The first thing to decide when implementing your persistence layer is which classes are to be persisted. If you need to persist a field/property then you must mark that class as persistable. In JPA there are three types of persistable classes.

• Entity - persistable class with full control over its persistence.
• MappedSuperclass - persistable class that will not be persisted into its own table simply providing some fields to be persisted. Consequently an inheritance tree cannot just have a mapped superclass on its own. Read more
• Embeddable - persistable class that is only persistable embedded into an entity class. Read more

To achieve the above aim with XML metadata, we do this

<entity class="org.datanucleus.test.Hotel">
    ...
</entity>

Alternatively, using JPA Annotations, like this

@Entity
public class Hotel
{
    ...
}

In the above example we have marked the class as an entity . We could equally have marked it as mapped-superclass (using annotation @MappedSuperclass, or XML element <mapped-superclass>) or as embeddable (using annotation @Embeddable, or XML element <embeddable>).

See also :-

• JPA XML reference
• JPA Annotations reference

Persistence Aware

With JPA you cannot access public fields of classes. DataNucleus allows an extension to permit this, but such classes need special enhancement. To allow this you need to

• Annotate the class that will access these public fields (assuming it isn't an Entity) with the DataNucleus extension annotation @PersistenceAware

You perform the annotation of the class as follows

@PersistenceAware
public class MyClassThatAccessesPublicFields
{
    ...
}

See also :-

• Annotations reference for @PersistenceAware

Read-Only

You can, if you wish, make a class read-only . This is a DataNucleus extension and you set it as follows

@Entity
@Extension(vendorName="datanucleus", key="read-only", value="true")
public class MyClass
{
    ...
}