JAXB and Inheritance - MOXy Extension @XmlDescriminatorNode/@XmlDescrimintatorValue

In previous blog posts I described how to leverage the xsi:type attribute and substitution groups to represent inheritance.  In this post I'll demonstrate how an upcoming EclipseLink 2.2 JAXB (MOXy) extension can be used to represent inheritance.  It leverages an attribute of your own choosing to represent the subtype.  If you are using a version of EclipseLink prior to 2.2 you can use the technique described here.

Java Model 

The model will contain an abstract super class for all types of contact information. We will use MOXy's @XmlDescriminatorNode annotation to specify the XML attribute we wish to use to indicate the appropriate subtype.

package blog.inheritance;

import org.eclipse.persistence.oxm.annotations.XmlDiscriminatorNode;

@XmlDiscriminatorNode("@classifier")
public abstract class ContactInfo {

}

Address and PhoneNumber will be the concrete implementations of ContactInfo. By default the type name will be used with the discriminator node, we can override this by using @XmlDescriminatorValue.

package blog.inheritance;

import org.eclipse.persistence.oxm.annotations.XmlDiscriminatorValue;

@XmlDiscriminatorValue("address-classifier")
public class Address extends ContactInfo {

    private String street;

    public String getStreet() {
        return street;
    }

    public void setStreet(String street) {
        this.street = street;
    }

}

package blog.inheritance;

import org.eclipse.persistence.oxm.annotations.XmlDiscriminatorValue;

@XmlDiscriminatorValue("phone-number-classifier")
public class PhoneNumber extends ContactInfo {

}

The Customer object can have different types of contact info set on it, so the property will refer to the super class.

package blog.inheritance;

import javax.xml.bind.annotation.XmlRootElement;

@XmlRootElement
public class Customer {

    private ContactInfo contactInfo;

    public ContactInfo getContactInfo() {
        return contactInfo;
    }

    public void setContactInfo(ContactInfo contactInfo) {
        this.contactInfo = contactInfo;
    }

}

Java Model - jaxb.properties

In order to specify that we are using the MOXy JAXB implementation we need to put a file called jaxb.properties in with our Address class with the following entry: 

javax.xml.bind.context.factory=org.eclipse.persistence.jaxb.JAXBContextFactory

Demo Code

We will use the following demo code to demonstrate the use of the descriminator node to represent inheritance.

package blog.inheritance;

import javax.xml.bind.JAXBContext;
import javax.xml.bind.Marshaller;

public class Demo {

    public static void main(String[] args) throws Exception {
        Customer customer = new Customer();
        Address address = new Address();
        address.setStreet("1 A Street");
        customer.setContactInfo(address);

        JAXBContext jc = JAXBContext.newInstance(Customer.class, Address.class, PhoneNumber.class);

        Marshaller marshaller = jc.createMarshaller();
        marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);
        marshaller.marshal(customer, System.out);
    }

}

 XML

The following is the resulting XML document.  Note that the Address object is marshalled to the contactInfo element with the classifier attribute containing the discriminator node value "address-classifier".

<customer>
   <contactInfo classifier="address-classifier">
      <street>1 A Street</street>
   </contactInfo>
</customer>

How does this All Work?

This is very similar to using the xsi:type attribute to represent the sub type.  However in this case we are not leveraging an XML schema concept.  This is not necessarily a bad thing, as there are legitimate use cases where this type of behaviour is required (check out this Stack Overflow post for an example).  It is also useful when you need to interact with XML binding tools with proprietary mechanisms for handling inheritance.

Further Reading

If you enjoyed this post you may also be interested in:

• JAXB and Inheritance - Using the xsi:type Attribute
• JAXB and Inheritance - Using Substitution Groups
• Ignoring Inheritance with @XmlTransient