Implementing Persistence with Data Classes

Adding the kotlin-jpa plugin to your build file makes it easy to use JPA.

data class Person(
    val name: String,
    val dob: LocalDate
)

From a JPA perspective, data classes have two issues.

1. JPA requires a no-argument constructor unless default values are provided for all properties, but data classes do not have a no-argument constructor.
2. Creating a data class with val properties results in an immutable object, which is not designed to work well with JPA objects.

No-Argument Constructor Issue #

Kotlin provides two plugins to solve the no-argument constructor issue. The no-arg plugin allows you to select classes to which a no-argument constructor will be added, and you can define annotations that trigger the addition of a no-argument constructor. The no-arg plugin automatically configures no-argument constructors for Kotlin entities.

plugins {
	kotlin("plugin.jpa") version "1.3.72"
}

dependencies {
	implementation("org.springframework.boot:spring-boot-starter-data-jpa")
}

You can use the no-arg plugin by adding the necessary syntax to your build file, similar to the kotlin-spring plugin. The compiler plugin adds a synthetic no-argument constructor to Kotlin classes. This means that you cannot call the synthetic no-argument constructor directly from Java or Kotlin. However, Spring can call the synthetic no-argument constructor using reflection.

The kotlin-jpa plugin is easier to use than the no-arg plugin. The kotlin-jpa plugin is built upon the no-arg plugin. The kotlin-jpa plugin adds a no-argument constructor to classes automatically marked with the following annotations:

• @Entity
• @Embeddable
• @MappedSuperClass

Difficulties Using Immutable Classes for JPA Entities #

You typically don't want to use immutable classes for JPA entities. Therefore, the Spring development team recommends using simple Kotlin classes with var properties for fields that you want to be mutable when used as entities.