JAXB : how to read the xsi:type value when unmarshalling

Question

I am using JAXB in the XSD / XML to Java direction. My XSD contains derived types and the XML I have to unmarshall contain xsi:type attributes. Looking at the code JAXB (the default Sun implementation) has generated (for the unmarshalling) there seems to appear no methods to get these attributes.

Is it because I can always do a getClass() on the unmarshalled Java object and find the actual class?

Still, would it not be the case that depending on the packages or classes I provide to the JAXBContext.newInstance some base class may be instantiated instead? (one that is a superclass of the class corresponding to the actual value of the xsi:type attribute). In such a case being able to read the value of the actual attribute as it appears in the XML instance could be needed.

Answer 1

The JAXB (JSR-222) implementation will take care of everything for you. JAXB considers that each class corresponds to a complex type. It has an algorithm for figuring out the type name, but you can override this using the @XmlType annotation. When an element is unmarshalled if it contains an xsi:type attribute then JAXB will look to see if there is a class associated with that type. If there is it will instantiate a class of that type, if not it will instantiate the type that corresponds to that element based on the mapping metadata supplied via annotations.

For More Information

• http://blog.bdoughan.com/2010/11/jaxb-and-inheritance-using-xsitype.html

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UPDATE

Below is an example that may help:

schema.xsd

In the XML schema below the complex type canadianAddress extends the complexType address .

<?xml version="1.0" encoding="UTF-8"?>
<schema 
    xmlns="http://www.w3.org/2001/XMLSchema" 
    targetNamespace="http://www.example.org/customer" 
    xmlns:tns="http://www.example.org/customer" 
    elementFormDefault="qualified">

    <element name="customer">
        <complexType>
            <sequence>
                <element name="address" type="tns:address"/>
            </sequence>
        </complexType>
    </element>

    <complexType name="address">
        <sequence>
            <element name="street" type="string"/>
        </sequence>
    </complexType>

    <complexType name="canadianAddress">
        <complexContent>
            <extension base="tns:address">
                <sequence>
                    <element name="postalCode" type="string"/>
                </sequence>
            </extension>
        </complexContent>
    </complexType>

</schema>

Demo

In the demo code below the XML will be converted to the JAXB model generated from the above XML schema, and then converted back to XML.

import java.io.File;
import javax.xml.bind.*;

public class Demo {

    public static void main(String[] args) throws Exception {
        JAXBContext jc = JAXBContext.newInstance("org.example.customer");

        Unmarshaller unmarshaller = jc.createUnmarshaller();
        File xml = new File("src/org/example/customer/input.xml");
        Customer customer = (Customer) unmarshaller.unmarshal(xml);

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

}

input.xml/Output

Below is the XML. The address element is qualified with xsi:type to indicate that it holds an instance of canadianAddress instead of just an address .

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<customer xmlns="http://www.example.org/customer">
    <address xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:type="canadianAddress">
        <street>1 A Street</street>
        <postalCode>Ontario</postalCode>
    </address>
</customer>