JPA 类继承机制---spec摘要

Entities support class inheritance, polymorphic associations, and polymorphic queries. They can extend non-entity classes, and non-entity classes can extend entity classes. Entity classes can be both abstract and concrete.

1) Abstract Entities
An abstract class may be declared an entity by decorating the class with @Entity. Abstract entities differ from concrete entities only in that they cannot be instantiated.
Abstract entities can be queried just like concrete queries. If an abstract entity is the target of a query, the query operates on all the concrete subclasses of the abstract entity.

2) Mapped Superclasses
Entities may inherit from superclasses that contain persistent state and mapping information,but are not entities. That is, the superclass is not decorated with the @Entity annotation, and is not mapped as an entity by the Java Persistence provider. These superclasses are most often used when you have state and mapping information common to multiple entity classes.
Mapped superclasses are not queryable, and can’t be used in EntityManager or Query operations. You must use entity subclasses of the mapped superclass in EntityManager or Query operations.Mapped superclasses can’t be targets of entity relationships.Mapped superclasses can be abstract or concrete.
Mapped superclasses do not have any corresponding tables in the underlying datastore. Entities that inherit from the mapped superclass define the table mappings

3)Non-Entity Superclasses
Entities may have non-entity superclasses, and these superclasses can be either abstract or concrete. The state of non-entity superclasses is non-persistent, and any state inherited from the non-entity superclass by an entity class is non-persistent.Non-entity superclasses may not be used in EntityManager or Query operations. Any mapping or relationship annotations in non-entity superclasses are ignored.

4) Entity InheritanceMapping Strategies
You can configure how the Java Persistence provider maps inherited entities to the underlying datastore by decorating the root class of the hierarchy with the
javax.persistence.Inheritance annotation. There are three mapping strategies that are used to map the entity data to the underlying database:
■ A single table per class hierarchy
■ A table per concrete entity class
■ A “join” strategy, where fields or properties that are specific to a subclass are mapped to a different table than the fields or properties that are common to the parent class