EclipseLink/UserGuide/MOXy/Advanced XML Schema Concepts/Substitution Groups and Choices

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Substitution Groups

With JAXB, you can use the element name, instead of using the xsi:type attribute, to represent inheritance by using XML schema substitution groups.

In this example, the Java model contains an abstract superclass for all types of contact information:

 
package blog.inheritance;
 
public abstract class ContactInfo {
 
}

The Address' and PhoneNumber classes are the concrete implementations of ContactInfo. Both classes use the @XmlRootElement annotation because the element name is used as the inheritance indicator.

package blog.inheritance;
 
import javax.xml.bind.annotation.XmlRootElement;
 
@XmlRootElement
public class Address extends ContactInfo {
 
    private String street;
 
    public String getStreet() {
        return street;
    }
 
    public void setStreet(String street) {
        this.street = street;
    }
 
}
 
@XmlRootElement
public class PhoneNumber extends ContactInfo {
 
}

Because the Customer object can have different types of contact information, the property refers to the superclass. The contactInfo' property contains the @XmlElementRef annotation to specify that the value type will be derived from the element name (and namespace URI).

 
package blog.inheritance;
 
import javax.xml.bind.annotation.XmlElementRef;
import javax.xml.bind.annotation.XmlRootElement;
 
@XmlRootElement
public class Customer {
 
    private ContactInfo contactInfo;
 
    @XmlElementRef
    public ContactInfo getContactInfo() {
        return contactInfo;
    }
 
    public void setContactInfo(ContactInfo contactInfo) {
        this.contactInfo = contactInfo;
    }
 
}

This schema represents the JAXB view of the object model. The schema type inheritance matches the Java class inheritance.

 
<xs:schema version="1.0" xmlns:xs="http://www.w3.org/2001/XMLSchema">
 
    <xs:element name="customer" type="customer"/>
 
    <xs:element name="contactInfo" type="contactInfo"/>
 
    <xs:element name="address" type="address"
        substitutionGroup="contactInfo"/>
 
    <xs:element name="phoneNumber" type="phoneNumber"
        substitutionGroup="contactInfo"/>
 
    <xs:complexType name="customer">
        <xs:sequence>
            <xs:element ref="contactInfo"/>
        </xs:sequence>
    </xs:complexType>
 
    <xs:complexType name="contactInfo" abstract="true">
        <xs:sequence/>
    </xs:complexType>
 
    <xs:complexType name="address">
        <xs:complexContent>
            <xs:extension base="contactInfo">
                <xs:sequence>
                    <xs:element name="street" type="xs:string" minOccurs="0"/>
                </xs:sequence>
            </xs:extension>
        </xs:complexContent>
    </xs:complexType>
 
    <xs:complexType name="phoneNumber">
        <xs:complexContent>
            <xs:extension base="contactInfo">
                <xs:sequence/>
            </xs:extension>
        </xs:complexContent>
    </xs:complexType>
 
</xs:schema>

Notice that each type has a corresponding global element. Additionally, the address and phoneNumber elements may be substituted for the contactInfo element.

Example

This example code demonstrates using the xsi:type attribute 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);
    }
 
}
 
The example produces the following XML:
 
<source lang="xml">
 
<customer>
    <address>
        <street>1 A Street</street>
    </address>
</customer>

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

How does this All Work?

Summary

Not all XML binding tools support inheritance. The ones that do often use different strategies. Some include the class name of the subclass as the qualifier, this strategy makes it difficult to send the resulting XML document to another tool. JAXB on the other hand leverages existing XML schema concepts to produce very portable XML documents. In a future post I'll discuss a MOXy extension for an alternate means of representing inheritance (check out the following for a sneak peak).

Version: 2.2.0 DRAFT
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