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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 Site
• Getting Started Guide
• Try Kotlin
• Kotlin Standard Library
• Issue Tracker
• Forum
• Kotlin Blog
• Subscribe to Kotlin YouTube channel
• Follow Kotlin on Twitter
• Public Slack channel
• TeamCity CI build

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.

• Kotlin Multiplatform Mobile for sharing code between Android and iOS
• Getting Started with Kotlin Multiplatform Mobile Guide
• Kotlin Multiplatform Benefits
• Share code on all platforms
• Share code on similar platforms

Editing Kotlin

• Kotlin IntelliJ IDEA Plugin
• Kotlin Eclipse Plugin
• Kotlin Sublime Text Package

Build environment requirements

In order to build Kotlin distribution you need to have:

• JDK 1.6, 1.7, 1.8 and 9

• Setup environment variables as following:

    JAVA_HOME="path to JDK 1.8"
    JDK_16="path to JDK 1.6"
    JDK_17="path to JDK 1.7"
    JDK_18="path to JDK 1.8"
    JDK_9="path to JDK 9"
  

For local development, if you're not working on bytecode generation or the standard library, it's OK to have only JDK 1.8 and JDK 9 installed, and to point JDK_16 and JDK_17 environment variables to your JDK 1.8 installation.

You also can use Gradle properties to setup JDK_* variables.

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 

Building

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

./gradlew <tasks-and-options>

or the following command on Windows:

gradlew <tasks-and-options>

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 dist/kotlinc/ folder
• ideaPlugin - assembles the Kotlin IDEA plugin distribution into dist/artifacts/ideaPlugin/Kotlin/ folder
• install - build and install all public artifacts into local maven repository
• runIde - build IDEA plugin and run IDEA with it
• coreLibsTest - build and run stdlib, reflect and kotlin-test tests
• gradlePluginTest - build and run gradle plugin tests
• compilerTest - build and run all compiler tests
• ideaPluginTest - build and run all IDEA plugin tests

To reproduce TeamCity build use -Pteamcity=true 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/ReadMe.md for details.

To build Kotlin/Native, see kotlin-native/README.md.

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 https://github.com/JetBrains/bunches/blob/master/ReadMe.md.

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 File -> New -> Module from Existing Sources... in the menu, and select build.gradle.kts 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 1.3.x version of the Kotlin plugin for working with the code. To make sure you have the latest version installed, use Tools -> Kotlin -> Configure Kotlin Plugin Updates.

Compiling and running

From this root project there are Run/Debug Configurations for running IDEA or the Generate Compiler Tests for example; so if you want to try out the latest and greatest IDEA plugin

• VCS -> Git -> Pull
• Run the IDEA run configuration in the project
• A child IntelliJ IDEA with the Kotlin plugin will then startup

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 verification-metadata.xml 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 help)

• Consider removing old versions from the file if you are updating dependencies.
• Leave meaningful origin 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 also-trust tag has to be added.
• If you’re adding a dependency with OS mentioning in an artifact name (darwin, mac, osx, linux, windows), remember to add counterparts for other platforms.

License

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

Contributing

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