Cooperative Cancellation: isActive and ensureActive
Make coroutines cancellable by checking isActive and calling ensureActive.
Cooperative Cancellation: isActive and ensureActive is a free Kotlin Academy lesson on CoddyKit. This is lesson 1 of 4. Any 3 lessons of this learning path are free to read in full — after that, CoddyKit PRO unlocks every lesson, plus hands-on practice with a built-in code editor and a 24/7 AI tutor. It is part of the Kotlin Academy learning path, and your progress syncs across the web and the CoddyKit app. The Kotlin Academy course includes 4 lessons in total.
Why Cooperative Cancellation?
Kotlin coroutines use cooperative cancellation: a cancelled coroutine is only stopped when it cooperates by checking its cancellation state. A tight CPU loop will not stop until it checks.
import kotlinx.coroutines.*
fun main() = runBlocking {
val job = launch {
var i = 0
while (true) { i++ } // never checks — never cancels!
}
delay(100)
job.cancel()
println("cancel sent but loop kept running until process ended")
}Checking isActive
isActive is a property on CoroutineScope that returns false once cancellation is requested. Checking it in a loop makes the coroutine cooperate.
import kotlinx.coroutines.*
fun main() = runBlocking {
val job = launch {
var i = 0
while (isActive) {
i++
}
println("Loop exited after $i iterations")
}
delay(10)
job.cancel()
job.join()
}ensureActive()
ensureActive() throws CancellationException if the coroutine is cancelled. It is a drop-in replacement for if (!isActive) throw CancellationException().
import kotlinx.coroutines.*
fun main() = runBlocking {
val job = launch {
try {
repeat(1_000_000) {
ensureActive() // throws if cancelled
// heavy computation here
}
} catch (e: CancellationException) {
println("Cancelled at iteration")
throw e
}
}
delay(5)
job.cancel()
job.join()
}yield() as a Cooperative Point
yield() suspends the coroutine briefly, allowing the scheduler to check cancellation and run other coroutines.
import kotlinx.coroutines.*
fun main() = runBlocking {
val job = launch {
repeat(10) { i ->
yield() // cooperative cancellation point + gives up scheduler
println("Step $i")
}
}
delay(2)
job.cancel()
job.join()
println("Done")
}isActive vs ensureActive
Use isActive for conditional logic (e.g., break out of a loop gracefully). Use ensureActive() when you want to propagate cancellation as an exception immediately.
import kotlinx.coroutines.*
fun main() = runBlocking {
launch {
repeat(100) { i ->
if (!isActive) {
println("Stopping at $i, cleaning up...")
return@launch
}
// work
}
}.also { delay(5); it.cancel(); it.join() }
}CancellationException is Special
CancellationException is not propagated to the parent — it signals normal cancellation. Never swallow it; always rethrow or let it propagate.
import kotlinx.coroutines.*
fun main() = runBlocking {
val job = launch {
try {
delay(1000)
} catch (e: CancellationException) {
println("Caught cancellation, rethrowing")
throw e // important!
}
}
delay(50)
job.cancel()
job.join()
println("Job state: ${job.isCancelled}")
}Cancellation in CPU-Bound Work
For CPU-bound work, insert ensureActive() or yield() at natural checkpoints to allow timely cancellation.
import kotlinx.coroutines.*
suspend fun heavyCompute(n: Int): Long {
var result = 0L
for (i in 0..n) {
ensureActive()
result += i
}
return result
}
fun main() = runBlocking {
val job = launch(Dispatchers.Default) {
println(heavyCompute(1_000_000))
}
delay(10)
job.cancel()
job.join()
}isActive in a Custom Suspend Function
You can access isActive inside any suspend function via coroutineContext.isActive or by using currentCoroutineContext().isActive.
import kotlinx.coroutines.*
suspend fun checkable() {
while (currentCoroutineContext().isActive) {
delay(10)
println("tick")
}
}
fun main() = runBlocking {
val job = launch { checkable() }
delay(35)
job.cancel()
job.join()
}Cancellation with Resource Cleanup
Even with cooperative cancellation, resources can be cleaned up in finally blocks — cancellation exception still propagates through them.
import kotlinx.coroutines.*
fun main() = runBlocking {
val job = launch {
try {
repeat(100) { i ->
ensureActive()
delay(20)
println("Working $i")
}
} finally {
println("Cleanup done")
}
}
delay(50)
job.cancel()
job.join()
}Polling with isActive
Polling loops are a perfect fit for isActive: keep polling until the coroutine is cancelled.
import kotlinx.coroutines.*
fun main() = runBlocking {
val job = launch {
while (isActive) {
println("Polling...")
delay(100)
}
println("Poller stopped")
}
delay(350)
job.cancel()
job.join()
}Difference from Thread.interrupted()
Unlike Java threads, Kotlin coroutines do not rely on interrupt flags. Cancellation is modeled via exceptions and scope state, making it composable and structured.
import kotlinx.coroutines.*
fun main() = runBlocking {
// Kotlin way: cooperative via isActive / ensureActive
launch {
ensureActive()
println("Kotlin coroutine: cancellation is explicit and structured")
}
}Quick Check
Which function throws immediately when a coroutine is cancelled?
Recap
Cooperative cancellation requires explicit check points: isActive for conditional exit, ensureActive() to throw immediately, yield() to suspend and give up the thread. Never swallow CancellationException.
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Frequently Asked Questions
Is the “Cooperative Cancellation: isActive and ensureActive” lesson free?
Yes — any 3 lessons of the Kotlin Academy learning path, including “Cooperative Cancellation: isActive and ensureActive”, are free to read in full here on the web. After that, CoddyKit PRO unlocks every lesson, plus interactive practice with a built-in code editor and a 24/7 AI tutor. The Kotlin Academy course includes 4 lessons in total.
What will I learn in “Cooperative Cancellation: isActive and ensureActive”?
Make coroutines cancellable by checking isActive and calling ensureActive. You practise Kotlin Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start Kotlin Academy?
No prior experience is required. Kotlin Academy on CoddyKit is structured for beginners through advanced learners, so you can start here or from the beginning and move at your own pace. This is lesson 1 of 4.
How long does the “Cooperative Cancellation: isActive and ensureActive” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this Kotlin Academy lesson?
Yes. Every Kotlin Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.
All lessons in this course
- Cooperative Cancellation: isActive and ensureActive
- withTimeout and withTimeoutOrNull
- Cleanup with finally and NonCancellable
- Cancellation Propagation in Coroutine Hierarchies