Spring read-only transaction Hibernate optimization

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Introduction

In this article, I’m going to explain how the Spring read-only transaction Hibernate optimization works.

After taking a look at what the Spring framework does when enabling the readOnly attribute on the @Transactional annotation, I realized that only the Hibernate flush mode is set to FlushType.MANUAL without propagating the read-only flag further to the Hibernate Session.

So, in the true spirit of open-source software developer, I decided it’s time to make a change.

Entity loaded state

When loading an entity, Hibernate extracts the loaded state from the underlying JDBC ResultSet. This process is called hydration on Hibernate terminology and is done by the Hibernate EntityPersister like this:

final Object[] values = persister.hydrate(
    rs,
    id,
    object,
    rootPersister,
    cols,
    fetchAllPropertiesRequested,
    session
);

The loaded state or hydrated state is needed by dirty checking mechanism to compare the current entity state with the loaded-time snapshot and determine if an UPDATE statement is needed to be executed at flush-time. Also, the detached state is used by the versionless optimistic locking mechanism to build the WHERE clause filtering predicates.

Therefore, upon loading an entity, the detached state is stored by the Hibernate Session unless the entity is loaded in read-only mode.

Read-only entities

By default, entities are loaded in read-write mode, meaning that the detached state is kept by the current Persistence Context until the entity is detached or if the JPA EntityManager or Hibernate Session is closed.

In order to load entities is the read-only mode, you can set either set the defaultReadOnly flag at the Session level or set the org.hibernate.readOnly JPA query hint.

To set the read-only for all entities loaded by a Hibernate Session either through a query or via direct fetching, you need to enable the defaultReadOnly property like this:

Session session = entityManager.unwrap(Session.class);
session.setDefaultReadOnly(true);

Or, if you have a default read-write Session and only want to load entities in read-only mode for a particular query, you can use the org.hibernate.readOnly JPA query hint as follows:

List<Post> posts = entityManager
.createQuery(
    "select p from Post p", Post.class)
.setHint(QueryHints.HINT_READONLY, true)
.getResultList();

Spring @Transactional annotation

Spring, just like Java EE, offers support for declarative transactions. Therefore, you can use the @Transactional annotation to mark the service layer method that should be wrapped in a transactional context.

The @Transactional annotation offers the readOnly attribute, which is false by default. The readOnly attribute can further be used by Spring to optimize the underlying data access layer operations.

Prior to Spring 5.1, when using Hibernate, the readOnly attribute of the @Transactional annotation was only setting the current Session flush mode to FlushType.MANUAL, therefore disabling the automatic dirty checking mechanism.

However, because the readOnly attribute did not propagate to the underlying Hibernate Session, I decided to create the SPR-16956 issue and provided a Pull Request as well, which after being Jürgenized, it got integrated, and available starting with Spring Framework 5.1.

Testing time

Let’s consider we have the following service and data access layer classes in our application:

Forum service and data access layer classes

The PostDAOImpl class is implemented like this:

@Repository
public class PostDAOImpl 
        extends GenericDAOImpl<Post, Long> 
        implements PostDAO {

    protected PostDAOImpl() {
        super(Post.class);
    }

    @Override
    public List<Post> findByTitle(String title) {
        return getEntityManager()
        .createQuery(
            "select p " +
            "from Post p " +
            "where p.title = :title", Post.class)
        .setParameter("title", title)
        .getResultList();
    }
}

While the ForumServiceImpl looks as follows:

@Service
public class ForumServiceImpl implements ForumService {

    @Autowired
    private PostDAO postDAO;

    @Autowired
    private TagDAO tagDAO;

    @PersistenceContext
    private EntityManager entityManager;

    @Override
    @Transactional
    public Post newPost(String title, String... tags) {
        Post post = new Post();
        post.setTitle(title);
        post.getTags().addAll(tagDAO.findByName(tags));
        return postDAO.persist(post);
    }

