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EclipseLink Solutions Guide for EclipseLink
Release 2.4
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Beta Draft: 2012-12-13

Making JAXB Beans Extensible

Use the @XmlVirtualAccessMethods annotation to specify that a JAXB bean is extensible. By using virtual properties in an extensible bean, you can specify mappings external to the bean. This allows you to modify the mappings without modifying the bean source file and without redeploying the bean's persistence unit.

In a multi-tenant (or SaaS) architecture, a single application runs on a server, serving multiple client organizations (tenants). Good multi-tenant applications allow per-tenant customizations. When these customizations are made to data, it can be difficult for the binding layer to handle them. JAXB is designed to work with domain models that have real fields and properties. EclipseLink Object-XML 2.3 (also known as MOXy) introduces the concept of virtual properties which can easily handle this use case. Virtual properties are defined by the Object-XML metadata file, and provide a way to extend a class without modifying the source.

This section has the following subsections:

Main Steps

To create and support an extensible JAXB bean:

Task 1: Configure the Bean

Configuring the bean consists of annotating the bean class with the @XmlVirtualAccessMethods, adding get and set methods for the property values, and adding a data structure to store the extended attributes and values.

Annotate the Bean Class with @Xml VirtualAccessMethods

Annotate the bean with @XmlVirtualAccessMethods to specify that it is extensible and to define virtual properties.

Table 10-2 describes the attributes available to the @XmlVirtualAccessMethods annotation.

Table 10-2 Attributes for the @XmlVirtualAccessMethods Annotation

Attribute Description

get

The name of the getter method to use for the virtual property. This method must take a single java.lang.String parameter and return a java.lang.Object.

Default: get

Required? No

set

The name of the setter method to use for the virtual property. This method must take a java.lang.String and a java.lang.Object parameter and return a java.lang.Object parameter.

Default: set

Required? No


Add get and set Methods to the Bean

Add get(String) and set(String, Object) methods to the bean. The get() method returns a value by property name and the set() method stores a value by property name. The default names for these methods are get and set, and they can be overridden with the @XmlVirtualAccessMethods annotation.

EclipseLink weaves these methods if weaving is enabled, which provides support for lazy loading, change tracking, fetch groups, and internal optimizations.

Add a Data Structure

Add a data structure to store the extended attributes and values, that is, the virtual mappings. These can then be mapped to the database. See "Task 2: Provide Additional Mappings".

A common way to store the virtual mappings is in a Map, but you can use other ways, as well. For example you could store the virtual mappings in a directory system.

When using field-based access, annotate the data structure with @XmlTransient so it cannot use it for another mapping. When using property-based access, @XmlTransient is unnecessary.

Use XML

As an alternative to, or in addition to, using @XmlVirtualAccessMethods, you can use the <access> and <access-methods> elements, for example:

<access>VIRTUAL</access>
<access-methods set-method="get" get-method="set"/>

XML to enable virtual access methods using get and set:

<xml-virtual-access-methods/>

XML to enable virtual access methods using put instead of set (default):

<xml-virtual-access-methods set-method="put"/>

XML to enable virtual access methods using retrieve instead of get (default):

<xml-virtual-access-methods get-method="retrieve"/>

XML to enable virtual access methods using retrieve and put instead of get and set (default):

<xml-virtual-access-methods get-method="retrieve" set-method="put"/>

Task 2: Provide Additional Mappings

To provide additional mappings, add the mappings to the eclipselink-oxm.xml file, for example:

<xml-element java-attribute="idNumber"/> 

Code Examples

The examples in this section illustrate how to use extensible JAXB beans. The example begins with the creation of a base class that other classes can extend. In this case the extensible classes are for Customers and PhoneNumbers. Mapping files are created for two separate tenants. Even though both tenants share several real properties, they will define virtual properties that are unique to their requirements.

Basic Setup

Example 10-5 illustrates a base class, ExtensibleBase, which other extensible classes can extend. In the example, the use of the @XmlTransient annotation prevents ExtensibleBase from being mapped as an inheritance relationship. The real properties represent the parts of the model that will be common to all tenants. The per-tenant extensions will be represented as virtual properties.

Example 10-5 A Base Class for Extensible Classes

package examples.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);
    }
}

Example 10-6 illustrates the definition of a Customer class. The Customer class is extensible because it inherits from a domain class that has been annotated with @XmlVirtualAccessMethods.

Example 10-6 An Extensible Customer Class

package examples.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;
    }
 
}

Example 10-7 illustrates an Address class. It is not necessary for every class in your model to be extensible. In this example, the Address class does not have any virtual properties.

Example 10-7 A Nonextensible Address Class

package examples.virtual;
 
public class Address {
 
    private String street;
 
    public String getStreet() {
        return street;
    }
 
    public void setStreet(String street) {
        this.street = street;
    }
 
}

Example 10-8 illustrates a PhoneNumber class. Like Customer, PhoneNumber will be an extensible class.

Example 10-8 An Extensible PhoneNumber Class

package examples.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;
    }
 
}

Define the Tenants

The examples in this section define two separate tenants. Even though both tenants share several real properties, the corresponding XML representation can be quite different due to virtual properties.

