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Generated Value

layout: default title: @GeneratedValue

Primary keys can be generated using a table, a sequence, or an identity column (if supported by the database).

When creating new entity objects, it is important to notify the persistence provider as to whether it should use your current id value for the INSERT, or depend upon a generation strategy instead.

Use the strategy element of the @GeneratedValue annotation to identify your choice.

  • Supported values include TABLE, SEQUENCE, IDENTITY, or AUTO, all of which are members of the GenerationType enumeration. (We will exclusively be using IDENTITY with MySQL.)

  • Place the annotation above the primary key field that you have mapped with @Id.

@Entity
public class Customer {
  @Id
  @GeneratedValue(strategy = GenerationType.IDENTITY)
  private int id;

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

  // ...
}

Notice that in the Customer Table (below), the id field is marked as PRI and auto_increment

  • PRI indicates that this is the primary key value.

  • auto_increment indicates that when a new value is inserted into the table, MySQL will handle the determination of the next available id, and assign it as the value to this field. This corresponds directly to the @GeneratedValue(strategy = GenerationType.IDENTITY) which tells the JPA that MySQL is handling Primary Key assignment.

mysql > desc Customer;
+------------+-------------+------+-----+---------+----------------+
| Field      | Type        | Null | Key | Default | Extra          |
+------------+-------------+------+-----+---------+----------------+
| id         | int(11)     | NO   | PRI | NULL    | auto_increment |
| first_name | varchar(50) | YES  |     | NULL    |                |
+------------+-------------+------+-----+---------+----------------+

Drill

Modify your Customer entity by adding the @GeneratedValue annotation above the id field.

  • Your JUnit test should still pass after you have made this change.

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