Introduction

In this article, we are going to see how to use a UUID entity identifier that is auto-generated by Hibernate either in the JVM or using database-specific UUID functions.

Our Post entity looks as follows:

The Post entity has a UUID identifier and a title. Now, let’s see how we can map the Post entity so that the UUID identifier be auto-generated for us.

GenerationType.AUTO

When using JPA, we can use the GenerationType.AUTO strategy of the @GeneratedValue annotation to assign numerical identifier automatically, based on an IDENTITY column, a SEQUENCE or the infamous TABLE generator.

However, few might know that GenerationType.AUTO can be used for UUID identifiers as well:

@Entity(name = "Post")
@Table(name = "post")
public class Post {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private UUID id;

    private String title;

    //Getters and setters omitted for brevity
}

Now, when persisting a Post entity:

Post post = new Post();
post.setTitle("High-Performance Java Persistence");

entityManager.persist(post);

Hibernate generates the following SQL INSERT statement:

INSERT INTO post (
    title, 
    id
) 
VALUES (
    'High-Performance Java Persistence', 
    'b5607d38-8fc1-43ef-b44e-34967083c80a'
)

Even batching works as expected:

for (int i = 0; i < 3; i++) {
    Post post = new Post();
    post.setTitle(
        String.format(
            "High-Performance Java Persistence, Part %d", 
            i + 1
        )
    );

    entityManager.persist(post);
}

Hibernate generating a single SQL INSERT statement with 3 bind parameter value sets:

Query:[
    "insert into post (title, id) values (?, ?)"
], 
Params:[
    (High-Performance Java Persistence, Part 1, 7176589b-a3ca-472f-bf00-c253c351ddcc), 
    (High-Performance Java Persistence, Part 2, a4269fb4-07c9-447a-9d65-f443c074de20), 
    (High-Performance Java Persistence, Part 3, e33962a0-d841-48b1-8f43-caf98116f3ee)
]

Database-generated UUID

Now, if we don’t want the UUID to be generated by the JVM, and want to use database-specific functions, we need to provide an implementation of the org.hibernate.id.UUIDGenerationStrategy interface:

public class PostgreSQLUUIDGenerationStrategy 
    implements UUIDGenerationStrategy {

    @Override
    public int getGeneratedVersion() {
        return 4;
    }

    @Override
    public UUID generateUUID(
            SharedSessionContractImplementor session) {
        return ((Session) session).doReturningWork(connection -> {
            try(
                Statement statement = connection.createStatement();
                ResultSet resultSet = statement.executeQuery(
                    "select uuid_generate_v4()" 
                )
            ) {
                while (resultSet.next()) {
                    return (UUID) resultSet.getObject(1);
                }
            }
            throw new IllegalArgumentException("Can't fetch a new UUID");
        });
    }
}

The getGeneratedVersion method defines what type of UUID we are generating, according to the IETF RFC 4122 standard. In our case, 4 stands for variant 4 (random) generator strategy.

Because we are using PostgreSQL, we also need to create the uuid-ossp extension before using the UUID-specific functions:

CREATE EXTENSION IF NOT EXISTS "uuid-ossp";

Now, we can call the uuid_generate_v4 function to get a variant 4 UUID based on IETF RFC 4122 specification.

To provide the PostgreSQLUUIDGenerationStrategy to our Post entity, we have to use the Hibernate-specific @GenericGenerator:

@Entity(name = "Post")
@Table(name = "post")
public class Post {

    @Id
    @GeneratedValue(
        strategy = GenerationType.AUTO, 
        generator = "pg-uuid"
    )
    @GenericGenerator(
        name = "pg-uuid", 
        strategy = "uuid2",
        parameters = @Parameter(
            name = "uuid_gen_strategy_class",
            value = "com.vladmihalcea.book.hpjp.hibernate.identifier.uuid.PostgreSQLUUIDGenerationStrategy"
        )
    )
    private UUID id;

    private String title;

    //Getters and setters omitted for brevity
}

The uuid2 strategy stands for org.hibernate.id.UUIDGenerator which we want to use instead of the legacy org.hibernate.id.UUIDHexGenerator that’s registered under the uuid name in Hibernate.

The @Parameter attribute is used to customize the UUID generation strategy via the uuid_gen_strategy_class parameter value which takes the fully-qualified class name of the org.hibernate.id.UUIDGenerationStrategy interface implementation.

And, that’s it!

Now, when persisting 3 Post entities, Hibernate generates the following SQL statements:

select uuid_generate_v4()
select uuid_generate_v4()
select uuid_generate_v4()

Query:[
    "insert into post (title, id) values (?, ?)"
], 
Params:[
    (High-Performance Java Persistence, Part 1, 9eb52a9b-fb81-4930-b0cd-079a447ed2ba), 
    (High-Performance Java Persistence, Part 2, 2a69ec7d-a147-4c71-8a20-9ba760de0149), 
    (High-Performance Java Persistence, Part 3, e7616832-bb4e-470a-8df4-0534ab56d960)
]

Notice the calls to the uuid_generate_v4 PostgreSQL function which is used to assign the UUID identifier values.

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Conclusion

Therefore, auto-generating a UUID identifier when using Hibernate is fairly easy.

You can either allow Hibernate to use the Java-based UUID generation strategy or you can delegate this task to the database. The latter option requires providing an implementation of the org.hibernate.id.UUIDGenerationStrategy which is rather straightforward.

Insert details about how the information is going to be processed

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