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.

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

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 JDK_16 and JDK_17 environment variables or use following SDK managers:

Alternatively, it is still possible to only provide required JDKs via environment variables (see gradle.properties for supported variable names). To ensure Gradle uses only JDKs from environmental variables - disable Gradle toolchain auto-detection by passing -Porg.gradle.java.installations.auto-detect=false option (or put it into $GRADLE_USER_HOME/gradle.properties).

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 kotlin.build.isObsoleteJdkOverrideEnabled=true to the local.properties 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

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
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 -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 1.6.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.

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 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.

Using -dev and -SNAPSHOT versions

We publish -dev and -SNAPSHOT versions frequently.

For -dev versions you can use the list of available versions and include this maven repository:

maven { url = uri("https://maven.pkg.jetbrains.space/kotlin/p/kotlin/bootstrap") }

For -SNAPSHOT versions that are updated daily, you can use the list of available versions and include this maven repository:

maven { url = uri("https://oss.sonatype.org/content/repositories/snapshots/") }

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.

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