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1. Overview

Identifiers in Hibernate represent the primary key of an entity. This implies the values are unique so that they can identify a specific entity, that they aren't null and that they won't be modified.

Hibernate provides a few different ways to define identifiers. In this article, we'll review each method of mapping entity ids using the library.

2. Simple Identifiers

The most straightforward way to define an identifier is by using the @Id annotation.

Simple ids are mapped using @Id to a single property of one of these types: Java primitive and primitive wrapper types, String, Date, BigDecimal and BigInteger.

Let's see a quick example of defining an entity with a primary key of type long:

@Entity
public class Student {

    @Id
    private long studentId;
    
    // standard constructor, getters, setters
}

3. Generated Identifiers

If we want to automatically generate the primary key value, we can add the @GeneratedValue annotation.

This can use four generation types: AUTO, IDENTITY, SEQUENCE and TABLE.

If we don't explicitly specify a value, the generation type defaults to AUTO.

3.1. AUTO Generation

If we're using the default generation type, the persistence provider will determine values based on the type of the primary key attribute. This type can be numerical or UUID.

For numeric values, the generation is based on a sequence or table generator, while UUID values will use the UUIDGenerator.

Let's first map an entity primary key using AUTO generation strategy:

@Entity
public class Student {

    @Id
    @GeneratedValue
    private long studentId;

    // ...
}

In this case, the primary key values will be unique at the database level.

Now we'll look at the UUIDGenerator, which was introduced in Hibernate 5.

In order to use this feature, we just need to declare an id of type UUID with @GeneratedValue annotation:

@Entity
public class Course {

    @Id
    @GeneratedValue
    private UUID courseId;

    // ...
}

Hibernate will generate an id of the form “8dd5f315-9788-4d00-87bb-10eed9eff566”.

3.2. IDENTITY Generation

This type of generation relies on the IdentityGenerator, which expects values generated by an identity column in the database. This means they are auto-incremented.

To use this generation type, we only need to set the strategy parameter:

@Entity
public class Student {

    @Id
    @GeneratedValue (strategy = GenerationType.IDENTITY)
    private long studentId;

    // ...
}

One thing to note is that IDENTITY generation disables batch updates.

3.3. SEQUENCE Generation

To use a sequence-based id, Hibernate provides the SequenceStyleGenerator class.

This generator uses sequences if our database supports them. It switches to table generation if they aren't supported.

In order to customize the sequence name, we can use the @GenericGenerator annotation with SequenceStyleGenerator strategy:

@Entity
public class User {
    @Id
    @GeneratedValue(generator = "sequence-generator")
    @GenericGenerator(
      name = "sequence-generator",
      strategy = "org.hibernate.id.enhanced.SequenceStyleGenerator",
      parameters = {
        @Parameter(name = "sequence_name", value = "user_sequence"),
        @Parameter(name = "initial_value", value = "4"),
        @Parameter(name = "increment_size", value = "1")
        }
    )
    private long userId;
    
    // ...
}

In this example, we've also set an initial value for the sequence, which means the primary key generation will start at 4.

SEQUENCE is the generation type recommended by the Hibernate documentation.

The generated values are unique per sequence. If we don't specify a sequence name, Hibernate will reuse the same hibernate_sequence for different types.

3.4. TABLE Generation

The TableGenerator uses an underlying database table that holds segments of identifier generation values.

Let's customize the table name using the @TableGenerator annotation:

@Entity
public class Department {
    @Id
    @GeneratedValue(strategy = GenerationType.TABLE, 
      generator = "table-generator")
    @TableGenerator(name = "table-generator", 
      table = "dep_ids", 
      pkColumnName = "seq_id", 
      valueColumnName = "seq_value")
    private long depId;

    // ...
}

In this example, we can see that we can also customize other attributes such as the pkColumnName and valueColumnName.

However, the disadvantage of this method is that it doesn't scale well and can negatively affect performance.

To sum up, these four generation types will result in similar values being generated but use different database mechanisms.

3.5. Custom Generator

Let's say we don't want to use any of the out-of-the-box strategies. In order to do that, we can define our custom generator by implementing the IdentifierGenerator interface.

We'll create a generator that builds identifiers containing a String prefix and a number:

public class MyGenerator 
  implements IdentifierGenerator, Configurable {

    private String prefix;

    @Override
    public Serializable generate(
      SharedSessionContractImplementor session, Object obj) 
      throws HibernateException {

        String query = String.format("select %s from %s", 
            session.getEntityPersister(obj.getClass().getName(), obj)
              .getIdentifierPropertyName(),
            obj.getClass().getSimpleName());

        Stream ids = session.createQuery(query).stream();

        Long max = ids.map(o -> o.replace(prefix + "-", ""))
          .mapToLong(Long::parseLong)
          .max()
          .orElse(0L);

        return prefix + "-" + (max + 1);
    }

    @Override
    public void configure(Type type, Properties properties, 
      ServiceRegistry serviceRegistry) throws MappingException {
        prefix = properties.getProperty("prefix");
    }
}

In this example, we override the generate() method from the IdentifierGenerator interface.

