Kotlin and Java sources

Kotlin sources and Java sources can be stored in the same folder, or they can be placed in different folders. The default convention is to use different folders:

project
    - src
        - main (root)
            - kotlin
            - java

The corresponding sourceSets property should be updated if you are not using the default convention:

sourceSets.main {
    java.srcDirs("src/main/myJava", "src/main/myKotlin")
}

sourceSets {
    main.kotlin.srcDirs += 'src/main/myKotlin'
    main.java.srcDirs += 'src/main/myJava'
}

Check for JVM target compatibility of related compile tasks

In the build module, you may have related compile tasks, for example:

• compileKotlin and compileJava

• compileTestKotlin and compileTestJava

For related tasks like these, the Kotlin Gradle plugin checks for JVM target compatibility. Different values of the jvmTarget attribute in the kotlin extension or task and targetCompatibility in the java extension or task cause JVM target incompatibility. For example: the compileKotlin task has jvmTarget=1.8, and the compileJava task has (or inherits) targetCompatibility=15.

Configure the behavior of this check by setting the kotlin.jvm.target.validation.mode property in the build.gradle file to:

• error – the plugin will fail the build; the default value for projects on Gradle 8.0+.

• warning – the Kotlin Gradle plugin will print a warning message; the default value for projects on Gradle less than 8.0.

• ignore – the plugin will skip the check and won't produce any messages.

To avoid JVM target incompatibility, configure a toolchain or align JVM versions manually.

plugins {
    kotlin("jvm") version "1.8.0"
}

plugins {
    id "org.jetbrains.kotlin.jvm" version "1.8.0"
}

Gradle Java toolchains support

Gradle 6.7 introduced Java toolchains support. Using this feature, you can:

• Use a JDK and a JRE that are different from the ones in Gradle to run compilations, tests, and executables.

• Compile and test code with a not-yet-released language version.

With toolchains support, Gradle can autodetect local JDKs and install missing JDKs that Gradle requires for the build. Now Gradle itself can run on any JDK and still reuse the remote build cache feature for tasks that depend on a major JDK version.

The Kotlin Gradle plugin supports Java toolchains for Kotlin/JVM compilation tasks. JS and Native tasks don't use toolchains. The Kotlin compiler always runs on the JDK the Gradle daemon is running on. A Java toolchain:

• Sets the -jdk-home option available for JVM targets.

• Sets the compilerOptions.jvmTarget to the toolchain's JDK version if the user doesn't set the jvmTarget option explicitly. If the user doesn't configure the toolchain, the jvmTarget field uses the default value. Learn more about JVM target compatibility.

• Sets the toolchain to be used by any Java compile, test and javadoc tasks.

• Affects which JDK kapt workers are running on.

Use the following code to set a toolchain. Replace the placeholder <MAJOR_JDK_VERSION> with the JDK version you would like to use:

kotlin {
    jvmToolchain {
        languageVersion.set(JavaLanguageVersion.of(<MAJOR_JDK_VERSION>)) // "8"
    }
    // Or shorter:
    jvmToolchain(<MAJOR_JDK_VERSION>) // "8"
}

kotlin {
    jvmToolchain {
        languageVersion.set(JavaLanguageVersion.of(<MAJOR_JDK_VERSION>)) // "8"
    }
    // Or shorter:
    jvmToolchain(<MAJOR_JDK_VERSION>) // "8"
}

Note that setting a toolchain via the kotlin extension updates the toolchain for Java compile tasks as well.

You can set a toolchain via the java extension, and Kotlin compilation tasks will use it:

java {
    toolchain {
        languageVersion.set(JavaLanguageVersion.of(<MAJOR_JDK_VERSION>)) // "8" 
    }
}

java {
    toolchain {
        languageVersion.set(JavaLanguageVersion.of(<MAJOR_JDK_VERSION>)) // "8" 
    }
}

To set any JDK (even local) for the specific task, use the Task DSL.

