How to map a composite identifier using an automatically @GeneratedValue with JPA and Hibernate
Introduction One of my readers asked me to answer the following StackOverflow question. While I already covered the best way to map composite identifiers with JPA and Hibernate, this use case is different because one column is automatically generated.
The best way to map a projection query to a DTO (Data Transfer Object) with JPA and Hibernate
Introduction In this article, I’m going to show you the best way to map a DTO projection when using JPA and Hibernate. While answering questions on the Hibernate forum, I stumbled on the following question. What was different than previous questions on the same topic was that the original poster realized the shortcoming of the JPA constructor expression which can be addressed elegantly by Hibernate. Because this has been a recurring theme on StackOverflow and the Hibernate forum, I decided to dedicate an article to the best way of mapping DTO projections… Read More
How to map a @ManyToOne association using a non-Primary Key column with JPA and Hibernate
Introduction While answering questions on the Hibernate forum, I stumbled on the following question about using the @ManyToOne annotation when the Foreign Key column on the client side references a non-Primary Key column on the parent side. In this article, you are going to see how to use the @JoinColumn annotation in order to accommodate non-Primary Key many-to-one associations.
What’s new in JPA 2.2 – Stream the result of a Query execution
Introduction In this article, we are going to see how the JPA 2.2 Stream query results are supported by Hibernate and the caveats of using database cursors just to limit the amount of data that needs to be fetched.
What’s new in JPA 2.2 – Java 8 Date and Time Types
Introduction In this article, we are going to see how JPA 2.2 Date/Time works, and which types you need to use depending on your business case requirements.
The best way to map the @DiscriminatorColumn with JPA and Hibernate
Introduction As previously explained, the SINGLE_TABLE inheritance is the most efficient entity inheritance strategy. However, for JPQL query such as this one: Hibernate generates a SQL query which filters by the associated discriminator column (e.g. DTYPE by default): So, because we are filtering by the discriminator column, we might want to index it or include it to speed up queries. However, the default STRING DiscriminatorType expects a VARCHAR column that must hold the longest entity subclass name. For the Announcement class, we need at least 12 bytes to store the entity class… Read More
The best way to use entity inheritance with JPA and Hibernate
Introduction Recently, my friend Lukas Eder wrote the following message on Twitter: Just like in any OOP (Object-Oriented Programming) language, entity inheritance is suitable for varying behavior rather than reusing data structures, for which we could use composition. The Domain Model compromising both data (e.g. persisted entities) and behavior (business logic), we can still make use of inheritance for implementing a behavioral software design pattern. In this article, I’m going to demonstrate how to use JPA inheritance as a means to implement the Strategy design pattern.
The best way to map the SINGLE_TABLE inheritance with JPA and Hibernate
Introduction Java, like any other object-oriented programming language, makes heavy use of inheritance and polymorphism. Inheritance allows defining class hierarchies that offer different implementations of a common interface. Conceptually, the Domain Model defines both data (e.g. persisted entities) and behavior (business logic). Nevertheless, inheritance is more useful for varying behavior rather than reusing data (composition is much more suitable for sharing structures). Even if the data (persisted entities) and the business logic (transactional services) are decoupled, inheritance can still help varying business logic (e.g. Visitor pattern). In this article, we are going… Read More
How to fix “wrong column type encountered” schema-validation errors with JPA and Hibernate
Introduction Mapping entities to database tables is usually a very straightforward process. However, if your mappings are rather unusual, you might bump into some rare issues like this one I found on the Hibernate forum. In this article, I’m going to explain the mapping between Java objects to JDBC and database column types, and how you can fix the issue described in the aforementioned Hibernate question.
