The data knowledge stack
Concurrency is not for the faint-hearted We all know concurrency programming is difficult to get it right. That’s why threading tasks are followed by extensive design and code review sessions. You never assign concurrent issues to inexperienced developers. The problem space is carefully analyzed, a design emerges and the solution is both documented and reviewed. That’s how threading related tasks are usually addressed. You will naturally choose a higher level abstraction since you don’t want to get tangled up in low-level details. That’s why the java.util.concurrent is usually better (unless you build… Read More
How to detect the Hibernate N+1 query problem during testing
Introduction In this article, you are going to learn how to automatically detect the N+1 query problem when using JPA and Hibernate using the Hypersistence Utils open-source project. With Hibernate, you manage entity state transitions, which are then translated to SQL statements. The number of generated SQL statements is affected by the current fetching strategy, Criteria queries, or Collection mappings, and you might not always get what you expected. Ignoring SQL statements is risky, and it may eventually put a heavy toll on the overall application performance. I’m a strong advocate of… Read More
JPA Criteria API can generate unexpected SQL statements
Introduction In this article, we are going to learn why you should always check the SQL statements generated by JPA Criteria API queries when using Hibernate. Criteria API is very useful for dynamically building queries, but that’s the only use case where I’d use it. Whenever you have a UI with N filters that may arrive in any M combinations, it makes sense to have an API to construct queries dynamically since concatenating strings is always a path I’m running away from. The question is, are you aware of the SQL queries… Read More
Hibernate integration testing strategies
Introduction I like integration testing. As I explained in this article, it’s a good way to check what SQL queries are generated by Hibernate behind the scenes. But integration tests require a running database server, and this is the first choice you have to make. Using a production-like local database server for Integration Testing For a production environment, I always prefer using incremental DDL scripts, since I can always know what version is deployed on a given server, and which scripts required to be deployed. I’ve been relying on Flyway to manage… Read More
How to fetch entities multiple levels deep with Hibernate
Introduction It’s quite common to retrieve a root entity along with its child associations on multiple levels. In our example, we need to load a Forest with its Trees and Branches and Leaves, and we will try to see have Hibernate behaves for three collection types: Sets, Indexed Lists, and Bags.
A beginner’s guide to Hibernate fetching strategies
Introduction When it comes to working with an ORM tool, everybody acknowledges the importance of database design and Entity-to-Table mapping. These aspects get a lot of attention, while things like fetching strategy might be simply put-off. In my opinion, the entity fetching strategy shouldn’t ever be separated from the entity mapping design, since it might affect the overall application performance unless properly designed. Before Hibernate and JPA got so popular, there was a great deal of effort put into designing each query, because you had to explicitly select all the joins you… Read More
A beginner’s guide to Hibernate flush operation order
Introduction As explained in this article, Hibernate shifts the developer mindset from SQL to entity state transitions. A JPA entity may be in one of the following states: New/Transient: the entity is not associated with a persistence context, be it a newly created object the database doesn’t know anything about. Persistent: the entity is associated with a persistence context (residing in the 1st Level Cache) and there is a database row representing this entity. Detached: the entity was previously associated with a persistence context, but the persistence context was closed, or the… Read More
How do Set and List collections behave with JPA and Hibernate
Introduction Hibernate is a great ORM tool, and it eases development considerably, but it has a lot of gotchas you must be aware of if you want to use it properly. On medium to large projects, it’s very common to have bidirectional parent-child associations, which allow us to navigate both ends of a given relationship. When it comes to controlling the persist/merge part of the association, there are two options available. One would be to have the @OneToMany end in charge of synchronizing the collection changes, but this is an inefficient approach…. Read More
