Mapping with Annotations - Beginning Hibernate: From Novice to Professional - page 114

Mapping an Embedded (Component) One-to-One Association
When all the fields of one entity are maintained within the same table as another, the
enclosed entity is referred to in Hibernate as a component . The EJB 3 standard refers to
such an entity as being embedded .
The @Embedded and @Embeddable attributes are used to manage this relationship. In this
book's database example, we associate an AuthorAddress class with an Author class in this way.
The AuthorAddress class is marked with the @Embeddable annotation. An embeddable
entity must be composed entirely of basic fields and attributes. An embeddable entity can
only use the @Basic , @Column , @Lob , @Temporal , and @Enumerated annotations. It cannot main-
tain its own primary key with the Id tag because its primary key is the primary key of the
enclosing entity.
The @Embeddable annotation itself is purely a marker annotation, and takes no additional
attributes, as demonstrated in Listing 6-11. Typically, the fields and properties of the embedd-
able entity need no further markup.
Listing 6-11. Marking an Entity for Embedding Within Other Entities
@Embeddable
public class AuthorAddress {
...
}
The enclosing entity then marks appropriate fields or getters in entities, making use of the
embeddable class with the @Embedded annotation, as shown in Listing 6-12.
Listing 6-12. Marking an Embedded Property
@Embedded
public AuthorAddress getAddress() {
return this.address;
}
The @Embedded annotation draws its column information from the embedded type, but
permits the overriding of a specific column or columns with the @AttributeOverride and
@AttributeOverrides tags (the latter to enclose an array of the former if multiple columns are
being overridden). For example, Listing 6-13 shows how to override the default column names
of the address and country attributes of AuthorAddress with columns named ADDR and NATION .
Listing 6-13. Overriding Default Attributes of an Embedded Property
@Embedded
@AttributeOverrides({
@AttributeOverride(name="address",column=@Column(name="ADDR")),
@AttributeOverride(name="country",column=@Column(name="NATION"))
})
public AuthorAddress getAddress() {
return this.address;
}