Extensible Models with EclipseLink JAXB (MOXy)

JAXB is designed to work with domain models that have real fields/properties.  In EclipseLink 2.3, MOXy introduces the concept of virtual properties.  Virtual properties are defined by the MOXy metadata file, and provide a way to extend a class without modifying the source.  In this post I'll describe what virtual properties are, and in upcoming posts I'll cover how they can be leveraged:

•  MOXy Extensible Models - Multi-Tenant Example

Java Model

ExtensibleBase

The @XmlVirtualAccessMethods annotation is used to specify that a class is extensible.  An extensible class is required to have a "get" method that returns a value by property name, and a "set" method that stores a value by property name.  The default names for these methods are "get" and "set", and can be overridden with the @XmlVirtualAccessMethods annotation.  Since we will have multiple extensible classes in this example we'll configure a base class for this behaviour that extensible classes can extend.  We will use the @XmlTransient annotation to prevent ExtensibleBase from being mapped as an inheritance relationship.

package blog.virtual;

import java.util.HashMap;
import java.util.Map;

import javax.xml.bind.annotation.XmlTransient;

import org.eclipse.persistence.oxm.annotations.XmlVirtualAccessMethods;

@XmlTransient
@XmlVirtualAccessMethods(setMethod="put")
public class ExtensibleBase {

    private Map<String, Object> extensions = new HashMap<String, Object>();

    public <T> T get(String property) {
        return (T) extensions.get(property);
    }

    public void put(String property, Object value) {
        extensions.put(property, value);
    }

}

Customer

The Customer class will be extensible since it inherits from a domain class that has been annotated with @XmlVirtualAccessMethods.

package blog.virtual;

import javax.xml.bind.annotation.XmlRootElement;

@XmlRootElement
public class Customer extends ExtensibleBase {

    private String firstName;
    private String lastName;
    private Address billingAddress;

    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 getBillingAddress() {
        return billingAddress;
    }

    public void setBillingAddress(Address billingAddress) {
        this.billingAddress = billingAddress;
    }

}

Address

It is not necessary to have every class in your model be extensible.  In this example the Address class will not have any virtual properties.

package blog.virtual;

public class Address {

    private String street;

    public String getStreet() {
        return street;
    }

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

}

PhoneNumber

PhoneNumber like Customer will be an extensible class.

package blog.virtual;

import javax.xml.bind.annotation.XmlValue;

public class PhoneNumber extends ExtensibleBase {

    private String number;

    @XmlValue
    public String getNumber() {
        return number;
    }

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

}

Mapping File

The metadata for the virtual properties is supplied through MOXy's XML mapping file.  Virtual properties are mapped in the same way as real properties.  Some additional information is required including type (since this can not be determined via reflection), and for collection properties a container type.

<?xml version="1.0"?>
<xml-bindings
    xmlns="http://www.eclipse.org/eclipselink/xsds/persistence/oxm"
    package-name="blog.virtual">
    <java-types>
        <java-type name="Customer">
            <xml-type prop-order="firstName middleName lastName billingAddress shippingAddress phoneNumbers"/>
            <java-attributes>
                <xml-attribute 
                    java-attribute="id" 
                    type="java.lang.Integer"/>
                <xml-element 
                    java-attribute="middleName" 
                    type="java.lang.String"/>
                <xml-element 
                    java-attribute="shippingAddress" 
                    type="blog.virtual.Address"/>
                <xml-element 
                    java-attribute="phoneNumbers" 
                    name="phoneNumber" 
                    type="blog.virtual.PhoneNumber" 
                    container-type="java.util.List"/>
            </java-attributes>
        </java-type>
        <java-type name="PhoneNumber">
            <java-attributes>
                <xml-attribute 
                    java-attribute="type" 
                    type="java.lang.String"/>
            </java-attributes>
        </java-type>
    </java-types>
</xml-bindings>

Demo

The get/set methods are used on the domain model to interact with the real properties and the accessors defined on the @XmlVirtualAccessMethods annotation are used to interact with the virtual properties.  The normal  JAXB mechanisms are used for marshal and unmarshal operations:

package blog.virtual;

import java.io.File;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

import javax.xml.bind.JAXBContext;
import javax.xml.bind.Marshaller;
import javax.xml.bind.Unmarshaller;

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>();
        properties.put(JAXBContextFactory.ECLIPSELINK_OXM_XML_KEY, "blog/virtual/binding.xml");
        JAXBContext jc = JAXBContext.newInstance(new Class[] {Customer.class, Address.class}, properties);

        Unmarshaller unmarshaller = jc.createUnmarshaller();
        File xml = new File("src/blog/virtual/input.xml");

        Customer customer = (Customer) unmarshaller.unmarshal(xml);
        customer.setLastName("Doe");
        customer.put("middleName", "Anne");

        PhoneNumber cellPhoneNumber = new PhoneNumber();
        cellPhoneNumber.setNumber("555-CELL");
        cellPhoneNumber.put("type", "CELL");
        customer.<List<PhoneNumber>>get("phoneNumbers").add(cellPhoneNumber);

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

}

XML 

As you can see below there is nothing special in XML related to real or virtual properties:

Input (input.xml)

<?xml version="1.0" encoding="UTF-8"?>
<customer id="123">
   <firstName>Jane</firstName>
   <lastName>Doh</lastName>
   <billingAddress>
      <street>1 Billing Street</street>
   </billingAddress>
   <shippingAddress>
      <street>2 Shipping Road</street>
   </shippingAddress>
   <phoneNumber type="WORK">555-WORK</phoneNumber>
   <phoneNumber type="HOME">555-HOME</phoneNumber>
</customer>

Output

<?xml version="1.0" encoding="UTF-8"?>
<customer id="123">
   <firstName>Jane</firstName>
   <middleName>Anne</middleName>
   <lastName>Doe</lastName>
   <billingAddress>
      <street>1 Billing Street</street>
   </billingAddress>
   <shippingAddress>
      <street>2 Shipping Road</street>
   </shippingAddress>
   <phoneNumber type="WORK">555-WORK</phoneNumber>
   <phoneNumber type="HOME">555-HOME</phoneNumber>
   <phoneNumber type="CELL">555-CELL</phoneNumber>
</customer>

Further Reading

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

• MOXy Extensible Models - Multi-Tenant Example
• Specifying EclipseLink MOXy as Your JAXB Provider  

 Other articles related to new MOXy features in EclipseLink 2.3:

• Overriding JAXB's "Name Mangling" Algorithm with EclipseLink MOXy
• Map to Element based on an Attribute Value with EclipseLink JAXB (MOXy)
• Using EclipseLink MOXy with a Metadata Repository