Java Reference
In-Depth Information
You probably have a pretty good idea of exactly how O/R mapping in JPA 2 works just by
glancing at
listing 2.8
, even if you have no familiarity with ORM tools such as Hibernate.
Think about the annotations that mirror relational concepts such as tables, columns, foreign
keys, and primary keys.
The
@Entity
annotation signifies the fact that the
Bid
class is a JPA entity
. The
@Table
annotation tells JPA that the
Bid
entity is mapped to the
BIDS
table
. Simil-
arly, the
@Column
annotations ( and ) indicate which
Bid
properties map to which
BIDS
table fields. The
@Id
annotation is somewhat special. It marks the
Id
property as
the primary key for the
Bid
entity . Just like a database record, a primary key uniquely
identifies an entity instance. The
@GeneratedValue
annotation is used to indicate
that the persistence provider should automatically generate the primary key when the en-
tity is saved into the database. The
Bid
entity is related to a number of other JPA entities
by holding direct object references, such the
Bidder
and
Item
entities. JPA allows such
object reference-based implicit relationships to be elegantly mapped to the database. In the
example, this is exactly what the
@ManyToOne
(
and
) and
@JoinColumn
(
and
) annotations do. In both cases, the
@ManyToOne
annotation indicates the nature
