Configuring Kotlin Support

Likewise, it is possible to customize the behavior of internal classes in the IntelliJ Platform using 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("..."+expensiveComputation());
}

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 { "..." + expensiveComputation() }

to receive all the benefits of lightweight logging while reducing the code verbosity.

With practice, you will be able to recognize many idioms in the IntelliJ Platform that can be simplified with Kotlin.

The IntelliJ Platform provides a type-safe DSL to build UI forms declaratively.

tip

Using UI Designer plugin with Kotlin is not supported.

Kotlin Coroutines are a lightweight and convenient alternative to threads with many advantages.

Adding Kotlin source files compilation support to a Gradle-based project requires adding and configuring the Kotlin JVM Gradle plugin.

Starting with Kotlin 1.4, a dependency on the standard library stdlib is added automatically (API Docs). In nearly all cases, it is not necessary to include it in the plugin distribution as the platform already bundles it.

To opt out, add this line in gradle.properties:

kotlin.stdlib.default.dependency = false

Plugins must always use the bundled library from the target IDE and not bundle their own version. Make sure it is not added via transitive dependencies either (see View and Debug Dependencies in the Gradle user guide).

Since 2024.2, a custom fork with additional patches is bundled.

See Kotlin Coroutines on how to use them in plugins.

In general, it is strongly advised to always use the bundled library version.

See Third-Party Software and Licenses for an overview of all bundled libraries.

The Kotlin Gradle plugin supports incremental compilation, which allows tracking changes in the source files, so the compiler handles only updated code.

Execution failed for task ':compileKotlin'.
> Failed to transform app.jar to match attributes {artifactType=classpath-entry-snapshot, org.gradle.libraryelements=jar, org.gradle.usage=java-runtime}.
   > Execution failed for ClasspathEntrySnapshotTransform: .../lib/app.jar.
      > Java heap space

kotlin.incremental.useClasspathSnapshot=false

Plugins may use Kotlin classes (class keyword) to implement declarations in the plugin configuration file. When registering an extension, the platform uses a dependency injection framework to instantiate these classes at runtime. For this reason, plugins must not use Kotlin objects (object keyword) to implement any plugin.xml declarations. Managing the lifecycle of extensions is the platform's responsibility, and instantiating these classes as Kotlin singletons may cause issues.

A notable exception is FileType (com.intellij.fileType extension point), see also the inspection descriptions below.

Problems are highlighted via these inspections (2023.2):

• Plugin DevKit | Code | Kotlin object registered as extension for Kotlin code

• Plugin DevKit | Plugin descriptor | Extension class is a Kotlin object for plugin.xml

A Kotlin companion object is always created once you try to load its containing class, and extension point implementations are supposed to be cheap to create. To avoid unnecessary classloading (and thus slowdown in IDE startup), companion object in extensions must only contain simple constants or logger. Anything else must be a top-level declaration or stored in an object.

Use inspection Plugin DevKit | Code | Companion object in extensions to highlight such problems (2023.3).

See Testing in K2 Locally.

The Analysis API is a powerful library for analyzing code in Kotlin. Built on top of the Kotlin PSI syntax tree, it provides access to various semantic information, including reference targets, expression types, declaration scopes, diagnostics, and more.

See Kotlin Analysis API Documentation for details.

How to shorten references