Persistence: Primary Keys, thesarfo

You can generate your own primary keys with Hibernate by implementing the IdentifierGenerator interface — the most important method to override is generate().

public class UUIDGenerator implements IdentifierGenerator {

    @Override
    public Object generate(SharedSessionContractImplementor sharedSessionContractImplementor, Object o) {
        return UUID.randomUUID().toString(); // this is just a basic implementation
    }
}

There are other methods you can implement based on your use case — for instance, a sign() method if you want to sign your primary key with a cryptographic algorithm.

On the id field:

@Id
@GenericGenerator(name = "UUIDGenerator", type = UUIDGenerator.class) // type is the class we created above; name can be anything
@GeneratedValue(generator = "UUIDGenerator")
private String id;

Composite primary key

Sometimes no single field is unique enough to identify an entity — you combine two or more fields into a composite primary key. There are two ways to define one: with an @IdClass, or with an @Embeddable id.

1. Option with @IdClass

Assuming we have a Product entity:

@Entity
public class Product {
    private String code;

    private long number;

    private String color;

    // getters and setters
}

To generate a composite primary key, create a ProductKey class containing the fields that make up the key.

Note: with composite primary keys, equals and hashCode are mandatory, to define the uniqueness of the fields.

public class ProductKey implements Serializable {

    private String code;
    private long number;

    // getters and setters
    // equals and hashcode
}

Then tell JPA where the primary key is:

@Entity
@IdClass(ProductKey.class) // telling jpa where our id class is
public class Product {
    @Id
    private String code;

    @Id
    private long number;

    private String color;

    // getters and setters
}

Creating a Product now derives its primary key from both code and number:

try {
    em.getTransaction().begin();

    Product p1 = new Product();
    p1.setCode("ABC");
    p1.setNumber(10);
    p1.setColor("Red");

    em.persist(p1);

    em.getTransaction().commit();
} finally {
    em.close();
}

2. Embeddable id

Create the key the same way:

@Embeddable // this means we can plug our class into our entity
public class StudentKey implements Serializable {
    private long number;
    private String code;

    // getters and setters
    // equals and hashcode
}

Then specify it as the primary key on the Student entity:

@Entity
public class Student {

    @EmbeddedId // insert our id from the embedded class
    private StudentKey id;

    private String name;

    // getters and setters
}

Creating a Student instance:

try {
    em.getTransaction().begin();

    // we create our key first
    StudentKey id = new StudentKey();
    id.setCode("ABC");
    id.setNumber(10);

    Student s = new Student();
    s.setId(id); // set our instance's id to the key
    s.setName("Ernest");

    em.persist(s);

    em.getTransaction().commit();
} finally {
    em.close();
}