document doc = db.parse(bis);
            Unmarshaller unmarshaller = context.createUnmarshaller();
            JAXBElement<?> result = unmarshaller.unmarshal(doc, Root.class);
            r = (Root) result.getValue();
        }

        assertTrue(r != null & r.getTheClass() != null && r.getTheClass() instanceof B);
    } catch (Exception e) {
        e.printStackTrace();
        fail(e.getMessage());
    }
}

I have tried making the unmarshalling namespace aware (as with JAXB xsi:type subclass unmarshalling not working which has not worked. I have tried using XmlElementRef which does not work either. I have tried the latest glassfish api and implementations downloaded from maven central (2.2.8). I have tried the MOXY eclipse persistence JAXB implementation. Neither have worked. I have tried unmarshalling without using a document builder which does not work as well. I have tried passing the Root.class and A.class into the JAXB context which also does not work.

I have a feeling that I have a fundamental misconception about what is going on. Any hints or ideas would be appreciated. Thank you.

• Chooks

Answer 1

UPDATE 2

You could use a library to determine the subclasses dynamically and pass that result to MOXy to build the JAXBContext. Below is an enhancement request that was entered suggesting Jandex for this purpose.

• https://bugs.eclipse.org/bugs/show_bug.cgi?id=412236

---

UPDATE 1

There was an issue prior to EclipseLink 1.2.0 (released October 23, 2009) where that exception could be thrown even though everything was setup correctly.

---

You just need to make the JAXBContext aware of the B class. One way to do this is to leverage the @XmlSeeAlso annotation on the A class.

import javax.xml.bind.annotation.XmlSeeAlso;

@XmlSeeAlso({B.class})
public abstract class A {

}

You can also include B in the classes used to bootstrap the JAXBContext:

JAXBContext jc = JAXBContext.newInstance(Root.class, B.class);