    @Override
    @Transactional(readOnly = true)
    public List<Post> findAllByTitle(String title) {
        List<Post> posts = postDAO.findByTitle(title);

        org.hibernate.engine.spi.PersistenceContext persistenceContext = getHibernatePersistenceContext();

        for(Post post : posts) {
            assertTrue(entityManager.contains(post));

            EntityEntry entityEntry = persistenceContext.getEntry(post);
            assertNull(entityEntry.getLoadedState());
        }

        return posts;
    }

    @Override
    @Transactional
    public Post findById(Long id) {
        Post post = postDAO.findById(id);

        org.hibernate.engine.spi.PersistenceContext persistenceContext = getHibernatePersistenceContext();

        EntityEntry entityEntry = persistenceContext.getEntry(post);
        assertNotNull(entityEntry.getLoadedState());

        return post;
    }

    private org.hibernate.engine.spi.PersistenceContext getHibernatePersistenceContext() {
        SharedSessionContractImplementor session = entityManager.unwrap(
            SharedSessionContractImplementor.class
        );
        return session.getPersistenceContext();
    }
}

We are interested in the findAllByTitle and findById service methods.

Notice that the findAllByTitle method is annotated with @Transactional(readOnly = true). When loading the Post entities matching the given title, Hibernate is going to fetch the entities in read-only mode, therefore discarding the loaded state, which we can validate via the Hibernate PersistenceContext.

On the other hand, the findById method uses the default read-write @Transactional annotation, and we can see that Hibernate PersistenceContext contains the detached state of the currently fetched Post entity.

When running the test that proves this new Spring 5.1 Hibernate optimization, everything works as expected:

@Test
public void test() {
    Post newPost = forumService.newPost(
        "High-Performance Java Persistence", 
        "hibernate", 
        "jpa"
    );
    assertNotNull(newPost.getId());

    List<Post> posts = forumService.findAllByTitle(
        "High-Performance Java Persistence"
    );
    assertEquals(1, posts.size());

    Post post = forumService.findById(newPost.getId());
    assertEquals(
        "High-Performance Java Persistence", 
        post.getTitle()
    );
}

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Conclusion

The main advantage of the Spring 5.1 read-only optimization for Hibernate is that we can save a lot of memory when loading read-only entities since the loaded state is discarded right away, and not kept for the whole duration of the currently running Persistence Context.

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8 Comments on “Spring read-only transaction Hibernate optimization

  1. Hi, what if we first call a method with readOnly transaction annotation, and then call a read\write method that updates the entity which was found by the read only method? is it a bad practice? what is the expected behavior?

    • It’s a bad practice, and the outcome depends on the current transaction propagation strategy. By default, the outer-most Tx will dictate the attributes, read-only in your case, so the read-writd method might either throw an exception or not do anything.

      • Thanks, i forgot to mention we use Propagation.SUPPORTS for the read only method. Is your answer stays the same in this case?

      • You’re welcome. I forgot to mention that I also offer training and consulting, which are much better suited to provide high-quality support than a blog post comment.

  2. I use a Spring version less than 5.1 with Oracle DB. Do I need to use an annotation @Transactional readonly?

    • Yes, you can still benefit from setting the read-only attribute as it will disable the Persistence Context auto flushing mechanism.

  3. I faced off a strange behaviour (or would it be a normal behaviour?).
    I have a service A with @Transaction (read-write) that calls others services B, C, D. These others services are @Transaction(readOnly=true).
    What I noticed is that always the service read-only Transaction is called, one connection is borrowed from the pool.
    Therefore there is a connection for a read-write transaction and one call for connection from the pool for each service with read-only transaction.
    I don’t know if that behaviour is a configuration from Spring or Hibernate, but in a big scale that seems a real performance problem. I’d expect one borrow for read-write transaction and other (only) for read-only transaction.

    Thanks!!

    • That’s not the typical behavior. By default, the transaction propagates since the TransactionInterceptor uses a thread-local context. In your case, you need to debug it and see why it happens like that.

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