Inheritance and custom types in Java

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You now know everything you need to know about the mapping of entities, properties, and inheritance hierarchies. You can already map complex domain models. In the second half of this chapter, we discuss another important feature that you should know by heart as a Hibernate user: the Hibernate mapping type system.
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The Hibernate type system
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In chapter 4, we first distinguished between entity and value types a central concept of ORM in Java. We must elaborate on that distinction in order for you to fully understand the Hibernate type system of entities, value types, and mapping types.
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Recapitulating entity and value types
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Entities are the coarse-grained classes in your system. You usually define the features of a system in terms of the entities involved. The user places a bid for an item is a typical feature definition; it mentions three entities. Classes of value types often don t even appear in the business requirements they re usually the fine-grained classes representing strings, numbers, and monetary amounts. Occasionally, value types do appear in feature definitions: the user changes billing address is one example, assuming that Address is a value type. More formally, an entity is any class whose instances have their own persistent identity. A value type is a class that doesn t define some kind of persistent identity. In practice, this means that entity types are classes with identifier properties, and value type classes depend on an entity. At runtime, you have a network of entity instances interleaved with value type instances. The entity instances may be in any of the three persistent lifecycle states: transient, detached, or persistent. We don t consider these lifecycle states to apply to the value type instances. (We ll come back to this discussion of object states in chapter 9.) Therefore, entities have their own lifecycle. The save() and delete() methods of the Hibernate Session interface apply to instances of entity classes, never to value type instances. The persistence lifecycle of a value type instance is completely tied to the lifecycle of the owning entity instance. For example, the username becomes persistent when the user is saved; it never becomes persistent independently of the user.
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In Hibernate, a value type may define associations; it s possible to navigate from a value type instance to some other entity. However, it s never possible to navigate from the other entity back to the value type instance. Associations always point to entities. This means that a value type instance is owned by exactly one entity when it s retrieved from the database; it s never shared. At the level of the database, any table is considered an entity. However, Hibernate provides certain constructs to hide the existence of a database-level entity from the Java code. For example, a many-to-many association mapping hides the intermediate association table from the application. A collection of strings (more accurately, a collection of value-typed instances) behaves like a value type from the point of view of the application; however, it s mapped to its own table. Although these features seem nice at first (they simplify the Java code), we have over time become suspicious of them. Inevitably, these hidden entities end up needing to be exposed to the application as business requirements evolve. The many-to-many association table, for example, often has additional columns added as the application matures. We re almost prepared to recommend that every database-level entity be exposed to the application as an entity class. For example, we would be inclined to model the many-to-many association as two one-to-many associations to an intervening entity class. We ll leave the final decision to you, however, and come back to the topic of many-to-many entity associations in the future chapters. Entity classes are always mapped to the database using <class>, <union-subclass>, <subclass>, and <joined-subclass> mapping elements. How are value types mapped You ve already met two different kinds of value type mappings: <property> and <component>. The value type of a component is obvious: It s the class that is mapped as embeddable. However, the type of a property is a more generic notion. Consider this mapping of the CaveatEmptor User and email address:
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