Extending JAXB - Representing Metadata as XML

In JAXB (JSR-222) metadata is applied to Java classes via standard annotations. There are times when using annotatations is not practical, and an alternate mechanism is required.  In this post I'll demonstrate how EclipseLink JAXB (MOXy) can leverage XML to represent the JAXB metadata.

An XML metadata representation is useful when:

• You cannot modify the domain model (it may come from a 3rd party).
• You do not want to introduce compile dependencies on JAXB APIs (if you are using a version of Java prior to Java SE 6).
• You want to apply multiple JAXB mappings to a domain model (you are limited to one representation with annotations).
• Your object model already contains so many annotations from other technologies that adding more would make the class unreadable.

Mapping File (binding.xml)

The mapping file contains the same information as the JAXB annotations.  Like with annotations you only need to specify metadata to override default behavior.

<?xml version="1.0"?>
<xml-bindings
    xmlns="http://www.eclipse.org/eclipselink/xsds/persistence/oxm"
    package-name="blog.bindingfile">
    <xml-schema 
        namespace="http://www.example.com/customer" 
        element-form-default="QUALIFIED"/>
    <java-types>
        <java-type name="Customer">
            <xml-root-element/>
            <xml-type prop-order="firstName lastName address phoneNumbers"/>
            <java-attributes>
                <xml-element java-attribute="firstName" name="first-name"/>
                <xml-element java-attribute="lastName" name="last-name"/>
                <xml-element java-attribute="phoneNumbers" name="phone-number"/>
            </java-attributes>
        </java-type>
        <java-type name="PhoneNumber">
            <java-attributes>
                <xml-attribute java-attribute="type"/>
                <xml-value java-attribute="number"/>
            </java-attributes>
        </java-type>
    </java-types>
</xml-bindings>

Domain Model

The following domain model will be used in this example.  Because the JAXB metadata is represented as XML, no annotations are used on the classes.

Customer 

package blog.bindingfile;

import java.util.List;
 
public class Customer {

    private String firstName;
    private String lastName;
    private Address address;
    private List<PhoneNumber> phoneNumbers;

    public String getFirstName() {
        return firstName;
    }

    public void setFirstName(String firstName) {
        this.firstName = firstName;
    }

    public String getLastName() {
        return lastName;
    }

    public void setLastName(String lastName) {
        this.lastName = lastName;
    }

    public Address getAddress() {
        return address;
    }

    public void setAddress(Address address) {
        this.address = address;
    }

    public List<PhoneNumber> getPhoneNumbers() {
        return phoneNumbers;
    }

    public void setPhoneNumbers(List<PhoneNumber> phoneNumbers) {
        this.phoneNumbers = phoneNumbers;
    }

}

Address 

package blog.bindingfile;

public class Address {

    private String street;

    public String getStreet() {
        return street;
    }

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

}

PhoneNumber 

package blog.bindingfile;

public class PhoneNumber {
 
    private String type;
    private String number;

    public String getType() {
        return type;
    }

    public void setType(String type) {
        this.type = type;
    }

    public String getNumber() {
        return number;
    }

    public void setNumber(String number) {
        this.number = number;
    }

}

XML (input.xml)

The following XML will be used in this example:

<?xml version="1.0" encoding="UTF-8"?>
<customer xmlns="http://www.example.com/customer">
    <first-name>Jane</first-name>
    <last-name>Doe</last-name>
    <address>
        <street>123 A Street</street>
    </address>
    <phone-number type="work">555-1111</phone-number>
    <phone-number type="cell">555-2222</phone-number>
</customer>

Demo Code

The XML mapping file is passed in via the properties parameter when the JAXBContext is instantiated.

package blog.bindingfile;
 
import java.io.File;
import java.util.*;
import javax.xml.bind.*;
import org.eclipse.persistence.jaxb.JAXBContextFactory;

public class Demo {

    public static void main(String[] args) throws Exception {
        Map<String, Object> properties = new HashMap<String, Object>(1);
        properties.put(JAXBContextFactory.ECLIPSELINK_OXM_XML_KEY, "blog/bindingfile/binding.xml");
        JAXBContext jc = JAXBContext.newInstance("blog.bindingfile", Customer.class.getClassLoader() , properties);

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

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

}

Specifying the MOXy JAXB Implementation (jaxb.properties)

In order to use this extension, you must use MOXy as your JAXB implementation.  To enable MOXy simply add a file named jaxb.properties in the same package as your domain classes with the following entry:

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

Further Reading

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

• MOXy's XML Metadata in a JAX-RS Service
• Using EclipseLink MOXy with a Metadata Repository