Core primitives to work with coroutines.

Coroutine builder functions:

Name Result Scope Description
launch Job CoroutineScope Launches coroutine that does not have any result
async Deferred CoroutineScope Returns a single value with the future result
produce ReceiveChannel ProducerScope Produces a stream of elements
actor SendChannel ActorScope Processes a stream of messages
runBlocking T CoroutineScope Blocks the thread while the coroutine runs

Coroutine dispatchers implementing CoroutineDispatcher:

Name Description
Dispatchers.Default Confines coroutine execution to a shared pool of background threads
Dispatchers.Unconfined Does not confine coroutine execution in any way
newSingleThreadContext Create new single-threaded coroutine context
newFixedThreadPoolContext Creates new thread pool of a fixed size
Executor.asCoroutineDispatcher Extension to convert any executor

More context elements:

Name Description
NonCancellable A non-cancelable job that is always active
CoroutineExceptionHandler Handler for uncaught exception

Synchronization primitives for coroutines:

Name Suspending functions Description
Mutex lock Mutual exclusion
Channel send, receive Communication channel (aka queue or exchanger)

Top-level suspending functions:

Name Description
delay Non-blocking sleep
yield Yields thread in single-threaded dispatchers
withContext Switches to a different context
withTimeout Set execution time-limit with exception on timeout
withTimeoutOrNull Set execution time-limit will null result on timeout
awaitAll Awaits for successful completion of all given jobs or exceptional completion of any
joinAll Joins on all given jobs

Cancellation support for user-defined suspending functions is available with suspendCancellableCoroutine helper function. NonCancellable job object is provided to suppress cancellation with withContext(NonCancellable) {...} block of code.

Select expression waits for the result of multiple suspending functions simultaneously:

Receiver Suspending function Select clause Non-suspending version
Job join onJoin isCompleted
Deferred await onAwait isCompleted
SendChannel send onSend offer
ReceiveChannel receive onReceive poll
ReceiveChannel receiveOrNull onReceiveOrNull poll
Mutex lock onLock tryLock
none delay onTimeout none

This module provides debugging facilities for coroutines (run JVM with -ea or -Dkotlinx.coroutines.debug options) and newCoroutineContext function to write user-defined coroutine builders that work with these debugging facilities.

This module provides a special CoroutineContext type TestCoroutineCoroutineContext that allows the writer of code that contains Coroutines with delays and timeouts to write non-flaky unit-tests for that code allowing these tests to terminate in near zero time. See the documentation for this class for more information.



General-purpose coroutine builders, contexts, and helper functions.


Channels – non-blocking primitives for communicating a stream of elements between coroutines.


Select expression to perform multiple suspending operations simultaneously until one of them succeeds.


Synchronization primitives (mutex).


Components to ease writing unit-tests for code that contains coroutines with delays and timeouts.


All Types