JPA Entities in Kotlin

This repository was originally used in the "Getting the Most from JPA with Kotlin" webinar. To follow the webinar flow step by step, checkout the "webinar-starting-point" branch and click on the image below to start watching.

JPA Entities on Kotlin Checklist

• To avoid significant performace issues and enable lazy loading for Many/One to One assosiations make sure you mark all JPA-related classes and their properties as open.

@Table(name = "project")
@Entity
open class Project {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "id", nullable = false)
    open var id: Long? = null

    @Column(name = "name", nullable = false)
    open var name: String? = null

    ...
}

• You may use the all-open compiler plugin to make all JPA-related classes and properties open by default. Make sure you configure it right, so it applys for all classes annotated as @Entity, @MappedSuperclass, @Embeddable.

<compilerPlugins>
   ...
   <plugin>all-open</plugin>
   ...
</compilerPlugins>
<pluginOptions>
   <option>all-open:annotation=javax.persistence.Entity</option>
   <option>all-open:annotation=javax.persistence.MappedSuperclass</option>
   <option>all-open:annotation=javax.persistence.Embeddable</option>
</pluginOptions> 

• Using primary constructors in JPA-related classes will cause the following exception org.hibernate.InstantiationException: No default constructor for entity. To resolve this issue you may manually define no-args constructor or use the kotlin-jpa compiler plugin, which ensures that no-args constructor will be generated in bytecode for each JPA-related class.

<compilerPlugins>
   ...
   <plugin>jpa</plugin>
   ...   
</compilerPlugins>

• To use data classes
  • Enable the all-open plugin as it was described above, because this is the only way to make data classes open in the compiled bytecode.
  • Override equals(), hashCode(), toString() in accordance with one of this articles by Vlad Mihalcea or Thorben Janssen.
• JPA Buddy is aware of all these things and always generate valid entities for you, including extra stuff like equals(), hashCode(), toString()!

Find more examples of JPA entities on Kotlin in the domain.kt file. It also includes detailed explanation of the different use cases inlined in the comments.