First, we want to find the highest number from the existing primary keys of the form prefix-XX. Then we add 1 to the maximum number found and append the prefix property to get the newly generated id value.

Our class also implements the Configurable interface so that we can set the prefix property value in the configure() method.

Next, let's add this custom generator to an entity.

For this, we can use the @GenericGenerator annotation with a strategy parameter that contains the full class name of our generator class:

@Entity
public class Product {

    @Id
    @GeneratedValue(generator = "prod-generator")
    @GenericGenerator(name = "prod-generator", 
      parameters = @Parameter(name = "prefix", value = "prod"), 
      strategy = "com.baeldung.hibernate.pojo.generator.MyGenerator")
    private String prodId;

    // ...
}

Also, notice we've set the prefix parameter to “prod”.

Let's see a quick JUnit test for a clearer understanding of the id values generated:

@Test
public void whenSaveCustomGeneratedId_thenOk() {
    Product product = new Product();
    session.save(product);
    Product product2 = new Product();
    session.save(product2);

    assertThat(product2.getProdId()).isEqualTo("prod-2");
}

Here the first value generated using the “prod” prefix was “prod-1”, followed by “prod-2”.

4. Composite Identifiers

Besides the simple identifiers we've seen so far, Hibernate also allows us to define composite identifiers.

A composite id is represented by a primary key class with one or more persistent attributes.

The primary key class must fulfill several conditions:

  • It should be defined using @EmbeddedId or @IdClass annotations.
  • It should be public, serializable and have a public no-arg constructor.
  • Finally, it should implement equals() and hashCode() methods.

The class's attributes can be basic, composite or ManyToOne, while avoiding collections and OneToOne attributes.

4.1. @EmbeddedId

Now let's look at how to define an id using @EmbeddedId.

First, we need a primary key class annotated with @Embeddable:

@Embeddable
public class OrderEntryPK implements Serializable {

    private long orderId;
    private long productId;

    // standard constructor, getters, setters
    // equals() and hashCode() 
}

Next, we can add an id of type OrderEntryPK to an entity using @EmbeddedId:

@Entity
public class OrderEntry {

    @EmbeddedId
    private OrderEntryPK entryId;

    // ...
}

Let's see how we can use this type of composite id to set the primary key for an entity:

@Test
public void whenSaveCompositeIdEntity_thenOk() {
    OrderEntryPK entryPK = new OrderEntryPK();
    entryPK.setOrderId(1L);
    entryPK.setProductId(30L);
        
    OrderEntry entry = new OrderEntry();
    entry.setEntryId(entryPK);
    session.save(entry);

    assertThat(entry.getEntryId().getOrderId()).isEqualTo(1L);
}

Here the OrderEntry object has an OrderEntryPK primary id formed of two attributes: orderId and productId.

4.2. @IdClass

The @IdClass annotation is similar to the @EmbeddedId. The difference with @IdClass is that the attributes are defined in the main entity class using @Id for each one. The primary key class will look the same as before.

Let's rewrite the OrderEntry example with an @IdClass:

@Entity
@IdClass(OrderEntryPK.class)
public class OrderEntry {
    @Id
    private long orderId;
    @Id
    private long productId;
    
    // ...
}

Then we can set the id values directly on the OrderEntry object:

@Test
public void whenSaveIdClassEntity_thenOk() {        
    OrderEntry entry = new OrderEntry();
    entry.setOrderId(1L);
    entry.setProductId(30L);
    session.save(entry);

    assertThat(entry.getOrderId()).isEqualTo(1L);
}

Note that for both types of composite ids, the primary key class can also contain @ManyToOne attributes.

Hibernate also allows defining primary keys made up of @ManyToOne associations combined with @Id annotation. In this case, the entity class should also fulfill the conditions of a primary key class.

However, the disadvantage of this method is that there's no separation between the entity object and the identifier.

5. Derived Identifiers

Derived identifiers are obtained from an entity's association using the @MapsId annotation.

First, let's create a UserProfile entity that derives its id from a one-to-one association with the User entity:

@Entity
public class UserProfile {

    @Id
    private long profileId;
    
    @OneToOne
    @MapsId
    private User user;

    // ...
}

Next, let's verify that a UserProfile instance has the same id as its associated User instance:

@Test
public void whenSaveDerivedIdEntity_thenOk() {        
    User user = new User();
    session.save(user);
       
    UserProfile profile = new UserProfile();
    profile.setUser(user);
    session.save(profile);

    assertThat(profile.getProfileId()).isEqualTo(user.getUserId());
}

6. Conclusion

In this article, we've seen the multiple ways we can define identifiers in Hibernate.

The full source code of the examples can be found over on GitHub.

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