114 CHAPTER 6 n MAPPING WITH ANNOTATIONS CHAPTER (Web hosting isp)
114 CHAPTER 6 n MAPPING WITH ANNOTATIONS CHAPTER 6 n MAPPING WITH ANNOTATIONS 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 AuthorAddressclass with an Authorclass in this way. The AuthorAddress class is marked with the @Embeddableannotation. 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 maintain 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 embeddable 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 @AttributeOverrideand @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 addressand 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; }
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