Question

I have some code:

@Id
@SequenceGenerator(name = "SOMETHING_SEQ")
@GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "SOMETHING_SEQ")
@Column(name = "SOMETHING", nullable = false)
private Long id;

How is hibernate providing my id?

I see in my database there a single sequence named ‘hibernate_sequence’ and no other hibernate ‘special tables’.

Answer 1

Actually, here your SOMETHING_SEQ is the name of sequence you configured somewhere in your hibernate config. And hibernate_sequence is the sequence name in the database. In configuration it would be looking something like below,

<sequence-generator name="SOMETHING_SEQ" 
    sequence-name="hibernate_sequence"
    allocation-size="<any_number_value>"/>

You can completely skip this configuration by using annotation instead. Then your @SequenceGenerator annotation would need to provide few more paramters. Below is the example.

@SequenceGenerator(name="SOMETHING_SEQ", sequenceName="hibernate_sequence", allocationSize=10)

For example multiple entity classes would do something like below,

@Entity
public class Entity1 {
  @Id
  @SequenceGenerator(name = "entity1Seq", sequenceName="ENTITY1_SEQ", allocationSize=1)
  @GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "entity1Seq")
  @Column(name = "ID", nullable = false)
  private Long id;

  ...
  ...

}

@Entity
public class Entity2 {
  @Id
  @SequenceGenerator(name = "entity2Seq", sequenceName="ENTITY2_SEQ", allocationSize=10)
  @GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "entity2Seq")
  @Column(name = "ID", nullable = false)
  private Long id;

  ...
  ...

}