Using Gradle

In order to build Kotlin with Gradle you should set up the kotlin-gradle plugin, apply it to your project and add kotlin-stdlib dependencies. Those actions may also be performed automatically in IntelliJ IDEA by invoking the Tools | Kotlin | Configure Kotlin in Project action.

Plugin and Versions

The kotlin-gradle-plugin compiles Kotlin sources and modules.

The version of Kotlin to use is usually defined as the kotlin_version property:

buildscript {
    ext.kotlin_version = '1.2.41'

    repositories {
        mavenCentral()
    }

    dependencies {
        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
    }
}

This is not required when using Kotlin Gradle plugin 1.1.1 and above with the Gradle plugins DSL.

Targeting the JVM

To target the JVM, the Kotlin plugin needs to be applied:

apply plugin: "kotlin"

Or, starting with Kotlin 1.1.1, the plugin can be applied using the Gradle plugins DSL:

plugins {
    id "org.jetbrains.kotlin.jvm" version "1.2.41"
}

The version should be literal in this block, and it cannot be applied from another build script.

Kotlin sources can be mixed with Java sources in the same folder, or in different folders. The default convention is using different folders:

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

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

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

Targeting JavaScript

When targeting JavaScript, a different plugin should be applied:

apply plugin: "kotlin2js"

This plugin only works for Kotlin files so it is recommended to keep Kotlin and Java files separate (if it's the case that the same project contains Java files). As with targeting the JVM, if not using the default convention, we need to specify the source folder using sourceSets:

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

In addition to the output JavaScript file, the plugin by default creates an additional JS file with binary descriptors. This file is required if you're building a re-usable library that other Kotlin modules can depend on, and should be distributed together with the result of translation. The generation is controlled by the kotlinOptions.metaInfo option:

compileKotlin2Js {
	kotlinOptions.metaInfo = true
}

Targeting Android

Android's Gradle model is a little different from ordinary Gradle, so if we want to build an Android project written in Kotlin, we need kotlin-android plugin instead of kotlin:

buildscript {
    ext.kotlin_version = '1.2.41'

    ...

    dependencies {
        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
    }
}
apply plugin: 'com.android.application'
apply plugin: 'kotlin-android'

Don't forget to configure the standard library dependency.

Android Studio

If using Android Studio, the following needs to be added under android:

android {
  ...

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

This lets Android Studio know that the kotlin directory is a source root, so when the project model is loaded into the IDE it will be properly recognized. Alternatively, you can put Kotlin classes in the Java source directory, typically located in src/main/java.

Configuring Dependencies

In addition to the kotlin-gradle-plugin dependency shown above, you need to add a dependency on the Kotlin standard library:

repositories {
    mavenCentral()
}

dependencies {
    compile "org.jetbrains.kotlin:kotlin-stdlib"
}

If you target JavaScript, use compile "org.jetbrains.kotlin:kotlin-stdlib-js" instead.

If you're targeting JDK 7 or JDK 8, you can use extended versions of the Kotlin standard library which contain additional extension functions for APIs added in new JDK versions. Instead of kotlin-stdlib, use one of the following dependencies:

compile "org.jetbrains.kotlin:kotlin-stdlib-jdk7"
compile "org.jetbrains.kotlin:kotlin-stdlib-jdk8"

In Kotlin 1.1.x, use kotlin-stdlib-jre7 and kotlin-stdlib-jre8 instead.

If your project uses Kotlin reflection or testing facilities, you need to add the corresponding dependencies as well:

compile "org.jetbrains.kotlin:kotlin-reflect"
testCompile "org.jetbrains.kotlin:kotlin-test"
testCompile "org.jetbrains.kotlin:kotlin-test-junit"

Starting with Kotlin 1.1.2, the dependencies with group org.jetbrains.kotlin are by default resolved with the version taken from the applied plugin. You can provide the version manually using the full dependency notation like compile "org.jetbrains.kotlin:kotlin-stdlib:$kotlin_version".

Annotation processing

See the description of Kotlin annotation processing tool (kapt).

Incremental compilation

Kotlin supports optional incremental compilation in Gradle. Incremental compilation tracks changes of source files between builds so only files affected by these changes would be compiled.

Starting with Kotlin 1.1.1, incremental compilation is enabled by default.

There are several ways to override the default setting:

1. add kotlin.incremental=true or kotlin.incremental=false line either to a gradle.properties or a local.properties file;

2. add -Pkotlin.incremental=true or -Pkotlin.incremental=false to gradle command line parameters. Note that in this case the parameter should be added to each subsequent build, and any build with disabled incremental compilation invalidates incremental caches.

When incremental compilation is enabled, you should see the following warning message in your build log:

Using kotlin incremental compilation

Note, that the first build won't be incremental.

