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12 Making JPA Entities and JAXB Beans Extensible

This chapter provides instructions for making JPA entities and JAXB beans extensible. Mappings can be added or modified externally, without modifying the entity or bean source file and without redeploying the persistence unit. This feature is useful in a Software-as-a-Service environment where multiple clients can share applications and data sources. It is also useful for customizing an application during installation rather than during development.

This chapter contains the following sections:

Use Case

Users want to establish a SaaS environment, where applications and data sources are shared by multiple clients.

Solution

Use the TopLink extensibility feature to extend JPA entities and JAXB beans by using external mappings.

Components

  • TopLink 12c Release 1 (12.1.2) or later.

    Note:

    TopLink's core functionality is provided by EclipseLink, the open source persistence framework from the Eclipse Foundation. EclipseLink implements Java Persistence API (JPA), Java Architecture for XML Binding (JAXB), and other standards-based persistence technologies, plus extensions to those standards. TopLink includes all of EclipseLink, plus additional functionality from Oracle.

12.1 Making JPA Entities Extensible

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

Extensible entities are useful in a multi-tenant (or SaaS) architecture where a shared, generic application can be used by multiple clients (tenants). Tenants have private access to their own data, and to data shared with other tenants.

Using extensible entities, you can:

  • Build an application where some mappings are common to all users and some mappings are user-specific.

  • Add mappings to an application after it is made available to a customer (even post-deployment).

  • Use the same EntityManagerFactory interface to work with data after mappings have changed.

  • Provide an additional source of metadata to be used by an application.

12.1.1 Main Tasks for Creating and Supporting an Extensible JPA Entity

To create and support an extensible JPA entity:

12.1.1.1 Task 1: Configure the Entity

Configure the entity by annotating the entity class with @VirtualAccessMethods (or using the XML <access-methods>), adding get and set methods for the property values, and adding a data structure to store the extended attributes and values, as described in the following sections:

12.1.1.1.1 Annotate the Entity Class with @VirtualAccessMethods

Annotate the entity with @VirtualAccessMethods to specify that it is extensible and to define virtual properties.

Table 12-1 describes the attributes available to the @VirtualAccessMethods annotation.

Table 12-1 Attributes for the @VirtualAccessMethods 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 parameter.

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


12.1.1.1.2 Add get and set Methods to the Entity

Add get(String) and set(String, Object) methods to the entity. 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 @VirtualAccessMethods annotation.

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

Note:

Weaving is not supported when using virtual access methods with OneToOne mappings. If attempted, an exception will be thrown.

12.1.1.1.3 Define Virtual Attribute Storage

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 Section 12.1.1.3, "Task 3: Provide Additional Mappings."

A common way to store the virtual mappings is in a Map object (as shown in Example 12-1), but you can also use other strategies.

When using field-based access, annotate the data structure with @Transient so the structure cannot be used for another mapping. When using property-based access, @Transient is unnecessary.

Example 12-1 illustrates an entity class that uses property access.

Example 12-1 Entity Class that Uses Property Access

@Entity
@VirtualAccessMethods
public class Customer{
 
    @Id
    private int id;
    ...
 
    @Transient
    private Map<String, Object> extensions;
 
    public <T> T get(String name) {
        return (T) extentions.get(name);
    }
 
    public Object set(String name, Object value) {
        return extensions.put(name, value);
    }
12.1.1.1.4 Use XML

As an alternative to, or in addition to, using the @VirtualAccessMethods annotation, you can use an access="VIRTUAL" attribute on a mapping element (such as <basic>), for example:

<basic name="idNumber" access="VIRTUAL" attribute-type="String">
    <column name="FLEX_COL1"/>
</basic>

To set virtual access methods as the defaults for the persistence unit, use the <access> and <access-methods> elements, for example:

