[
https://issues.apache.org/jira/browse/JDO-764?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16103024#comment-16103024
]
Andy Jefferson commented on JDO-764:
------------------------------------
Why does the patch result in things like this
@PersistenceCapable(table="...")
@PersistenceCapable(detachable="true", identityType=IdentityType.DATASTORE)
@DatastoreIdentity(strategy=IdGeneratorStrategy.IDENTITY,
column="DATASTORE_IDENTITY")
@Discriminator(strategy=DiscriminatorStrategy.CLASS_NAME,
column="DISCRIMINATOR", indexed="true")
@Inheritance(strategy=InheritanceStrategy.NEW_TABLE)
public class XXX {...}
You have duplicated @PersistenceCapable, so which will be used ? likely the
first that is presented to the accessing code by Java.
No "merging" will take place in DataNucleus code, because it should never
expect such things. The proposal to support "meta-annotations" was not a
proposal to support merging of duplicated annotations (with normal annotation
usage you will never get duplications, so there is no basic requirement to
support merging there)
> Allow JDO annotations to be used in meta-annotations
> ----------------------------------------------------
>
> Key: JDO-764
> URL: https://issues.apache.org/jira/browse/JDO-764
> Project: JDO
> Issue Type: Improvement
> Components: api, specification
> Affects Versions: JDO 3.1
> Reporter: Andy Jefferson
> Fix For: JDO 3.2
>
> Attachments: jdo-764.patch
>
>
> 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
--
This message was sent by Atlassian JIRA
(v6.4.14#64029)
