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Kotlin for Plugin Developers

1. Why Kotlin?

Using Kotlin to write plugins for the IntelliJ Platform is very similar to writing plugins in Java. Existing plugin developers can get started by converting boilerplate Java classes to their Kotlin equivalents by using the J2K compiler bundled with the IntelliJ Platform (versions 143.+), and developers can easily mix and match Kotlin classes with their existing Java code.

In addition to null safety and type-safe builders, the Kotlin language offers a number of convenient features for plugin development, which make plugins easier to read and simpler to maintain. Much like Kotlin for Android, the IntelliJ Platform makes extensive use of callbacks, which are easy to express as lambdas in Kotlin.

Likewise, it is easy to customize the behavior of internal classes in IntelliJ IDEA, with extensions. For example, it is common practice to guard logging statements to avoid the cost of parameter construction, leading to the following ceremony when using the log:

if(logger.isDebugEnabled()) {
  logger.debug("...");
}

We can achieve the same result more succinctly in Kotlin, by declaring the following extension method:

inline fun Logger.debug(lazyMessage: () -> String) {
  if (isDebugEnabled) {
    debug(lazyMessage())
  }
}

Now we can directly write logger.debug { "..." } to receive all the benefits of lightweight logging, with none of the verbosity. With practice, you will be able to recognize many idioms in the IntelliJ Platform that can be simplified with Kotlin. To learn more about building IntelliJ Platform plugins with Kotlin, this tutorial will help you get started.

2. Adding Kotlin Support

Plugins targeting the IntelliJ Platform versions 143 and above are easy to migrate: just start writing Kotlin. The necessary Kotlin plugins and libraries are already bundled with the IDE, requiring no further configuration. For version 142 or below, you will need to install and configure dependencies to the Kotlin runtime (in addition to installing the Kotlin plugin itself, for code assistance and tooling support). For detailed instructions, please refer to the Kotlin documentation.

3. Kotlin Gradle Plugin

For plugins already using the Gradle Build System, or those that require precise control over the Kotlin build process, we recommend using the kotlin-gradle-plugin. This Gradle plugin greatly simplifies building Kotlin projects in a controlled and reproducible manner.

As plugins often target a range of IntelliJ Platform releases, it is important to package your plugin with its own Kotlin runtime to ensure backward- and forward- compatibility. Should the default Kotlin runtime shipped with the IntelliJ Platform change during the course of your plugin's targeted version range, the plugin itself will remain unaffected (assuming no breaking changes to the IntelliJ Platform API). For example, your build.gradle file may look like so:

buildscript {
    repositories {
        mavenCentral()
    }
    dependencies {
        classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:$kotlin_version"
    }
}

apply plugin: 'org.jetbrains.intellij'
apply plugin: 'kotlin'

intellij {
    version 'LATEST-TRUNK-SNAPSHOT'
    pluginName 'Example'
}

group 'com.example'
version '0.0.1'

repositories {
    mavenCentral()
}

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

4. Examples

There are a number of open source Kotlin projects built on the IntelliJ Platform. For a readily available source of up to date examples and applications of the Kotlin language for building developer tools with the IntelliJ Platform, developers may look to these projects for inspiration: