Introduction

In this article, we are going to see what options Spring Data JPA offers for entity locking.

We will see how we can apply a shared or exclusive row-level lock upon fetching one or multiple entities.

JPA LockModeType

As I explained in this article, JPA offers several explicit locking options that can be applied at entity fetching time via the LockModeType enumeration.

If you want to apply a pessimistic lock, then you can use one of the following options:

• LockModeType.PESSIMISTIC_READ to apply a shared or read lock on the associated table records.
• LockModeType.PESSIMISTIC_WRITE to apply an exclusive or write lock on the associated table records.

If the underlying database does not support shared locks, then the PESSIMISTIC_READstrategy will fall back to a PESSIMISTIC_WRITE since this option is widely supported.

Direct entity locking

If we want to lock an entity upon fetching it, then the JPA EntityManager offers a find method variant that allows us to pass the LockModeType:

Post post = entityManager.find(Post.class, id, lockMode);

However, while the Spring Data JpaRepository doesn’t offer this option, we can easily add it using a custom Spring Data Repository.

For instance, we can have our PostRepository extend a CustomPostRepository beside the JpaRepository:

@Repository
public interface PostRepository 
        extends JpaRepository<Post, Long>, CustomPostRepository {
}

In the CustomPostRepository interface, we can define a lockById method like this:

public interface CustomPostRepository {

    Post lockById(Long id, LockModeType lockMode);
}

While Spring Data JPA can provide an implementation for query methods or methods annotated with the @Query annotation, for our CustomPostRepository interface, we will have to provide the CustomPostRepositoryImpl implementation that Spring Data JPA will use when creating the PostRepository Java object instance at runtime.

The CustomPostRepositoryImpl looks as follows:

public class CustomPostRepositoryImpl 
        implements CustomPostRepository {
    
    @PersistenceContext
    private EntityManager entityManager;

    @Override
    public Post lockById(Long id, LockModeType lockMode) {
        return entityManager.find(Post.class, id, lockMode);
    }
}

Having the implementation of the lockById method in place, we can now test it.

To apply a shared lock on PostgreSQL, we will have to use the LockModeType.PESSIMISTIC_READ strategy, like this:

Post postWithSharedLock = postRepository.lockById(
    1L, 
    LockModeType.PESSIMISTIC_READ
);

And, Hibernate will execute the following SQL query:

SELECT 
    p.id AS id1_0_0_,
    p.slug AS slug2_0_0_,
    p.title AS title3_0_0_
FROM 
    post p
WHERE 
    p.id = 1
FOR SHARE

To acquire an exclusive lock, we can pass the PESSIMISTIC_WRITE option:

Post postWithExclusiveLock = postRepository.lockById(
    2L, 
    LockModeType.PESSIMISTIC_WRITE
);

And, Hibernate will use a FOR UPDATE clause instead:

SELECT 
    p.id AS id1_0_0_,
    p.slug AS slug2_0_0_,
    p.title AS title3_0_0_
FROM 
    post p
WHERE 
    p.id = 2
FOR UPDATE

Query entity locking

We can also apply row-level locks to all the records returned by a given SQL query, and for that, Spring Data JPA offers the @Lock annotation.

For instance, in the PostCommentRepository, we can define a lockAllByPostId method that applies a read lock on all the child PostComment rows that are associated with a given Post parent record:

@Repository
public interface PostCommentRepository 
        extends JpaRepository<PostComment, Long> {

    @Query("""
        select pc
        from PostComment pc
        where pc.post.id = :postId
        """)
    @Lock(LockModeType.PESSIMISTIC_READ)
    List<PostComment> lockAllByPostId(
        @Param("postId") Long postId
    );
}

And when executing the lockAllByPostId method:

List<PostComment> commentWithLock = postCommentRepository.lockAllByPostId(1L);

Hibernate will run the following SQL query on PostgreSQL:

SELECT 
    pc.id as id1_1_, 
    pc.post_id as post_id3_1_, 
    pc.review as review2_1_ 
FROM 
    post_comment pc 
WHERE 
    pc.post_id = 1
FOR SHARE OF pc

Cool, right?

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Conclusion

Locking entities is very easy with Spring Data JPA, and it works for both direct entity fetching as it does for queries.

In fact, I think it would be a good idea to add the lockById method to the BaseJpaRepository utility offered by the Hypersistence Utils project.

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