@JvmDefault and how add compiler option

Question

I need to make a default void method in a Kotlin interface. I ran into a problem and a hint said Usage of @JvmDefault is only allowed with -Xjvm-default option.

Where do I need to write this Xjvm-default?

Answer 1

Additional relevant failure output can be: is not abstract and does not override abstract method.

About passing the -Xjvm-default flag:

maven

in the plugin configuration node for kotlin-maven-plugin add

<args>
  <arg>-Xjvm-default=all</arg>
</args>

IDEA

in Preferences (via dev.to): Build, Execution, Deployment -> Compiler -> Kotlin Compiler -> Additional command line parameters add -Xjvm-default=all

in Project Structure: Modules -> Kotlin -> Additional command line parameters add -Xjvm-default=all

@JvmDefault should not be necessary with kotlin > 1.4 but there may be special constellations in which the manual should help determine which combination of annotation and flag is required.

Answer 2

In root build.gradle add below code

allprojects {
    //Support @JvmDefault
    tasks.withType(org.jetbrains.kotlin.gradle.tasks.KotlinCompile).all {
        kotlinOptions {
            freeCompilerArgs = ['-Xjvm-default=enable'] //enable or compatibility
            jvmTarget = "1.8"
    }
}

Answer 3

This question regarding @JvmDefault seems rather similar to this one.

The -Xjvm-default=enable flag has to be passed to the compiler. Since I assume that you do not work with kotlinc (the Kotlin compiler) directly on the command-line: This Q&A contains information on how to pass flags to the Kotlin compiler when using Gradle.

Answer 4

@Target([AnnotationTarget.FUNCTION, AnnotationTarget.PROPERTY]) annotation class JvmDefault

Specifies that a JVM default method should be generated for non-abstract Kotlin interface member.

Usages of this annotation require an explicit compilation argument to be specified: either -Xjvm-default=enable or -Xjvm-default=compatibility.

with -Xjvm-default=enable, only default method in interface is generated for each @JvmDefault method. In this mode, annotating an existing method with @JvmDefault can break binary compatibility, because it will effectively remove the method from the DefaultImpls class.

with -Xjvm-default=compatibility, in addition to the default interface method, a compatibility accessor is generated in the DefaultImpls class, that calls the default interface method via a synthetic accessor. In this mode, annotating an existing method with @JvmDefault is binary compatible, but results in more methods in bytecode.

Removing this annotation from an interface member is a binary incompatible change in both modes.

Generation of default methods is only possible with JVM target bytecode version 1.8 (-jvm-target 1.8) or higher.

@JvmDefault methods are excluded from interface delegation.

https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.jvm/-jvm-default/index.html

if you use gradle then add this parameter to gradle script

https://kotlinlang.org/docs/reference/using-gradle.html

for example add this to gradle:

kotlinOptions {
    freeCompilerArgs = ['-Xjvm-default=compatibility']
}

otherwise if you use Kotlinc command line compiler add -Xjvm-default=compatibility after your command in command line

Answer 5

The new answer since Kotlin 1.4 would be:

tasks.withType(KotlinCompile::class).all {
    kotlinOptions {
        jvmTarget = "11"

        // For creation of default methods in interfaces
        freeCompilerArgs = listOf("-Xjvm-default=all")
    }
}

You can then even leave off the @JvmDefault completely!

Cf. https://kotlinlang.org/docs/reference/java-to-kotlin-interop.html#default-methods-in-interfaces.