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EclipseLink Solutions Guide for EclipseLink
Release 2.5
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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:

Main Tasks for Creating and Supporting an Extensible JPA Entity

To create and support an extensible JPA entity:

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:

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


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.


NoteNote:

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


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 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);
    }

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>

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".

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>

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."

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"/>

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"/>

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.

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);
    }
 
...
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