Taming the Lazy Beast: Reactive DTOs and Hibernate Relationships in Quarkus

Building reactive applications with Quarkus and Hibernate Reactive is a fantastic way to achieve high performance and responsiveness. However, you’ll inevitably encounter the “lazy loading” challenge when dealing with entity relationships and Data Transfer Objects (DTOs). This post will guide you through the common pitfalls and provide practical solutions to ensure smooth data flow in your reactive Quarkus applications.

The Lazy Loading Conundrum

Hibernate, by default, employs lazy fetching for related entities. This means that related data is only loaded when you explicitly access it. While this conserves resources in many scenarios, it can lead to problems in reactive environments:

• LazyInitializationException: When using Uni or Multi (SmallRye Mutiny’s reactive types), the reactive stream might complete before lazy-loaded data is accessed, resulting in a session closure and the dreaded LazyInitializationException.
• DTO Serialization Issues: Attempting to serialize an entity with uninitialized lazy relations into a DTO (often for JSON responses) will also trigger this exception.

Why does this happen in Reactive Contexts?

Reactive programming is about non-blocking operations. When you use Uni and Multi you are creating a stream of data that will be processed asynchronously. The Hibernate session may close before the lazy loaded relationships are accessed.

Conquering the Lazy Beast: Practical Solutions

Here’s how to tackle lazy loading and ensure your DTOs are populated correctly in your reactive Quarkus applications:

1. Explicit Fetching with JPQL or Criteria API (The Recommended Approach)

This approach provides the most control and performance benefits. By explicitly fetching related entities in your queries, you guarantee that the necessary data is loaded before the reactive stream completes.

• Example using JPQL:

public Uni<List<MyDTO>> findMyDTOsWithRelations() {
  return getEntityManager()
      .createQuery(
          "SELECT DISTINCT e FROM MyEntity e JOIN FETCH e.relatedEntity",
          MyEntity.class)
      .getResultList()
      .map(myEntities
          -> myEntities.stream()
                 .map(myEntity
                     -> new MyDTO(myEntity.name,
                         myEntity.relatedEntity
                             .relatedName) // Example of accessing the related
                                           // entity.
                     )
                 .collect(Collectors.toList()));
}

• Why it works: The JOIN FETCH clause instructs Hibernate to eagerly load the relatedEntity along with the MyEntity.

2. DTO Projection with Joins

Leverage Panache’s projection capabilities to directly retrieve related data into your DTOs using joins.

• Example:

public Uni<List<MyDTO>> findMyDTOsWithRelationsProjection() {
  return find(
      "SELECT e.name, r.relatedName FROM MyEntity e JOIN e.relatedEntity r")
      .project(MyDTO.class)
      .list();
}

• Benefits: This approach is efficient as it retrieves only the required data.

3. Eager Fetching (Use with Caution)

You can change the fetching strategy to eager fetching using annotations like @ManyToOne(fetch = FetchType.EAGER).

• Caveats: Eager fetching can lead to performance issues if you have many related entities, as it loads all related data regardless of whether you need it. Use this sparingly.

4. Manual DTO Population

For more complex scenarios, retrieve the main entity and related entities in separate reactive calls, then combine the results into your DTO.

Key Considerations

• DISTINCT: Use DISTINCT in JPQL queries with joins to prevent duplicate results.
• Testing: Write comprehensive tests to ensure your DTOs are populated correctly.
• Performance Analysis: Monitor your application’s performance to identify potential bottlenecks.

In Conclusion

By understanding the challenges of lazy loading and adopting the appropriate solutions, you can build robust and efficient reactive Quarkus applications that seamlessly handle entity relationships and DTOs. Remember that explicit fetching and projections offer the best balance of performance and control. Happy coding!