java - JAXB inheritance, unmarshal to subclass of marshaled class

Question

I'm using JAXB to read and write XML. What I want is to use a base JAXB class for marshalling and an inherited JAXB class for unmarshalling. This is to allow a sender Java application to send XML to another receiver Java application. The sender and receiver will share a common JAXB library. I want the receiver to unmarshall the XML into a receiver specific JAXB class which extends the generic JAXB class.

Example:

This is the common JAXB class which is used by the sender.

@XmlRootElement(name="person")
public class Person {
    public String name;
    public int age;
}

This is the receiver specific JAXB class used when unmarshalling the XML. The receiver class has logic specific to the receiver application.

@XmlRootElement(name="person")
public class ReceiverPerson extends Person {
    public doReceiverSpecificStuff() ...
}

Marshalling works as expected. The problem is with unmarshalling, it still unmarshals to Person despite the JAXBContext using the package name of the subclassed ReceiverPerson.

JAXBContext jaxbContext = JAXBContext.newInstance(package name of ReceiverPerson);

What I want is to unmarshall to ReceiverPerson. The only way I've been able to do this is to remove @XmlRootElement from Person. Unfortunately doing this prevents Person from being marshaled. It's as if JAXB starts at the base class and works its way down until it finds the first @XmlRootElement with the appropriate name. I've tried adding a createPerson() method which returns ReceiverPerson to ObjectFactory but that doesn't help.

Answer 1

The following snippet is a method of a Junit 4 test with a green light:

@Test
public void testUnmarshallFromParentToChild() throws JAXBException {
  Person person = new Person();
  int age = 30;
  String name = "Foo";
  person.name = name;
  person.age= age;

  // Marshalling
  JAXBContext context = JAXBContext.newInstance(person.getClass());
  Marshaller marshaller = context.createMarshaller();

  StringWriter writer = new StringWriter();
  marshaller.marshal(person, writer);

  String outString = writer.toString();

  assertTrue(outString.contains("</person"));

  // Unmarshalling
  context = JAXBContext.newInstance(Person.class, RecieverPerson.class);
  Unmarshaller unmarshaller = context.createUnmarshaller();
  StringReader reader = new StringReader(outString);
  RecieverPerson reciever = (RecieverPerson)unmarshaller.unmarshal(reader);

  assertEquals(name, reciever.name);
  assertEquals(age, reciever.age);
}

The important part is the use of the JAXBContext.newInstance(Class... classesToBeBound) method for the unmarshalling context:

 context = JAXBContext.newInstance(Person.class, RecieverPerson.class);

With this call, JAXB will compute a reference closure on the classes specified and will recognize RecieverPerson. The test passes. And if you change the parameters order, you'll get a java.lang.ClassCastException (so they must be passed in this order).