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3.5.2 Using components
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So far, the classes of our object model have all been entity classes with their own lifecycle and identity. The User class, however, has a special kind of association with the Address class, as shown in figure 3.5. In object modeling terms, this association is a kind of aggregation a part of relationship. Aggregation is a strong form of association: It has additional semantics with regard to the lifecycle of objects. In our case, we have an even stronger
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Mapping persistent classes
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firstname : String lastname : String username : String password : String email : String ranking : int created : Date billing home
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Address
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street : String zipCode : String city : String
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Figure 3.5 Relationships between User and Address using composition
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form, composition, where the lifecycle of the part is dependent on the lifecycle of the whole. Object modeling experts and UML designers will claim that there is no difference between this composition and other weaker styles of association when it comes to the Java implementation. But in the context of ORM, there is a big difference: a composed class is often a candidate value type. We now map Address as a value type and User as an entity. Does this affect the implementation of our POJO classes Java itself has no concept of composition a class or attribute can t be marked as a component or composition. The only difference is the object identifier: A component has no identity, hence the persistent component class requires no identifier property or identifier mapping. The composition between User and Address is a metadata-level notion; we only have to tell Hibernate that the Address is a value type in the mapping document. Hibernate uses the term component for a user-defined class that is persisted to the same table as the owning entity, as shown in listing 3.7. (The use of the word component here has nothing to do with the architecture-level concept, as in software component.)
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Listing 3.7 Mapping the User class with a component Address
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<class name="User" table="USER"> <id name="id" column="USER_ID" type="long"> <generator class="native"/> </id> <property
Fine-grained object models
name="username" column="USERNAME" type="string"/> <component name="homeAddress" class="Address">
Declare persistent attributes
<property name="street" type="string" column="HOME_STREET" notnull="true"/> <property name="city" type="string" column="HOME_CITY" not-null="true"/> <property name="zipcode" type="short" column="HOME_ZIPCODE" not-null="true"/> </component> <component name="billingAddress" class="Address">
Reuse component class
<property name="street" type="string" column="BILLING_STREET" notnull="true"/> <property name="city" type="string" column="BILLING_CITY" not-null="true"/> <property name="zipcode" type="short" column="BILLING_ZIPCODE" not-null="true"/> </component> ... </class>
We declare the persistent attributes of Address inside the <component> element. The property of the User class is named homeAddress. We reuse the same component class to map another property of this type to the same table.
Mapping persistent classes
Figure 3.6 shows how the attributes of the Address class are persisted to the same table as the User entity. Notice that in this example, we have modeled the composition association as unidirectional. We can t navigate from Address to User. Hibernate supports both unidirectional and bidirectional compositions; however, unidirectional composition is far more common. Here s an example of a bidirectional mapping:
Figure 3.6 Table attributes of User <component with Address component name="homeAddress" class="Address"> <parent name="user"/> <property name="street" type="string" column="HOME_STREET"/> <property name="city" type="string" column="HOME_CITY"/> <property name="zipcode" type="short" column="HOME_ZIPCODE"/> </component>
The <parent> element maps a property of type User to the owning entity, in this example, the property is named user. We then call Address.getUser() to navigate in the other direction. A Hibernate component may own other components and even associations to other entities. This flexibility is the foundation of Hibernate s support for finegrained object models. (We ll discuss various component mappings in chapter 6.) However, there are two important limitations to classes mapped as components:
Shared references aren t possible. The component Address doesn t have its own database identity (primary key) and so a particular Address object can t be referred to by any object other than the containing instance of User. There is no elegant way to represent a null reference to an Address. In lieu of an elegant approach, Hibernate represents null components as null values in all mapped columns of the component. This means that if you store a component object with all null property values, Hibernate will return a null component when the owning entity object is retrieved from the database.
Support for fine-grained classes isn t the only ingredient of a rich domain model. Class inheritance and polymorphism are defining features of object-oriented models.
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