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'.
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; ... ... } 
hibernate-seq, and of course when you said Hibernate to generate it, you must not provide any yourself. Or even if you provide some value per say, that value would get overwritten.How is hibernate providing my id?
Well, you explicitly told the JPA engine to generate identifier automatically (with the @GeneratedValue annotation) using a strategy of type SEQUENCE indicating that a database sequence should be used to generate the identifier. In case you wonder, sequences are database specific objects (e.g. Oracle) that can be used to generate unique integers.
I see in my database there a single sequence named 'hibernate_sequence'
You didn't use the sequenceName annotation element in your @SequenceGenerator to specify the name of the database sequence object to use so Hibernate created a default sequence object during schema generation (which defaults to hibernate_sequence). To specify a sequence, do it like this:
@Id @GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "my_entity_seq_gen") @SequenceGenerator(name = "my_entity_seq_gen", sequenceName="MY_ENTITY_SEQ") private Long id; In order to name the sequence you have to set the sequenceName in your @SequenceGenerator annotation:
@GeneratedValue(name="gen", strategy = GeneratorType.SEQUENCE) @SequenceGenerator(name="gen", sequenceName="Sequence_Name", allocationSize = 1) @Id public Long getId() { // ... } Of note, if you are using a pre-existing generator, your allocationSize must match the allocation size of that generator.
In Oracle you don't have the auto_increment type as in MySQL. So, to generate an auto_increment column you need to use a sequence.
This is an example of how you can achieve this.
create table test (id number, testdata varchar2(255)); create sequence test_seq start with 1 increment by 1 nomaxvalue; create trigger test_trigger before insert on test for each row begin select test_seq.nextval into :new.id from dual; end; So you create a sequence and use a trigger before each row is inserted to add its id.
So hibernate must be doing something like this, or instead of using the trigger doing
insert into test values(test_seq.nextval, 'no trigger needed!'); Note: Example taken from here
nextval on insert.