For example, Hibernate does not throw an exception when it encounters a final entity class. Some JPA providers don’t enforce this rule. Id ".jpa" version "1.5.21"Īs per the JPA specification, all JPA-related classes and properties must be open. Alternatively and preferably, use the kotlin-jpa compiler plugin which ensures that no-args constructor is generated in the bytecode for each JPA-related class: or enable the plugin, simply add it to the dependencies of kotlin-maven-plugin and to compilerPlugins: To resolve this issue, you may manually define a no-args constructor in all entities. However, adding a primary constructor we lose the default one, so if you try to use it with Hibernate, you get the following exception: : No default constructor for entity. Primary constructors are one of the most loved features in Kotlin. equals() and hashCode() implementations must take into account the mutable nature of entities.Otherwise we might hit unexpected performance issues or a LazyInitializationException. All lazy associations must be loaded only when explicitly requested.These requirements are enough to make entities work, but we need additional rules to make them work well: No methods or persistent instance variables of the entity class may be final. The no-arg constructor must be public or protected.Ģ. The entity class may have other constructors as well. The entity class must have a no-arg constructor. The JPA Specification provides its own set of restrictions, here are the two most important to us:ġ. To work, and work well, they need to satisfy certain requirements, let’s start by defining them. Rules for JPA EntitiesĮntities are not regular DTOs. This article will be mostly focused on Hibernate as it is an undoubtable leader among all JPA implementations. Spoiler alert: data classes are not the best option for entity classes. Let’s look at how to avoid the common pitfalls and make the most of using Kotlin. There is no JPA without entities, and defining them in Kotlin comes with some caveats. Picking JPA for a fresh Kotlin application also makes sense, as it is a mature technology familiar to the developers. Such migrations is one of the reasons why we might have to work with JPA in Kotlin. The latter allows developers to migrate their projects to Kotlin without having to rewrite the entire codebase. Kotlin is great: it’s more concise and expressive than Java, it allows for safer code and offers seamless interoperability with Java.
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