Setting JDK version with the Task DSL

The Task DSL allows setting any JDK version for any task implementing the UsesKotlinJavaToolchain interface. At the moment, these tasks are KotlinCompile and KaptTask. If you want Gradle to search for the major JDK version, replace the <MAJOR_JDK_VERSION> placeholder in your build script:

val service = project.extensions.getByType<JavaToolchainService>()
val customLauncher = service.launcherFor {
    it.languageVersion.set(JavaLanguageVersion.of(<MAJOR_JDK_VERSION>)) // "8"
}
project.tasks.withType<UsesKotlinJavaToolchain>().configureEach {
    kotlinJavaToolchain.toolchain.use(customLauncher)
}

JavaToolchainService service = project.getExtensions().getByType(JavaToolchainService.class)
Provider<JavaLauncher> customLauncher = service.launcherFor {
    it.languageVersion.set(JavaLanguageVersion.of(<MAJOR_JDK_VERSION>)) // "8"
}
tasks.withType(UsesKotlinJavaToolchain::class).configureEach { task ->
    task.kotlinJavaToolchain.toolchain.use(customLauncher)
}

Or you can specify the path to your local JDK and replace the placeholder <LOCAL_JDK_VERSION> with this JDK version:

tasks.withType<UsesKotlinJavaToolchain>().configureEach {
    kotlinJavaToolchain.jdk.use(
        "/path/to/local/jdk", // Put a path to your JDK
        JavaVersion.<LOCAL_JDK_VERSION> // For example, JavaVersion.17
    )
}

Associate compiler tasks

You can associate compilations by setting up such a relationship between them that one compilation uses the compiled outputs of the other. Associating compilations establishes internal visibility between them.

The Kotlin compiler associates some compilations by default, such as the test and main compilations of each target. If you need to express that one of your custom compilations is connected to another, create your own associated compilation.

To make the IDE support associated compilations for inferring visibility between source sets, add the following code to your build.gradle(.kts):

val integrationTestCompilation = kotlin.target.compilations.create("integrationTest") {
    associateWith(kotlin.target.compilations.getByName("main"))
}

integrationTestCompilation {
    kotlin.target.compilations.create("integrationTest") {
        associateWith(kotlin.target.compilations.getByName("main"))
    }
}

Here, the integrationTest compilation is associated with the main compilation that gives access to internal objects from functional tests.

Kotlin and Java sources for JavaScript

This plugin only works for Kotlin files, so it is recommended that you keep Kotlin and Java files separate (if the project contains Java files). If you don't store them separately, specify the source folder in the sourceSets block:

kotlin {
    sourceSets["main"].apply {    
        kotlin.srcDir("src/main/myKotlin") 
    }
}

kotlin {
    sourceSets {
        main.kotlin.srcDirs += 'src/main/myKotlin'
    }
}

Dependency types

Choose the dependency type based on your requirements.

Dependency on the standard library

A dependency on the standard library (stdlib) is added automatically to each source set. The version of the standard library used is the same as the version of the Kotlin Gradle plugin.

For platform-specific source sets, the corresponding platform-specific variant of the library is used, while a common standard library is added to the rest. The Kotlin Gradle plugin selects the appropriate JVM standard library depending on the compilerOptions.jvmTarget compiler option of your Gradle build script.

If you declare a standard library dependency explicitly (for example, if you need a different version), the Kotlin Gradle plugin won't override it or add a second standard library.

