Kotlin Builder

JVM Builder for Kotlin Data Classes

Introduction

This is an annotation processor for Kotlin data classes to generate a builder for use from other JVM languages. This provides an alternative to an all-args constructor or generating telescopic constructors.

Usage is as simple as adding the @JvmBuilder annotation to the class of your choice, for example:

@JvmBuilder
data class Person(val name: String, val age: Int)

After applying kapt, a Person_Builder class will be generated allowing you to construct an object from other language, e.g. in Java this would be:

new Person_Builder()
    .name("Henry")
    .age(15)
    .build()

Nullability

The annotation processor will generate a builder that accepts nullable fields. As you're calling this from another JVM language, this is ok as they typically don't have the notion of nullability. During the build process, nullability will be checked and an IllegalStateException will be raised informing you of the mistake as illustrated n the following, taken from the examples:

Kotlin:

@JvmBuilder
data class Param1Int(val param1: Int)

Groovy Test:

    def "will error if trying to build and a value isn't provided for a non-null field"() {
        when:
            new Param1Int_Builder().param1(null).build()

        then:
            thrown(IllegalStateException)
    }

Default Values

If defaults have been provided in the data class, these can be omitted from the builder chain and the original value will be provided, as shown in this example:

Kotlin:

@JvmBuilder
data class Param1Default(val defaultString: String = "The D. Fault")

Groovy Test:

    def "uses defaults when provided"() {
        expect:
            new Param1Default_Builder().build().defaultString == "The D. Fault"
    }

If a parameter does not have a default value and is not nullable, an IllegalStateException will be raised on build.

Static builder() method

It is normal for the class the builder is for to generate a static builder() method to provide easy access to creating a builder instance. As there is no officially supported way during annotation processing to modify existing classes this is not provided, but can be achieved with the following:

@JvmBuilder
data class BuilderMethodProvided(val param1: Int = 1, val param2: String = "Default") {
    companion object {
        @JvmStatic fun builder() = BuilderMethodProvided_Builder()
    }
}

Obviously this won't compile until you have run kapt to generate the builder.

Custom setter prefix

As of version 1.1.0 it is possible to provide a custom prefix for the generated setter methods.

@JvmBuilder(setterPrefix = "with")
data class CustomSetterSpec(val param1: String)

new CustomSetterSpec_Builder()
    .withParam1("ignored")
    .build()

Please note that prior to this version the default prefix was with. This has been removed so that no default is provided.

Getting Started

Kotlin Builder is available from maven central and must be added as a kapt and compile dependency to your project. The example/ directory has a fully working example to get you started.

The builder uses runtime reflection for nullability and default parameters, so this must be provided on your runtime classpath, shown below:

apply plugin: 'kotlin'
apply plugin: 'kotlin-kapt'

def kotlinbuilderVersion = "1.1.0"

dependencies {
  compileOnly "com.masabi.kotlinbuilder:masabi-kotlinbuilder:$kotlinbuilderVersion"
  
  implementation (
        "org.jetbrains.kotlin:kotlin-stdlib:$kotlin_version",
        "org.jetbrains.kotlin:kotlin-reflect:$kotlin_version"
    )

    kapt(
        "com.masabi.kotlinbuilder:kotlinbuilder-processor:$kotlinbuilderVersion"
    )
}

Change Log

Version 1.1.0 (03-09-2018)

• Split the annotation out from the processor to avoid classpath leaking (jffiorillo)
• Removed "with" as the default setter prefix (jffiorillo)
• Provide customer setter prefix

Version 1.0.1 (23-07-2018)

• Fixed issue where nullable values weren't being overriden

Version 1.0.0 (20-07-2018)

• Initial release