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 JDO 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 in the
base
class for the inheritance tree. This is then used for all persistent classes
in the tree.
|
See also :-
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
.
See also :-
Where one of the fields that is primary-key of your class is a persistable object you have
something known as
compound identity
since the identity of this class contains the
identity of a related class. Please refer to the docs for
Compound Identity
By choosing
application identity
you are controlling the process of identity generation for this
class. This does not mean that you have a lot of work to do for this. JDO2 and JPA1 define many ways of
generating these identities and DataNucleus supports all of these and provides some more of its own besides.
See also :-
When using
application identity
, the class has associated field(s) that equate to the identity.
As a result you can simply access the values for these field(s).
Alternatively you could use a JDO identity-independent way
Object id = pm.getObjectId(obj);
Object id = JDOHelper.getObjectId(obj);
JDO allows implementations to support the changing of the identity of a persisted object.
This is an optional feature and DataNucleus doesn't currently support it.
If you have the JDO identity then you can access the object with that identity like this
Object obj = pm.getObjectById(id);
If you are using SingleField identity then you can access it from the object class name
and the key value like this
Object obj = pm.getObjectById(MyClass.class, mykey);
If you are using your own PK class then the
mykey
value is the toString() form
of the identity of your PK class.
