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

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.

However, while auto-generating a UUID identifier when using Hibernate is fairly easy, the standard UUIDs are not a good fit for Primary Keys, as you are better off using a time-sorted TSID instead. For more details about this topic, check out this article.

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