Programming language: Kotlin
License: GNU General Public License v3.0 or later
Tags: Misc    
Latest version: v0.18.5

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Kotlin Beta JetBrains official project GitHub license Maven Central

Note on Beta status: the plugin is in its active development phase and changes from release to release. We do provide a compatibility of atomicfu-transformed artifacts between releases, but we do not provide strict compatibility guarantees on plugin API and its general stability between Kotlin versions.

Atomicfu is a multiplatform library that provides the idiomatic and effective way of using atomic operations in Kotlin.

Table of contents


  • Code it like a boxed value atomic(0), but run it in production efficiently:
    • as java.util.concurrent.atomic.AtomicXxxFieldUpdater on Kotlin/JVM
    • as a plain unboxed value on Kotlin/JS
  • Multiplatform: write common Kotlin code with atomics that compiles for Kotlin JVM, JS, and Native backends:
    • Compile-only dependency for JVM and JS (no runtime dependencies)
    • Compile and runtime dependency for Kotlin/Native
  • Use Kotlin-specific extensions (e.g. inline loop, update, updateAndGet functions).
  • Use atomic arrays, user-defined extensions on atomics and locks (see more features).
  • Tracing operations for debugging.


Let us declare a top variable for a lock-free stack implementation:

import kotlinx.atomicfu.* // import top-level functions from kotlinx.atomicfu

private val top = atomic<Node?>(null) 

Use top.value to perform volatile reads and writes:

fun isEmpty() = top.value == null  // volatile read
fun clear() { top.value = null }   // volatile write

Use compareAndSet function directly:

if (top.compareAndSet(expect, update)) ... 

Use higher-level looping primitives (inline extensions), for example:

top.loop { cur ->   // while(true) loop that volatile-reads current value 

Use high-level update, updateAndGet, and getAndUpdate, when possible, for idiomatic lock-free code, for example:

fun push(v: Value) = top.update { cur -> Node(v, cur) }
fun pop(): Value? = top.getAndUpdate { cur -> cur?.next } ?.value

Declare atomic integers and longs using type inference:

val myInt = atomic(0)    // note: integer initial value
val myLong = atomic(0L)  // note: long initial value   

Integer and long atomics provide all the usual getAndIncrement, incrementAndGet, getAndAdd, addAndGet, and etc operations. They can be also atomically modified via += and -= operators.


Apply plugin

Gradle configuration

Gradle configuration is supported for all platforms, minimal version is Gradle 6.8.

In top-level build file:


buildscript {
    repositories {

    dependencies {

apply(plugin = "kotlinx-atomicfu")


buildscript {
    repositories {
    dependencies {
        classpath 'org.jetbrains.kotlinx:atomicfu-gradle-plugin:0.18.5'

apply plugin: 'kotlinx-atomicfu'

Maven configuration

Maven configuration is supported for JVM projects.

Declare atomicfu version


Declare provided dependency on the AtomicFU library


Configure build steps so that Kotlin compiler puts classes into a different classes-pre-atomicfu directory, which is then transformed to a regular classes directory to be used later by tests and delivery.

Build steps

    <!-- compile Kotlin files to staging directory -->
    <!-- transform classes with AtomicFU plugin -->
            <!-- "VH" to use Java 9 VarHandle, "BOTH" to produce multi-version code -->

Usage constraints

  • Declare atomic variables as private val or internal val. You can use just (public) val, but make sure they are not directly accessed outside of your Kotlin module (outside of the source set). Access to the atomic variable itself shall be encapsulated.
  • Only simple operations on atomic variables directly are supported.
    • Do not read references on atomic variables into local variables, e.g. top.compareAndSet(...) is ok, while val tmp = top; tmp... is not.
    • Do not leak references on atomic variables in other way (return, pass as params, etc).
  • Do not introduce complex data flow in parameters to atomic variable operations, i.e. top.value = complex_expression and top.compareAndSet(cur, complex_expression) are not supported (more specifically, complex_expression should not have branches in its compiled representation). Extract complex_expression into a variable when needed.
  • Use the following convention if you need to expose the value of atomic property to the public:
private val _foo = atomic<T>(initial) // private atomic, convention is to name it with leading underscore
public var foo: T by _foo            // public delegated property (val/var)

Transformation modes

Basically, Atomicfu library provides an effective usage of atomic values by performing the transformations of the compiled code. For JVM and JS there 2 transformation modes available:

  • Post-compilation transformation that modifies the compiled bytecode or *.js files.
  • IR transformation that is performed by the atomicfu compiler plugin.

Atomicfu compiler plugin

Compiler plugin transformation is less fragile than transformation of the compiled sources as it depends on the compiler IR tree.

