[#javax-jdo-annotation-Discrimnator]
= @Discriminator (`jdo`)

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The `@javax.jdo.annotation.Discriminator` is used by JDO/DataNucleus to specify how to discriminate between subclasses of an inheritance hierarchy.

It is valid to add a `@Discriminator` for any class, even those not part of an explicitly mapped inheritance hierarchy.
Apache Causeway also checks for this annotation, and if present will use the `@Discriminator#value()` as the object type, a unique alias for the object's class name.

[NOTE]
====
Causeway parses the `@Discriminator` annotation from the Java source code; it does not query the JDO metamodel.
This means that it the `@Discriminator` annotation must be used rather than the equivalent `<discriminator>` link:https://www.datanucleus.org/products/accessplatform_6_0/jdo/metadata_xml.html[XML metadata].
====

This value is used internally to generate a string representation of an objects identity (the `Oid`).
This can appear in several contexts, including:

* as the value of `Bookmark#getObjectType()` and in the `toString()` value of `Bookmark`
(see xref:refguide:applib:index/services/bookmark/BookmarkService.adoc[BookmarkService])
* in the serialization of `OidDto` in the xref:refguide:schema:cmd.adoc[command] and xref:refguide:schema:ixn.adoc[interaction] schemas
* in the URLs of the xref:vro:ROOT:about.adoc[RestfulObjects viewer]
* in the URLs of the xref:vw:ROOT:about.adoc[Web UI (Wicket viewer)]
* in XML snapshots generated by the xref:refguide:applib:index/services/xmlsnapshot/XmlSnapshotService.adoc[XmlSnapshotService]

== Examples

For example:

[source,java]
----
@javax.jdo.annotations.Discriminator(value="custmgmt.Customer")
public class Customer {
    ...
}
----

has an object type of `custmgmt.Customer`.

== Precedence

The rules of precedence for determining a domain object's object type are:

1. xref:refguide:applib-ant:Discriminator.adoc[@Discriminator]
2. `@DomainObject#logicalTypeName`
3. xref:refguide:applib-ant:PersistenceCapable.adoc[@PersistenceCapable], if at least the `schema` attribute is defined.
+
If both `schema` and `table` are defined, then the value is "`schema.table`".
If only `schema` is defined, then the value is "`schema.className`".

4. Fully qualified class name of the entity.

[TIP]
====
This might be obvious, but to make explicit: we recommend that you always specify an object type for your domain objects.

Otherwise, if you refactor your code (change class name or move package), then any externally held references to the OID of the object will break.
At best this will require a data migration in the database; at worst it could cause external clients accessing data through the xref:vro:ROOT:about.adoc[Restful Objects] viewer to break.
====

[NOTE]
====
If the object type is not unique across all domain classes then the framework will fail-fast and fail to boot.
An error message will be printed in the log to help you determine which classes have duplicate object tyoes.
====
