DataNucleus Access Platform - Application Identity

With application identity you are taking control of the specification of id's to DataNucleus. Application identity requires a primary key class (unless using SingleFieldIdentity, where one is provided for you) , and each persistent capable class may define a different class for its primary key, and different persistent capable classes can use the same primary key class, as appropriate. With application identity the field(s) of the primary key will be present as field(s) of the class itself. To specify that a class is to use application identity , you add the following to the MetaData for the class.

<class name="MyClass" identity-type="application" objectid-class="MyIdClass">
    <field name="myPrimaryKeyField" primary-key="true"/>
    ...
</class>

For JDO2 we specify the identity-type and objectid-class . The objectid-class is the class defining the identity for this class. Alternatively, if we are using annotations

@PersistenceCapable(objectIdClass=MyIdClass.class)
public class MyClass
{
    @Persistent(primaryKey="true")
    private long myPrimaryKeyField;
}

When you have an inheritance hierarchy, you should specify the identity type and any primary-key fields in the base class for the inheritance tree. This is then used for all persistent classes in the tree.

See also :-

Primary Key

Using application identity requires the use of a Primary Key class. In JDO 1.0 it was necessary to always provide this class. In JDO 2.0 an in-built class is available where the identity is defined in a single field. This is referred to as SingleFieldIdentity . DataNucleus supports this builtin class. Where the class has multiple fields that form the primary key a Primary Key class must be provided. In JPA1 when there is a single primary key field you dont need to specify the primary key class. If there are more than 1 id field then you define the id-class .