[JDO-764] Allow JDO annotations to be used in meta-annotations

• Type: Improvement
• Status: Closed
• Priority: Major
• Resolution: Fixed
• Affects Version/s: JDO 3.1
• Fix Version/s: JDO 3.2
• Component/s: api, specification
• Assignee: Unassigned
• Reporter: Andy Jefferson
• Created: 25/May/17 08:51
• Updated: 17/Feb/22 19:54
• Resolved: 16/Nov/17 18:11

Description

By default annotations are used directly in a persistable class. Java additionally allows annotations to be formed of other annotations. This is particularly useful where a user has a particular combination of annotations to set on a class/field/method and wants to simply annotate with an abbreviated form. For example, specifying attributes of an annotation

@PersistenceCapable(detachable="true", identityType="datastore", embeddedOnly="true")

or formed of multiple annotations

@PersistenceCapable(detachable="true")
@Extension(vendorName="datanucleus", key="multitenancy-column-name", value="TENANT")

These can be represented as meta-annotations like this

@Target(TYPE)
@Retention(RUNTIME)
@PersistenceCapable(detachable="true", identityType="datastore", embeddedOnly="true")
public @interface DatastoreIdPersistable
{
}

@Target(TYPE)
@Retention(RUNTIME)
@PersistenceCapable(detachable="true")
@Extension(vendorName="datanucleus", key="multitenancy-column-name", value="TENANT")
public @interface MultitenantPersistable
{
}

and the user can subsequently just annotate their persistable class as

@DatastoreIdPersistable
public class MyClass1

{...}


@MultitenantPersistable
public class MyClass2 {...}

The work required to support this in the JDO spec is simply to update the following annotations to add @Target(

{ElementType.ANNOTATION_TYPE}

)

The annotations requiring this are
@Element, @EmbeddedId, @Key, @NotPersistent, @Order, @Persistent, @Serialized, @Transactional, and @Value (all other annotations already have @Target(

{ElementType.TYPE}

) which already permits their usage in meta-annotations.

The same is proposed for JPA 2.2, see https://github.com/javaee/jpa-spec/issues/43