EclipseLink/UserGuide/sandbox/gelernter/Extensible Domain Models

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EclipseLink JPA

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This topic is currently under review.

Extensible Entities

EL NewIn.png New in version 2.3.



.

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 Software-as-a-Service) environment where a shared, generic application can be used by multiple clients (tenants). Tenants have private access to their own data, as well as to data shared with other tenants. See also Single-Table Multi-Tenancy.

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 to work with data after mappings have changed.
  • Provide an additional source of metadata to be used by an application.

To create and support an extensible entity,

  1. Configure the entity. See Configuring the Entity.
  2. Include flexible columns in the database table to store the additional data. See Designing the Schema.
  3. Specify extended mappings in the eclipselink-orm.xml file. See Providing Additional Mappings
  4. Configure persistence.xml. See Configuring persistence.xml.

Configuring the Entity

To configure the entity,

  1. Annotate with @VirtualAccessMethods
  2. Add get() and set() methods
  3. Add a data structure

Annotate with @VirtualAccessMethods

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

@VirtualAccessMethods Attributes
Attribute Description Default Required?
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. get Yes
set The name of the setter method to use for the virtual property This method must take a java.lang.String parameter and return a java.lang.Object parameter. set Yes


Add get() and set() Methods

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. You must use the the get(String) and set(String, Object) signatures, or else weaving will not work.

Elug note icon.png

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

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 Providing Additional Mappings.

A common way to store the virtual mappings is in a Map (as shown in the examples in this topic), 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 @Transient so it cannot use it for another mapping. When using property-based access, @Transient' is unnecessary.

Example

The following example shows an entity 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);
    }

Using XML

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

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

'REVIEWERS: // Is this correct? Is this adequate?//'

Designing the Schema

Provide database tables with extra columns for storing flexible mapping data. For example, the following Customer table includes two predefined columns, ID and NAME, and three flexible columns, FLEX_COL1, FLEX_COL2, FLEX_COL3:

  • CUSTOMER
    • INTEGER ID
    • VARCHAR NAME
    • VARCHAR FLEX_COL1
    • VARCHAR FLEX_COL2
    • VARCHAR FLEX_CO31

You can then specify which of those flex columns should be used to persist an extended attribute, as described below, in Providing Additional Mappings.

Providing Additional Mappings

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

<basic name="idNumber" attribute-type="String">
  <column name="FLEX_COL1"/>
  <access-methods get-method="get" set-method="set"/>
</basic>

//REVIEWERS: Are there any limitations on the types of mappings that support flexible mappings? Also, do you think anything more should be said about what you have to do in eclipselink-orm.xml?//

  • I think that part of this will be addressed by anything we do to document using <access-methods> to specify Virtual mappings--Tom.ware.oracle.com 16:04, 28 June 2011 (UTC)
  • The XML file simply gets treated as another XML file in the list of XML files. As long as you obey all the rules related to what can be overridden, you can use any kind of mapping. The challenge in using non-virtual mappings is how to have the data structures that support them make sense when the document is not there. e.g. if you're going to have an extension that uses an instance variable, for the instances of the application that don't use that extension file, how is that instance variable treated - JPA will likely try to use it for a mapping using its defaulting-rules --Tom.ware.oracle.com 16:04, 28 June 2011 (UTC)

Configuring persistence.xml

Configure persistence unit properties in persistence.xml to indicate that the application should retrieve the flexible mappings from the eclipselink-orm.xml file,. For example:

//REVIEWERS Did I get that intro right? Would different wording be better here? The design doc said “Use persistence unit properties to get your application to use the file."//

  • Both persistence unit propeties and persistence.xml are legitimate use cases. We should describe both. persistence.xml allows either a default, or a single-user file that can be changed. persistence unit properties allow specification of the file at runtime and provides a more dynamic experience.--Tom.ware.oracle.com 16:06, 28 June 2011 (UTC)
<property name="eclipselink.metadata-source" value="XML"/>
<property name="eclipselink.metadata-source.xml.url" value="foo://bar"/>

//REVIEWERS What more can be said about these? See my related questions below, under Configuring the EntityManagerFactory and the Metadata Repository.//

  • Maybe the two sections should go together. We could mention that by default we support using a file at a URL, but it is possible to also override how the repository works and then go into details.--Tom.ware.oracle.com 17:20, 28 June 2011 (UTC)

Examples

The following examples illustrate variations on configuring extensible entities.

Example 1

Example 1 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.
  • Extensions are mapped in a portable way by specifying @Transient.

Example 1

@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 2

Example 2 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.
  • Extensions are mapped in a portable way by specifying @Transient.
  • The XML for extended mapping indicates which get() and set() method to use.

Example 2

@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" attribute-type="String">
      <column name="FLEX_1"/>
      <access-methods get-method="getExtension" set-method="setExtension"/>
    </basic>

Example 3

Example 3 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.


@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);
    }
 
...

Configuring the 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. The current release supports XML repositories.

Specify the class to use and any configuration information for the metadata repository through persistence unit properties. The entity manager factory 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: *org.eclipse.persistence.internal.jpa.extensions.MetadataRepository *org.eclipse.persistence.internal.jpa.extensions.XMLMetadataRepository

Example

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

<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"/>

Refreshing the Metadata Repository

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

refreshMetadata takes a Map of properties, and that map of properties can be used to override the properties previously defined for the metadata-source.


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