JAXB and Inheritance - Using the xsi:type Attribute

In a previous blog post I was challenged by an Xstream user to describe how JAXB handles inheritance.  In this post I'll describe one strategy that can be used, and how easily this can be done with JAXB.  I'll discuss alternate strategies in future posts.

Java Model 

The model will contain an abstract super class for all types of contact information.

package blog.inheritance;

public abstract class ContactInfo {

}

Address and PhoneNumber will be the concrete implementations of ContactInfo.

package blog.inheritance;

public class Address extends ContactInfo {

    private String street;

    public String getStreet() {
        return street;
    }

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

}

package blog.inheritance;

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;
    }

}

Demo Code

We will use the following demo code to demonstrate the use of 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);
    }

}

 XML

The following is the resulting XML document.  Note the use of the xsi:type attribute on the contactInfo element.

<customer>
    <contactInfo 
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
    xsi:type="address">
        <street>1 A Street</street>
    </contactInfo>
</customer>

How does this All Work?

First we will take a look at the schema that represents JAXB's view on this object model.  There is an inheritance among the schema types that matches the inheritance among the Java classes.  The contactInfo element in the customer type is of type contactInfo.  When we produce an XML document we can use the xsi:type attribute to cast the element to any of the sub-types.

<xs:schema version="1.0" xmlns:xs="http://www.w3.org/2001/XMLSchema">

    <xs:element name="customer" type="customer"/>

    <xs:complexType name="customer">
        <xs:sequence>
            <xs:element name="contactInfo" type="contactInfo" minOccurs="0"/>
        </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>

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 how represent inheritance using the element name by leveraging the XML schema concept of substitution groups.

Further Reading

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

• JAXB and Inheritance - Using Substitution Groups
• JAXB and Inheritance - MOXy Extension @XmlDescriminatorNode/@XmlDescrimintatorValue
• Ignoring Inheritance with @XmlTransient