Hibernate/JPA - Primary Key Generation Strategies

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The JPA specification supports 4 different primary key generation strategies that generate the primary key values programmatically or use database features, like auto-incremented columns or sequences.

4 Options to Generate Primary Keys

The JPA specification supports 4 different primary key generation strategies that generate the primary key values programmatically or use database features, like auto-incremented columns or sequences. The only thing you have to do is to add the @GeneratedValue annotation to your primary key attribute and choose a generation strategy.

@Id
@GeneratedValue
@Column(name = "id", updatable = false, nullable = false)
private Long id;

1. GenerationType.AUTO

The GenerationType.AUTO is the default generation type and lets the persistence provider choose the generation strategy.
For example, consider we have a Student JPA entity class with GenerationType.AUTO as generation type:

@Entity
@Table(name = "student")
public class Student {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    @Column(name = "id", updatable = false, nullable = false)
    private Long id;

    @Column(name = "first_name")
    private String firstName;

    @Column(name = "last_name")
    private String lastName;

    @Column(name = "email")
    private String email;

    public Student() {

    }

   // getter and setters
}

If you use Hibernate as your persistence provider, it selects a generation strategy based on the database-specific dialect. For most popular databases, it selects GenerationType.SEQUENCE which I will explain in a further section.

2. GenerationType.IDENTITY

The GenerationType.IDENTITY is the easiest to use but not the best one from a performance point of view. It relies on an auto-incremented database column and lets the database generate a new value with each insert operation. From a database point of view, this is very efficient because the auto-increment columns are highly optimized, and it doesn’t require any additional statements.

For example, consider we have a Student JPA entity class with GenerationType.IDENTITY as a generation type:

@Entity
@Table(name = "student")
public class Student {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "id", updatable = false, nullable = false)
    private Long id;

    @Column(name = "first_name")
    private String firstName;

    @Column(name = "last_name")
    private String lastName;

    @Column(name = "email")
    private String email;

    public Student() {

    }

   // getter and setters
}

3. GenerationType.SEQUENCE

The GenerationType.SEQUENCE is to generate primary key values and uses a database sequence to generate unique values.

It requires additional select statements to get the next value from a database sequence. But this has no performance impact on most applications. And if your application has to persist a huge number of new entities, you can use some Hibernate-specific optimizations to reduce the number of statements.

For example, consider we have a Student JPA entity class with GenerationType.SEQUENCE as generation type:

@Entity
@Table(name = "student")
public class Student {

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

    @Column(name = "first_name")
    private String firstName;

    @Column(name = "last_name")
    private String lastName;

    @Column(name = "email")
    private String email;

    public Student() {

    }

   // getter and setters
}

If you don’t provide any additional information, Hibernate will request the next value from its default sequence. You can change that by referencing the name of a @SequenceGenerator in the generator attribute of the @GeneratedValue annotation. The @SequenceGenerator annotation lets you define the name of the generator, the name, and schema of the database sequence, and the allocation size of the sequence.

@Id
@GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "book_generator")
@SequenceGenerator(name="book_generator", sequenceName = "book_seq", allocationSize=50)
@Column(name = "id", updatable = false, nullable = false)
private Long id;

4.GenerationType.TABLE

The GenerationType.TABLE gets only rarely used nowadays. It simulates a sequence by storing and updating its current value in a database table which requires the use of pessimistic locks that put all transactions into sequential order. This slows down your application, and you should, therefore, prefer the GenerationType.SEQUENCE, if your database supports sequences, which most popular databases do.

For example, consider we have a Student JPA entity class with GenerationType.TABLE as generation type:

@Entity
@Table(name = "student")
public class Student {

    @Id
    @GeneratedValue(strategy = GenerationType.TABLE)
    @Column(name = "id", updatable = false, nullable = false)
    private Long id;

    @Column(name = "first_name")
    private String firstName;

    @Column(name = "last_name")
    private String lastName;

    @Column(name = "email")
    private String email;

    public Student() {

    }

   // getter and setters
}

You can use the @TableGenerator annotation in a similar way as the already explained @SequenceGenerator annotation to specify the database table which Hibernate shall use to simulate the sequence.

@Id
@GeneratedValue(strategy = GenerationType.TABLE, generator = "book_generator")
@TableGenerator(name="book_generator", table="id_generator", schema="bookstore")
@Column(name = "id", updatable = false, nullable = false)
private Long id;

Summary

As you’ve seen, JPA offers 4 different ways to generate primary key values:

1. GenerationType.AUTO: Hibernate selects the generation strategy based on the used dialect,
2. GenerationType.IDENTITY: Hibernate relies on an auto-incremented database column to generate the primary key,
3. GenerationType.SEQUENCE: Hibernate requests the primary key value from a database sequence,
4. GenerationType.TABLE: Hibernate uses a database table to simulate a sequence.

GitHub Repository

The complete source code of this article is available on my GitHub Repository - https://github.com/RameshMF/Hibernate-ORM-Tutorials

Conclusion

In this article,  we have seen 4 different primary key generation strategies that generate the primary key values programmatically or use database features, like auto-incremented columns or sequences.

You can learn more about Hibernate ORM Framework at Hibernate Tutorial

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