Kotlin Multiplatform

This page contains examples of using Mill to build Kotlin Multiplatform projects, sharing code between platforms such as the JVM and Kotlin/JS.

Kotlin Multiplatform support is experimental. It currently supports JVM and Kotlin/JS platforms, and requires Kotlin 2.0 or later.

Hello Kotlin Multiplatform

This example shows a minimal Kotlin Multiplatform module foo, compiled for the JVM and for Kotlin/JS running on Node.js.

build.mill (download, browse)

package build
import mill.*, kotlinlib.*, kotlinlib.js.*, kotlinlib.multiplatform.*

object foo extends KotlinMultiplatformModule {
  def kotlinVersion = "2.2.20"

  object jvm extends PlatformModule {
    def mainClass = Some("foo.MainKt")

    object test extends PlatformTests, TestModule.Junit4 {
      def mvnDeps = Seq(mvn"org.jetbrains.kotlin:kotlin-test-junit:${this.kotlinVersion()}")
    }
  }

  object js extends PlatformModule, KotlinJsModule {
    def kotlinJsModuleKind = ModuleKind.ESModule
    def kotlinJsRunTarget = Some(RunTarget.Node)

    object test extends PlatformTests, KotlinTestPackageTests
  }
}

A KotlinMultiplatformModule contains one PlatformModule per target platform. Its sources are laid out by source set, as in the Kotlin Gradle plugin:

foo/src/
  commonMain/kotlin/   shared by all platforms
  jvmMain/kotlin/      only for `foo.jvm`
  jvmMain/java/        Java sources, only for `foo.jvm`
  jsMain/kotlin/       only for `foo.js`
  commonTest/kotlin/   tests shared by all platforms
  jvmTest/kotlin/      tests only for `foo.jvm.test`

Each platform module compiles its own source set together with commonMain, so expect declarations in commonMain can be implemented by actual declarations in jvmMain and jsMain. The source set used by a platform module is named after it, e.g. object jvm reads jvmMain, while the compiler is chosen by the traits it extends, e.g. KotlinJsModule for Kotlin/JS.

> ./mill foo.jvm.run
Hello from JVM

> ./mill foo.js.run
Hello from JS

> ./mill foo.jvm.test
...foo.GreetingTest...greetingMentionsPlatform...
...foo.JvmPlatformTest...platformIsJvm...

> ./mill foo.js.test

> cat out/foo/js/test/testForked.dest/test-report.xml
...name="Mocha Tests" tests="1" failures="0" errors="0"...