Revision as of 16:09, 19 April 2011

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Handling Inheritance

EclipseLink MOXy provides several ways to represent your inheritance hierarchy in XML.

Using xsi:type

By default, EclipseLink will use the xsi:type attribute to represent inheritance in XML.

In this example an abstract super class (ContactInfo) contains all types of contact information. Address and PhoneNumber are the concrete implementations of ContactInfo.

public abstract class ContactInfo {
}
 
public class Address extends ContactInfo {
 
   private String street;
   ... 
 
}
 
public class PhoneNumber extends ContactInfo {
 
   private String number;
   ...
 
}

Because the Customer object can have different types of contact information, its property refers to the superclass.

 
@XmlRootElement
public class Customer {
 
   private ContactInfo contactInfo;
   ... 
 
}

Marshalling an example Customer would produce the following XML:

<customer>
   <contactInfo 
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
      xsi:type="address">
      <street>323 Main Street</street>
   </contactInfo>
</customer>

Note the xsi:type attribute on the contactInfo element.

Using Substitution Groups

Another way to model inheritance in XML is through XML Schema's substitution groups functionality. Using this approach, the element name itself determines which subclass to use.

Taking the same example from above, we will add @XmlRootElement annotations to each of the subclasses, which will act as the inheritance indicator.

public abstract class ContactInfo {
}
 
@XmlRootElement
public class Address extends ContactInfo {
 
   private String street;
   ... 
 
}
 
@XmlRootElement
public class PhoneNumber extends ContactInfo {
 
   private String number;
   ...
 
}

Using this approach, marshalling an example Customer would produce the following XML:

<customer>
   <address>
      <street>323 Main Street</street>
   </address>
</customer>

Note that the Address object is marshalled to the address element.

Using MOXy Exentions: @XmlDescriminatorNode/@XmlDescrimintatorValue

You can use the @XmlDescriminatorNode and @XmlDescrimintatorValue MOXy extensions avaialable in EclipseLink 2.2 JAXB to represent inheritance. With these extensions, you can select the attribute to represent the subtype.

In this example an abstract super class ('ContactInfo) contains all types of contact information. The ContactInfo uses the @XmlDescriminatorNode annotation to specify the XML attribute (classifier) to indicate the subtype.

Address and PhoneNumber are the concrete implementations of ContactInfo. The @XmlDescriminatorValue is used to override the default type name.

package blog.inheritance;
 
import org.eclipse.persistence.oxm.annotations.XmlDiscriminatorNode;
import org.eclipse.persistence.oxm.annotations.XmlDiscriminatorValue;
 
@XmlDiscriminatorNode("@classifier")
public abstract class ContactInfo {
 
}
 
@XmlDiscriminatorValue("address-classifier")
public class Address extends ContactInfo {
 
    private String street;
 
    public String getStreet() {
        return street;
    }
 
    public void setStreet(String street) {
        this.street = street;
    }
 
}
 
@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.

jaxb.properties

To use the MOXy JAXB implementation, use a jaxb.properties file with the Address class that contains the following: javax.xml.bind.context.factory=org.eclipse.persistence.jaxb.JAXBContextFactory

In the following example, the descriminator represents 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);
    }
 
}

The above sample produces the following XML.

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

Notice that Address'is marshalled to the contactInfo element. Its classifier attribute contains the discriminator node value address-classifier.

	MOXy User Guide	Simple Values
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