Coroutines support

Coroutines support is an experimental feature in Kotlin 1.2, so the Kotlin compiler reports a warning when you use coroutines in your project. To turn off the warning, add the following block to your build.gradle file:

kotlin {
    experimental {
        coroutines 'enable'
    }
}

Module names

The Kotlin modules that the build produces are named accordingly to the archivesBaseName property of the project. If a project has a broad name like lib or jvm, which is common for subprojects, the Kotlin output files related to the module (*.kotlin_module) might clash with those from third-party modules with the same name. This causes problems when a project is packaged into a single archive (e.g. APK).

To avoid this, consider setting a unique archivesBaseName manually:

archivesBaseName = 'myExampleProject_lib'

Gradle Build Cache support (since 1.2.20)

The Kotlin plugin supports Gradle Build Cache (Gradle version 4.3 and above is required; caching is disabled with lower versions).

The kapt annotation processing tasks are not cached by default since annotation processors run arbitrary code that may not necessarily transform the task inputs into the outputs, might access and modify the files that are not tracked by Gradle etc. To enable caching for kapt anyway, add the following lines to the build script:

kapt {
    useBuildCache = true
}

To disable the caching for all Kotlin tasks, set the system property flag kotlin.caching.enabled to false (run the build with the argument -Dkotlin.caching.enabled=false).

Compiler Options

To specify additional compilation options, use the kotlinOptions property of a Kotlin compilation task.

When targeting the JVM, the tasks are called compileKotlin for production code and compileTestKotlin for test code. The tasks for custom source sets of are called accordingly to the compile<Name>Kotlin pattern.

The names of the tasks in Android Projects contain the build variant names and follow the pattern compile<BuildVariant>Kotlin, for example, compileDebugKotlin, compileReleaseUnitTestKotlin.

When targeting JavaScript, the tasks are called compileKotlin2Js and compileTestKotlin2Js respectively, and compile<Name>Kotlin2Js for custom source sets.

To configure a single task, use its name. Examples:

compileKotlin {
    kotlinOptions.suppressWarnings = true
}

compileKotlin {
    kotlinOptions {
        suppressWarnings = true
    }
}

It is also possible to configure all Kotlin compilation tasks in the project:

tasks.withType(org.jetbrains.kotlin.gradle.tasks.KotlinCompile).all {
    kotlinOptions {
        // ...
    }
}

A complete list of options for the Gradle tasks follows:

Attributes common for JVM, JS, and JS DCE

Name	Description	Possible values	Default value
allWarningsAsErrors	Report an error if there are any warnings		false
suppressWarnings	Generate no warnings		false
verbose	Enable verbose logging output		false
freeCompilerArgs	A list of additional compiler arguments		[]

Attributes common for JVM and JS

Name	Description	Possible values	Default value
apiVersion	Allow to use declarations only from the specified version of bundled libraries	"1.0", "1.1", "1.2", "1.3 (EXPERIMENTAL)"
languageVersion	Provide source compatibility with specified language version	"1.0", "1.1", "1.2", "1.3 (EXPERIMENTAL)"

Attributes specific for JVM

Name	Description	Possible values	Default value
javaParameters	Generate metadata for Java 1.8 reflection on method parameters		false
jdkHome	Path to JDK home directory to include into classpath, if differs from default JAVA_HOME
jvmTarget	Target version of the generated JVM bytecode (1.6 or 1.8), default is 1.6	"1.6", "1.8"	"1.6"
noJdk	Don't include Java runtime into classpath		false
noReflect	Don't include Kotlin reflection implementation into classpath		true
noStdlib	Don't include Kotlin runtime into classpath		true

Attributes specific for JS

Name	Description	Possible values	Default value
friendModulesDisabled	Disable internal declaration export		false
main	Whether a main function should be called	"call", "noCall"	"call"
metaInfo	Generate .meta.js and .kjsm files with metadata. Use to create a library		true
moduleKind	Kind of a module generated by compiler	"plain", "amd", "commonjs", "umd"	"plain"
noStdlib	Don't use bundled Kotlin stdlib		true
outputFile	Output file path
sourceMap	Generate source map		false
sourceMapEmbedSources	Embed source files into source map	"never", "always", "inlining"
sourceMapPrefix	Prefix for paths in a source map
target	Generate JS files for specific ECMA version	"v5"	"v5"
typedArrays	Translate primitive arrays to JS typed arrays		true

Generating documentation

To generate documentation for Kotlin projects, use Dokka; please refer to the Dokka README for configuration instructions. Dokka supports mixed-language projects and can generate output in multiple formats, including standard JavaDoc.

OSGi

For OSGi support see the Kotlin OSGi page.

Examples

The following examples show different possibilities of configuring the Gradle plugin:

• Kotlin
• Mixed Java and Kotlin
• Android
• JavaScript