How to retry JPA transactions after an OptimisticLockException
Introduction In this article, I will discuss how we can implement an automatic Retry mechanism when dealing with the JPA OptimisticLockException. You can find the introductory part here and the MongoDB implementation here.
21st century logging
I think logging should get more attention than we are currently giving it. When designing an application a great deal of effort goes into modeling the customer business logic, making sure all use cases are covered and handled properly. The business model is mapped to a persistence storage (be at a RDBMS or a NoSQL solution), frameworks are being chosen: web, middle-ware, batch jobs, and probably SLF4J with log4j or logback. This has been the case of almost all applications I’ve been involved with, and logging was always a second-class citizen, relying… Read More
How to implement Equals and HashCode for JPA entities
Introduction Every Java object inherits the equals and hashCode methods, yet they are useful only for Value objects, being of no use for stateless behavior-oriented objects. While comparing references using the “==” operator is straightforward, for object equality things are a little bit more complicated.
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
Lock processing logic by customer
Introduction In the current application we are developing there was one use case where we wanted to synchronize message processing by message provider (customer generating those messaging). The flow looks something like this: So messages may come randomly since there are more customer jobs running in parallel, but we want to ensure that messages belonging to the same customer are processed one after the other (analog to the Serializable database isolation level) while allowing messages coming from different customers to be processed in parallel.
