Mapping with Annotations - Beginning Hibernate: From Novice to Professional - page 103

• AUTO : Hibernate decides which generator type to use, based on the database's support
for primary key generation.
• IDENTITY : The database is responsible for determining and assigning the next primary
key.
• SEQUENCE : Some databases support a SEQUENCE column type. See the “Generating Primary
Key Values with @SequenceGenerator ” section later in the chapter.
• TABLE : This type keeps a separate table with the primary key values. See the “Generating
Primary Key Values with @TableGenerator ” section later in the chapter.
You will notice that the available values for the strategy attribute do not exactly match
the values for Hibernate's primary key generators for XML mapping. If you need to use
Hibernate-specific primary key generation strategies, you can use some of the Hibernate
extensions described at the end of this chapter—but as always, you risk forfeiting portabil-
ity of your application to other EJB 3 environments when taking advantage of Hibernate-
specific features.
For the Book class, we are going to use the default key generation strategy. Letting
Hibernate determine which generator type to use makes your code portable between dif-
ferent databases. Because we want Hibernate to use property access to our POJO, we must
annotate the getter method for the identifier, not the field that it accesses:
@Id
@GeneratedValue
public int getId() {
return id;
}
Generating Primary Key Values with @SequenceGenerator
As noted in the section on the @Id tag, we can declare the primary key property as being
generated by a database sequence. A sequence is a database object that can be used as a
source of primary key values. It is similar to the use of an identity column type, except that
a sequence is independent of any particular table and can therefore be used by multiple
tables.
To declare the specific sequence object to use and its properties, you must include an
@SequenceGenerator annotation on the annotated field. Here's an example:
@Id
@SequenceGenerator(name="seq1",sequenceName="HIB_SEQ")
@GeneratedValue(strategy=SEQUENCE,generator="seq1")
public int getId() {
return id;
}
Here, a sequence generation annotation named seq1 has been declared. This refers to the
database sequence object called HIB_SEQ . The name seq1 is then referenced as the generator
attribute of the @GeneratedValue annotation.