If you do not need a standard library at all, you can add the opt-out option to the gradle.properties:

kotlin.stdlib.default.dependency=false

 `kotlin.stdlib.jdk.variants.version.alignment=false`

Set dependencies on test libraries

The kotlin.test API is available for testing Kotlin projects on all supported platforms. Add the dependency kotlin-test to the commonTest source set, so that the Gradle plugin can infer the corresponding test dependencies for each test source set:

• kotlin-test-common and kotlin-test-annotations-common for common source sets

• kotlin-test-junit for JVM source sets

• kotlin-test-js for Kotlin/JS source sets

Kotlin/Native targets do not require additional test dependencies, and the kotlin.test API implementations are built-in.

kotlin {
    sourceSets {
        val commonTest by getting {
            dependencies {
                implementation(kotlin("test")) // This brings all the platform dependencies automatically
            }
        }
    }
}

kotlin {
    sourceSets {
        commonTest {
            dependencies {
                implementation kotlin("test") // This brings all the platform dependencies automatically
            }
        }
    }
}

You can use the kotlin-test dependency in any shared or platform-specific source set as well.

For Kotlin/JVM, Gradle uses JUnit 4 by default. Therefore, the kotlin("test") dependency resolves to the variant for JUnit 4, namely kotlin-test-junit.

You can choose JUnit 5 or TestNG by calling useJUnitPlatform() or useTestNG() in the test task of your build script. The following example is for a Kotlin Multiplatform project:

kotlin {
    jvm {
        testRuns["test"].executionTask.configure {
            useJUnitPlatform()
        }
    }
    sourceSets {
        val commonTest by getting {
            dependencies {
                implementation(kotlin("test"))
            }
        }
    }
}

kotlin {
    jvm {
        testRuns["test"].executionTask.configure {
            useJUnitPlatform()
        }
    }
    sourceSets {
        commonTest {
            dependencies {
                implementation kotlin("test")
            }
        }
    }
}

The following example is for a JVM project:

dependencies {
    testImplementation(kotlin("test"))
}

tasks {
    test {
        useTestNG()
    }
}

dependencies {
    testImplementation 'org.jetbrains.kotlin:kotlin-test'
}

test {
    useTestNG()
}

Learn how to test code using JUnit on the JVM.

If you need to use a different JVM test framework, disable automatic testing framework selection by adding the line kotlin.test.infer.jvm.variant=false to the project's gradle.properties file. After doing this, add the framework as a Gradle dependency.

If you have used a variant of kotlin("test") in your build script explicitly and your project build stopped working with a compatibility conflict, see this issue in the Compatibility Guide.

Set a dependency on a kotlinx library

If you use a kotlinx library and need a platform-specific dependency, you can use platform-specific variants of libraries with suffixes such as -jvm or -js, for example, kotlinx-coroutines-core-jvm. You can also use the library's base artifact name instead – kotlinx-coroutines-core.

kotlin {
    sourceSets {
        val jvmMain by getting {
            dependencies {
                implementation("org.jetbrains.kotlinx:kotlinx-coroutines-core-jvm:1.6.4")
            }
        }
    }
}

kotlin {
    sourceSets {
        jvmMain {
            dependencies {
                implementation 'org.jetbrains.kotlinx:kotlinx-coroutines-core-jvm:1.6.4'
            }
        }
    }
}

If you use a multiplatform library and need to depend on the shared code, set the dependency only once, in the shared source set. Use the library's base artifact name, such as kotlinx-coroutines-core or ktor-client-core.

kotlin {
    sourceSets {
        val commonMain by getting {
            dependencies {
                implementation("org.jetbrains.kotlinx:kotlinx-coroutines-core:1.6.4")
            }
        }
    }
}

kotlin {
    sourceSets {
        commonMain {
            dependencies {
                implementation 'org.jetbrains.kotlinx:kotlinx-coroutines-core:1.6.4'
            }
        }
    }
}

Set dependencies at top level

Alternatively, you can specify the dependencies at top level, using the following pattern for the configuration names: <sourceSetName><DependencyType>. This can be helpful for some Gradle built-in dependencies, like gradleApi(), localGroovy(), or gradleTestKit(), which are not available in the source sets' dependency DSL.

dependencies {
    "commonMainImplementation"("com.example:my-library:1.0")
}

dependencies {
    commonMainImplementation 'com.example:my-library:1.0'
}