Kotlin Compiler

2.3.21 verified Fri May 01 auth: no javascript

The official Kotlin compiler from JetBrains, version 2.3.21. Kotlin is a statically typed programming language that targets the JVM, JavaScript, Wasm, and native platforms. The compiler includes support for multiplatform projects, coroutines, and a rich standard library. Released on a frequent cadence with stable releases every few months and previews. Key differentiators include full interoperability with Java, concise syntax, and safety features like null safety and sealed classes. This package provides the command-line compiler (kotlinc) and is also used as a dependency by build tools like Gradle and Maven.

Common errors

error error: unresolved reference: kotlinx ↓

cause Missing dependency for kotlinx.coroutines or kotlinx.serialization in Gradle/Maven.

fix

Add implementation 'org.jetbrains.kotlinx:kotlinx-coroutines-core:1.7.3' (or similar) to build.gradle.

error FAILURE: Build failed with an exception. * What went wrong: Could not set unknown property 'jvmTarget' for task ':compileKotlin' of type org.jetbrains.kotlin.gradle.tasks.KotlinCompile. ↓

cause Using the deprecated compileKotlin closure without typed configuration.

fix

Use tasks.named('compileKotlin', org.jetbrains.kotlin.gradle.tasks.KotlinCompile) { kotlinOptions.jvmTarget = '1.8' }.

Warnings

breaking Kotlin 2.0+ uses K2 compiler by default, which may cause issues with some code that relied on old compiler behavior. ↓

fix Enable K1 compiler via -Xuse-k1 compiler flag or update code to be compatible with K2.

gotcha Using 'kotlin' Gradle plugin version must match compiler version (e.g., Kotlin 2.3.21 plugin requires compiler 2.3.21). ↓

fix Ensure plugin and compiler versions are consistent. Use version catalog or ext.kotlinVersion.

deprecated Kotlin Android Extensions plugin is deprecated in Kotlin 1.8 and removed in 2.0. ↓

fix Migrate to Jetpack View Binding or Compose.

Install

npm install kotlin-compiler

yarn add kotlin-compiler

pnpm add kotlin-compiler

Imports

kotlin
wrong
```
import org.jetbrains.kotlin.*
```
correct
```
import kotlin.*
```
Standard library imports use the 'kotlin' root package. Use 'import kotlin.*' over 'org.jetbrains.kotlin.*' which is for internal compiler APIs.
kotlinx.coroutines
wrong
```
import kotlin.coroutines.*
```
correct
```
import kotlinx.coroutines.*
```
Coroutines are in the 'kotlinx.coroutines' package, not 'kotlin.coroutines' (which is for low-level primitives).
KotlinCompile task (Gradle)
wrong
```
compileKotlin { kotlinOptions.jvmTarget = "1.8" }
```
correct
```
tasks.withType<KotlinCompile> { kotlinOptions.jvmTarget = "1.8" }
```
Use typed task configuration (KotlinCompile) instead of the untyped 'compileKotlin' for better IDE support.

Quickstart

Shows basic compilation of a Kotlin source file to a runnable JAR using the command-line compiler.

// hello.kt
fun main() {
    println("Hello, Kotlin!")
}

// Compile: kotlinc hello.kt -include-runtime -d hello.jar
// Run: java -jar hello.jar