Introduction

I’ve already written about the Open Session in View Anti-Pattern, so now it’s time to add another Hibernate fetching bad practices. Although the hibernate.enable_lazy_load_no_trans configuration property is a lesser-known setting, it’s good to know why you shouldn’t employ it in your data access layer code.

What does it do?

By default, this property is disabled and, to enable it, you need to provide the following configuration property:

<property 
    name="hibernate.enable_lazy_load_no_trans" 
    value="true"/>

Considering the following entities:

With this configuration property in place, the following code snippets can be executed without throwing any LazyInitializationException:

List<PostComment> comments = null;

EntityManager entityManager = null;
EntityTransaction transaction = null;
try {
    entityManager = entityManagerFactory()
        .createEntityManager();
    transaction = entityManager.getTransaction();
    transaction.begin();

    comments = entityManager.createQuery(
        "select pc " +
        "from PostComment pc " +
        "where pc.review = :review", PostComment.class)
    .setParameter("review", review)
    .getResultList();

    transaction.commit();
} catch (Throwable e) {
    if ( transaction != null && transaction.isActive())
        transaction.rollback();
    throw e;
} finally {
    if (entityManager != null) {
        entityManager.close();
    }
}
for(PostComment comment : comments) {
    LOGGER.info("The post title is '{}'", 
        comment.getPost().getTitle());
}

Without the hibernate.enable_lazy_load_no_trans configuration property in place, the comment.getPost().getTitle() line would throw a LazyInitializationException because the comments collection was not initialized, and the Persistence Context is already closed, along with the database connection that fetched the post entity.

Behind the scenes, a temporary Session is opened just for initializing every post association. Every temporary Session implies acquiring a new database connection, as well as a new database transaction.

The more association being loaded lazily, the more additional connections are going to be requested which puts pressure on the underlying connection pool. Each association being loaded in a new transaction, the transaction log is forced to flush after each association initialization.

Online Workshops

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• Caching Best Practices with JPA and Hibernate (2.5 hours) on the 30th of September
• High-Performance SQL (4 hours) on the 6th of October in collaboration with Voxxed Days Ticino
• High-Performance SQL (12 hours) starting on the 28th of October in collaboration with Bouvet

Conclusion

Just like Open Session in View, the hibernate.enable_lazy_load_no_trans configuration property is an anti-pattern as well because it only treats the symptoms and does not solve the actual cause of the LazyInitializationException.

By properly initializing all lazy associations prior to closing the initial Persistence Context, and switching to DTO projections where entities are not even necessary, the LazyInitializationException is prevented in a much more efficient way.

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