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The Kotlin Programming Language.

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official project TeamCity (simple build status) Maven Central GitHub license Revved up by Gradle Enterprise

Kotlin Programming Language

Welcome to Kotlin!
It is an open-source, statically typed programming language supported and developed by JetBrains and open-source contributors.

Some handy links:

Kotlin Multiplatform capabilities

Support for multiplatform programming is one of Kotlin’s key benefits. It reduces time spent writing and maintaining the same code for different platforms while retaining the flexibility and benefits of native programming.

Editing Kotlin

Build environment requirements

This repository is using Gradle toolchains feature to select and auto-provision required JDKs from AdoptOpenJdk project.

Unfortunately AdoptOpenJdk project does not provide required JDK 1.6 and 1.7 images, so you could either download them manually and provide path to installation via

environment variables or use following SDK managers: - Asdf-vm - Jabba - SDKMAN!

Alternatively, it is still possible to only provide required JDKs via environment variables (see for supported variable names). To ensure Gradle uses only JDKs from environmental variables - disable Gradle toolchain auto-detection by passing
option (or put it into

For local development, if you're not working on the standard library, it's OK to avoid installing JDK 1.6 and JDK 1.7. Add
to the
file, so build will only use JDK 1.8+. Note, that in this case, build will have Gradle remote build cache misses for some tasks.

Note: The JDK 6 for MacOS is not available on Oracle's site. You can install it by

$ brew tap homebrew/cask-versions
$ brew install --cask java6

On Windows you might need to add long paths setting to the repo:

git config core.longpaths true 


The project is built with Gradle. Run Gradle to build the project and to run the tests using the following command on Unix/macOS:


or the following command on Windows:


On the first project configuration gradle will download and setup the dependencies on

  • intellij-core
    is a part of command line compiler and contains only necessary APIs.
  • idea-full
    is a full blown IntelliJ IDEA Community Edition to be used in the plugin module.

These dependencies are quite large, so depending on the quality of your internet connection you might face timeouts getting them. In this case you can increase timeout by specifying the following command line parameters on the first run:

./gradlew -Dhttp.socketTimeout=60000 -Dhttp.connectionTimeout=60000

Important gradle tasks

  • clean
    - clean build results
  • dist
    - assembles the compiler distribution into
  • install
    - build and install all public artifacts into local maven repository
  • coreLibsTest
    - build and run stdlib, reflect and kotlin-test tests
  • gradlePluginTest
    - build and run gradle plugin tests
  • compilerTest
    - build and run all compiler tests

To reproduce TeamCity build use

flag. Local builds don't run proguard and have jar compression disabled by default.

OPTIONAL: Some artifacts, mainly Maven plugin ones, are built separately with Maven. Refer to libraries/ for details.

To build Kotlin/Native, see kotlin-native/

Building for different versions of IntelliJ IDEA and Android Studio

Kotlin plugin is intended to work with several recent versions of IntelliJ IDEA and Android Studio. Each platform is allowed to have a different set of features and might provide a slightly different API. Instead of using several parallel Git branches, the project stores everything in a single branch, but files may have counterparts with version extensions (*.as32, *.172, *.181). The primary file is expected to be replaced with its counterpart when targeting a non-default platform.

A More detailed description of this scheme can be found at

Usually, there's no need to care about multiple platforms as all features are enabled everywhere by default. Additional counterparts should be created if there's an expected difference in behavior or an incompatible API usage is required and there's no reasonable workaround to save source compatibility. Kotlin plugin contains a pre-commit check that shows a warning if a file has been updated without its counterparts.

Development for some particular platform is possible after 'switching' that can be done with the Bunch Tool from the command line.

cd kotlin-project-dir

switching to IntelliJ Idea 2019.1

bunch switch 191

Working with the project in IntelliJ IDEA

Working with the Kotlin project requires at least IntelliJ IDEA 2019.1. You can download IntelliJ IDEA 2019.1 here.

After cloning the project, to import the project in IntelliJ choose the project directory in the Open project dialog. Then, after project opened, select

Module from Existing Sources...
in the menu, and select
file in the project's root folder.

In the import dialog, select

use default gradle wrapper

To be able to run tests from IntelliJ easily, check

Delegate IDE build/run actions to Gradle
and choose
Gradle Test Runner
in the Gradle runner settings after importing the project.

At this time, you can use the latest released

version of the Kotlin plugin for working with the code. To make sure you have the latest version installed, use
Configure Kotlin Plugin Updates

For handy work with compiler tests it's recommended to use Kotlin Compiler Test Helper

Dependency verification

We have a dependencies verification feature enabled in the repository for all Gradle builds. Gradle will check hashes (md5 and sha256) of used dependencies and will fail builds with

Dependency verification failed
errors when local artifacts are absent or have different hashes listed in the verification-metadata.xml file.

It's expected that

should only be updated with the commits that modify the build. There are some tips how to perform such updates:
  • Use auto-generation for getting an initial list of new hashes (verify updates relate to you changes).

./gradlew -M sha256,md5 help

(any other task may be used instead of


  • Consider removing old versions from the file if you are updating dependencies.
  • Leave meaningful
    attribute (instead of
    Generated by Gradle
    ) if you did some manual verification of the artifact.
  • Always do manual verification if several hashes are needed and a new
    tag has to be added.
  • If you’re adding a dependency with OS mentioning in an artifact name (
    ), remember to add counterparts for other platforms.


Kotlin is distributed under the terms of the Apache License (Version 2.0). See license folder for details.


Please be sure to review Kotlin's contributing guidelines to learn how to help the project.

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