To turn on IR transformation set these properties in your gradle.properties file:

For Kotlin >= 1.7.20

kotlinx.atomicfu.enableJvmIrTransformation=true // for JVM IR transformation
kotlinx.atomicfu.enableJsIrTransformation=true // for JS IR transformation

For Kotlin >= 1.6.20 and Kotlin < 1.7.20

kotlinx.atomicfu.enableIrTransformation=true // only JS IR transformation is supported

Also for JS backend make sure that ir or both compiler mode is set:

kotlin.js.compiler=ir // or both

Options for post-compilation transformation

Some configuration options are available for post-compilation transform tasks on JVM and JS.

To set configuration options you should create atomicfu section in a build.gradle file, like this:

atomicfu {
  dependenciesVersion = '0.18.5'

JVM options

To turn off transformation for Kotlin/JVM set option transformJvm to false.

Configuration option jvmVariant defines the Java class that replaces atomics during bytecode transformation. Here are the valid options:

  • FU โ€“ atomics are replaced with AtomicXxxFieldUpdater.
  • VH โ€“ atomics are replaced with VarHandle, this option is supported for JDK 9+.
  • BOTH โ€“ multi-release jar file will be created with both AtomicXxxFieldUpdater for JDK <= 8 and VarHandle for JDK 9+.

JS options

To turn off transformation for Kotlin/JS set option transformJs to false.

Here are all available configuration options (with their defaults):

atomicfu {
  dependenciesVersion = '0.18.5' // set to null to turn-off auto dependencies
  transformJvm = true // set to false to turn off JVM transformation
  jvmVariant = "FU" // JVM transformation variant: FU,VH, or BOTH
  transformJs = true // set to false to turn off JVM transformation

More features

AtomicFU provides some additional features that you can use.

Arrays of atomic values

You can declare arrays of all supported atomic value types. By default arrays are transformed into the corresponding java.util.concurrent.atomic.Atomic*Array instances.

If you configure variant = "VH" an array will be transformed to plain array using VarHandle to support atomic operations.

val a = atomicArrayOfNulls<T>(size) // similar to Array constructor

val x = a[i].value // read value
a[i].value = x // set value
a[i].compareAndSet(expect, update) // do atomic operations

User-defined extensions on atomics

You can define you own extension functions on AtomicXxx types but they must be inline and they cannot be public and be used outside of the module they are defined in. For example:

private inline fun AtomicBoolean.tryAcquire(): Boolean = compareAndSet(false, true)


This project includes kotlinx.atomicfu.locks package providing multiplatform locking primitives that require no additional runtime dependencies on Kotlin/JVM and Kotlin/JS with a library implementation for Kotlin/Native.

  • SynchronizedObject is designed for inheritance. You write class MyClass : SynchronizedObject() and then use synchronized(instance) { ... } extension function similarly to the synchronized function from the standard library that is available for JVM. The SynchronizedObject superclass gets erased (transformed to Any) on JVM and JS, with synchronized leaving no trace in the code on JS and getting replaced with built-in monitors for locking on JVM.

  • ReentrantLock is designed for delegation. You write val lock = reentrantLock() to construct its instance and use lock/tryLock/unlock functions or lock.withLock { ... } extension function similarly to the way jucl.ReentrantLock is used on JVM. On JVM it is a typealias to the later class, erased on JS.

Note that package kotlinx.atomicfu.locks is experimental explicitly even while atomicfu is experimental itself, meaning that no ABI guarantees are provided whatsoever. API from this package is not recommended to use in libraries that other projects depend on.

Tracing operations

You can debug your tests tracing atomic operations with a special trace object:

private val trace = Trace()
private val current = atomic(0, trace)

fun update(x: Int): Int {           
    // custom trace message
    trace { "calling update($x)" }
    // automatic tracing of modification operations 
    return current.getAndAdd(x)

All trace messages are stored in a cyclic array inside trace.

You can optionally set the size of trace's message array and format function. For example, you can add a current thread name to the traced messages:

private val trace = Trace(size = 64) {   
    index, // index of a trace message 
    text   // text passed when invoking trace { text }
    -> "$index: [${Thread.currentThread().name}] $text" 

trace is only seen before transformation and completely erased after on Kotlin/JVM and Kotlin/JS.

Kotlin Native support

Atomic references for Kotlin/Native are based on FreezableAtomicReference and every reference that is stored to the previously frozen (shared with another thread) atomic is automatically frozen, too.

Since Kotlin/Native does not generally provide binary compatibility between versions, you should use the same version of Kotlin compiler as was used to build AtomicFU. See [gradle.properties](gradle.properties) in AtomicFU project for its kotlin_version.

*Note that all licence references and agreements mentioned in the kotlinx.atomicfu README section above are relevant to that project's source code only.