Tenant 1

The first tenant is an online sporting goods store that requires the following extensions to its model:

  • Customer ID

  • Customer's middle name

  • Shipping address

  • A collection of contact phone numbers

  • Type of phone number (that is, home, work, or cell)

The metadata for the virtual properties is supplied through Object-XML's XML mapping file. Virtual properties are mapped in the same way as real properties. Some additional information is required, including type (since this cannot be determined through reflection), and for collection properties, a container type. The virtual properties defined below for Customer are middleName, shippingAddress, and phoneNumbers. For PhoneNumber, the virtual property is the type property.

Example 10-9 illustrates the binding-tenant1.xml mapping file.

Example 10-9 Defining Virtual Properties for Tenant 1

<?xml version="1.0"?>
<xml-bindings
    xmlns="http://www.eclipse.org/eclipselink/xsds/persistence/oxm"
    package-name="examples.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="examples.virtual.Address"/>
                <xml-element
                    java-attribute="phoneNumbers"
                    name="phoneNumber"
                    type="examples.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>

The get and 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. Example 10-10 illustrates the Customer class code for tenant 1 to obtain the data associated with virtual properties.

Example 10-10 Tenant 1 Code to Provide the Data Associated with Virtual Properties

...
Customer customer = new Customer();
 
//Set Customer's real properties
customer.setFirstName("Jane");
customer.setLastName("Doe");
 
Address billingAddress = new Address();
billingAddress.setStreet("1 Billing Street");
customer.setBillingAddress(billingAddress);
 
//Set Customer's virtual 'middleName' property
customer.put("middleName", "Anne");
 
//Set Customer's virtual 'shippingAddress' property
Address shippingAddress = new Address();
shippingAddress.setStreet("2 Shipping Road");
customer.put("shippingAddress", shippingAddress);
 
List<PhoneNumber> phoneNumbers = new ArrayList<PhoneNumber>();
customer.put("phoneNumbers", phoneNumbers);
 
PhoneNumber workPhoneNumber = new PhoneNumber();
workPhoneNumber.setNumber("555-WORK");
//Set the PhoneNumber's virtual 'type' property
workPhoneNumber.put("type", "WORK");
phoneNumbers.add(workPhoneNumber);
 
PhoneNumber homePhoneNumber = new PhoneNumber();
homePhoneNumber.setNumber("555-HOME");
//Set the PhoneNumber's virtual 'type' property
homePhoneNumber.put("type", "HOME");
phoneNumbers.add(homePhoneNumber);
 
Map<String, Object> properties = new HashMap<String, Object>();
properties.put(JAXBContextFactory.ECLIPSELINK_OXM_XML_KEY, "examples/virtual/binding-tenant1.xml");
JAXBContext jc = JAXBContext.newInstance(new Class[] {Customer.class, Address.class}, properties);
 
Marshaller marshaller = jc.createMarshaller();
marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);
marshaller.marshal(customer, System.out);
...
 

Example 10-11 illustrates the XML output from the Customer class for tenant 1.

Example 10-11 XML Output from the Customer Class for Tenant 1

<?xml version="1.0" encoding="UTF-8"?>
<customer>
   <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>
</customer>

Tenant 2

The second tenant is a streaming media provider that offers on-demand movies and music to its subscribers. It requires a different set of extensions to the core model:

  • A single contact phone number

For this tenant, the mapping file is also used to customize the mapping of the real properties.

Example 10-12 illustrates the binding-tenant2.xml mapping file.

Example 10-12 Defining Virtual Properties for Tenant 2

<?xml version="1.0"?>
<xml-bindings
    xmlns="http://www.eclipse.org/eclipselink/xsds/persistence/oxm"
    package-name="examples.virtual">
    <xml-schema namespace="urn:tenant1" element-form-default="QUALIFIED"/>
    <java-types>
        <java-type name="Customer">
            <xml-type prop-order="firstName lastName billingAddress phoneNumber"/>
            <java-attributes>
                <xml-attribute java-attribute="firstName"/>
                <xml-attribute java-attribute="lastName"/>
                <xml-element java-attribute="billingAddress" name="address"/>
                <xml-element
                    java-attribute="phoneNumber"
                    type="examples.virtual.PhoneNumber"/>
            </java-attributes>
        </java-type>
    </java-types>
</xml-bindings>

Example 10-13 illustrates the tenant 2 Customer class code to obtain the data associated with virtual properties.

Example 10-13 Tenant 2 Code to Provide the Data Associated with Virtual Properties

...
Customer customer = new Customer();
customer.setFirstName("Jane");
customer.setLastName("Doe");
 
Address billingAddress = new Address();
billingAddress.setStreet("1 Billing Street");
customer.setBillingAddress(billingAddress);
 
PhoneNumber phoneNumber = new PhoneNumber();
phoneNumber.setNumber("555-WORK");
customer.put("phoneNumber", phoneNumber);
 
Map<String, Object> properties = new HashMap<String, Object>();
properties.put(JAXBContextFactory.ECLIPSELINK_OXM_XML_KEY, "examples/virtual/binding-tenant2.xml");
JAXBContext jc = JAXBContext.newInstance(new Class[] {Customer.class, Address.class}, properties);
 
Marshaller marshaller = jc.createMarshaller();
marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);
marshaller.marshal(customer, System.out);
...

Example 10-14 illustrates the XML output from the Customer class for tenant 2.

Example 10-14 XML Output from the Customer Class for Tenant 2

<?xml version="1.0" encoding="UTF-8"?>
<customer xmlns="urn:tenant1" firstName="Jane" lastName="Doe">
   <address>
      <street>1 Billing Street</street>
   </address>
   <phoneNumber>555-WORK</phoneNumber>
</customer>
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