<persistence-unit-metadata>
           <xml-mapping-metadata-complete/>
           <exclude-default-mappings/>
           <persistence-unit-defaults>
                <access>VIRTUAL</access>
                <access-methods set-method="get" get-method="set"/>
              </persistence-unit-defaults>
   </persistence-unit-metadata>

12.1.1.2 Task 2: Design the Schema

Provide database tables with extra columns to store virtual attribute values. For example, the following Customer table includes two predefined columns, ID and NAME, and three columns for storing the attribute values, EXT_1, EXT_2, EXT_3:

CUSTOMER table

  • INTEGER ID

  • VARCHAR NAME

  • VARCHAR EXT_1

  • VARCHAR EXT_2

  • VARCHAR EXT_3

You can then specify which of the FLEX columns should be used to persist an extended attribute, as described in "Task 3: Provide Additional Mappings".

12.1.1.3 Task 3: Provide Additional Mappings

To provide additional mappings, add the mappings with the column and access-methods attributes to the eclipselink-orm.xml file, for example:

<basic name="idNumber" access="VIRTUAL" attribute-type="String">
  <column name="FLEX_COL1"/>
</basic>

12.1.1.4 Task 4: Externalizing Extensions Using a MetaDataSource

Configure persistence unit properties to indicate that the application should retrieve the flexible mappings from the eclipselink-orm.xml file. You can set persistence unit properties using the persistence.xml file or by setting properties on the EntityManagerFactory interface, as described in the following sections.

For more information about external mappings, see Chapter 13, "Using an External MetaData Source."

12.1.1.4.1 Configure the persistence.xml File

In the persistence.xml file, use the eclipselink.metadata-source property to use the default eclipselink-orm.xml file. Use the eclipselink.metadata-source.xml.url property to use a different file at the specified location, for example:

<property name="eclipselink.metadata-source" value="XML"/>
<property name="eclipselink.metadata-source.xml.url" value="foo://bar"/>
12.1.1.4.2 Configure EntityManagerFactory and the Metadata Repository

Extensions are added at bootstrap time through access to a metadata repository. The metadata repository is accessed through a class that provides methods to retrieve the metadata it holds. EclipseLink includes a metadata repository implementation that supports XML repositories.

Specify the class to use and any configuration information for the metadata repository through persistence unit properties. The EntityManagerFactory interface integrates additional mapping information from the metadata repository into the metadata it uses to bootstrap.

You can provide your own implementation of the class to access the metadata repository. Each metadata repository access class must specify an individual set of properties to use to connect to the repository.

You can subclass either of the following classes:

  • org.eclipse.persistence.internal.jpa.extensions.MetadataRepository

  • org.eclipse.persistence.internal.jpa.extensions.XMLMetadataRepository

In the following example, the properties that begin with com.foo are subclasses defined by the developer.

<property name="eclipselink.metadata-source" value="com.foo.MetadataRepository"/>
<property name="com.foo.MetadataRepository.location" value="foo://bar"/>
<property name="com.foo.MetadataRepository.extra-data" value="foo-bar"/>
12.1.1.4.3 Refresh the Metadata Repository

If you change the metadata and you want an EntityManager instance based on the new metadata, you must call the refreshMetadata() method on the EntityManagerFactory interface to refresh the data. The next EntityManager instance will be based on the new metadata.

The refreshMetadata() method takes a map of properties that can be used to override the properties previously defined for the metadata-source element.

12.1.2 Code Examples

Example 12-2 illustrates the following:

  • Field access is used for non-extension fields.

  • Virtual access is used for extension fields, using defaults (get(String) and set(String, Object)).

  • The get(String) and set(String, Object) methods will be woven, even if no mappings use them, because of the presence of @VirtualAccessMethods.

These items are illustrated in bold font.

Example 12-2 Virtual Access Using Default get and set Method Names

@Entity
@VirtualAccessMethods
public class Address {
 
    @Id
    private int id;
 
    @Transient
    private Map<String, Object> extensions;
 
    public int getId(){
        return id;
    }
 
    public <T> T get(String name) {
        return (T) extentions.get(name);
    }
 
    public Object set(String name, Object value) {
        return extensions.put(name, value);
    }
. 
.
.
 

