Issue with sequence why two entity are sharing the same sequence when generating schema with hbm2ddl ?

Question

I am using hbm2ddl in my hibernate based application to generate the db schema. The value of hibernate.hbm2ddl.auto property is create-drop.

I am using @Entity annotations for my POJO classes.

@Entity 
public class testTable1 {
     @Id     
     @GeneratedValue(strategy = GenerationType.SEQUENCE)
     Long id; 
}

@Entity 
public class testTable2 {
     @Id     
     @GeneratedValue(strategy = GenerationType.SEQUENCE)
     Long id; 
}

However on executing the code I keep getting continuously incremental Id values. e.g. for 2 tables the Id (i.e. Prim Key) should start each with 1. But after inserting records in Table 1, the sequence goes from next value for Table 2. It should start again from 1 for table 2. I tried GenerationType.SEQUENCE & GenerationType.AUTO. nothing works :-(

Answer 1

You are using the Global sequence generator that hibernate provide by default when no generator is provided as specifed by the JPA Spec. To have a private generator you should declare a private generator with the annotation @SequenceGenerator and set the generator attribute of the @GeneratedValue annotation

Extracted from javadoc

@GeneratedValue
(Optional) The name of the primary key generator to use as specified in the SequenceGenerator or TableGenerator annotation.

Defaults to the id generator supplied by persistence provider.

SequenceGenerator
This annotation defines a primary key generator that may be referenced by name when a generator element is specified for the GeneratedValue annotation. A sequence generator may be specified on the entity class or on the primary key field or property. The scope of the generator name is global to the persistence unit (across all generator types).

Example:

@SequenceGenerator(name="EMP_SEQ", sequenceName="private_sequence")

Hibernate recommends that new projects use hibernate.id.new_generator_mappings=true as the new generators are more efficient and closer to the JPA 2 specification semantic

Section 1.3. Properties
2.2.3. Mapping identifier properties

Complete example

@Entity
@SequenceGenerator(name="PRIVATE_SEQ", sequenceName="private_sequence")
public class test {

    @Id
    @GeneratedValue(strategy = GenerationType.SEQUENCE, generator="PRIVATE_SEQ")
    Long id;
}

Answer 2

 @Entity
    public class MyEntity2 {
        
        @Id
        @GeneratedValue(strategy = GenerationType.SEQUENCE, generator="myentity2_seq")
        @SequenceGenerator(name = "myentity2_seq",sequenceName = "myentity2_seq_table")
        private int id;

    @Column(length = 100)
    private String name;
//setters & getters ...
}

In the Database, it will create myentity2_seq_table Table for maintaining ids. we can customize table creation using initialValue = 100, allocationSize = 10 in SequenceGenerator.