Example 12-3 illustrates the following:

  • Field access is used for non-extension fields.

  • The @VirtualAccessMethods annotation overrides methods to be used for getting and setting.

  • The get(String) and set(String, Object) methods will be woven, even if no mappings use them, because of the presence of @VirtualAccessMethods.

  • The XML for extended mapping indicates which get() and set() method to use.

These items are illustrated in bold font.

Example 12-3 Overriding get and set Methods

@Entity
@VirtualAccessMethods(get="getExtension", set="setExtension")
public class Address {
 
    @Id
    private int id;
 
    @Transient
    private Map<String, Object> extensions;
 
    public int getId(){
        return id;
    }
 
    public <T> T getExtension(String name) {
        return (T) extensions.get(name);
    }
 
    public Object setExtension(String name, Object value) {
        return extensions.put(name, value);
    }
 
    ...
 
    <basic name="name"  access="VIRTUAL" attribute-type="String">
      <column name="FLEX_1"/>
    </basic>
 

Example 12-4 illustrates the following:

  • Property access is used for non-extension fields.

  • Virtual access is used for extension fields, using defaults (get(String) and set(String, Object)).

  • The extensions are mapped in a portable way. @Transient is not required, because property access is used.

  • The get(String) and set(String, Object) methods will be woven, even if no mappings use them, because of the presence of @VirtualAccessMethods.

These items are illustrated in bold font.

Example 12-4 Using Property Access

@Entity
@VirtualAccessMethods
public class Address {
 
    private int id;
 
    private Map<String, Object> extensions;
 
    @Id
    public int getId(){
        return id;
    }
 
    public <T> T get(String name) {
        return (T) extensions.get(name);
    }
 
    public Object set(String name, Object value) {
        return extensions.put(name, value);
    }
 
...

12.2 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 extensions to JAXB introduce 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:

12.2.1 Main Steps

To create and support an extensible JAXB bean:

12.2.1.1 Task 1: Configure the Bean

Configure the bean by 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. Alternatively, you can use the <xml-virtual-access-methods> element in eclipselink-orm.xml.

12.2.1.1.1 Annotate the Bean Class with @Xml VirtualAccessMethods

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

Table 12-2 describes the attributes available to the @XmlVirtualAccessMethods annotation.

Table 12-2 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


12.2.1.1.2 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.

12.2.1.1.3 Define Virtual Attribute Storage

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.

12.2.1.1.4 Use XML

As an alternative to, or in addition to, using @XmlVirtualAccessMethods, you can use the XML equivalents, for example:

  • 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"/>
    

12.2.1.2 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"/> 

12.2.2 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.

12.2.2.1 Basic Setup

Example 12-5 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 12-5 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 12-6 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 12-6 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 12-7 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 12-7 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 12-8 illustrates a PhoneNumber class. Like Customer, PhoneNumber will be an extensible class.

Example 12-8 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;
    }
 
}

12.2.2.2 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 captured in the eclipselink-oxm.xml mapping file or in files using the eclipselink-orm.xml schema. 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 12-9 illustrates the binding-tenant1.xml mapping file.

Example 12-9 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 12-10 illustrates the Customer class code for tenant 1 to obtain the data associated with virtual properties.

Example 12-10 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 12-11 illustrates the XML output from the Customer class for tenant 1.

Example 12-11 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 12-12 illustrates the binding-tenant2.xml mapping file.

Example 12-12 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 12-13 illustrates the tenant 2 Customer class code to obtain the data associated with virtual properties.

Example 12-13 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 12-14 illustrates the XML output from the Customer class for tenant 2.

Example 12-14 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>

12.3 Additional Resources

See the following resources for more information about the technologies and tools used to implement the solutions in this chapter: