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Cancellation and timeouts

This section covers coroutine cancellation and timeouts.

Cancelling coroutine execution

In a long-running application you might need fine-grained control on your background coroutines. For example, a user might have closed the page that launched a coroutine and now its result is no longer needed and its operation can be cancelled. The launch function returns a Job that can be used to cancel running coroutine:

fun main(args: Array<String>) = runBlocking {
    val job = launch {
        repeat(1000) { i ->
            println("I'm sleeping $i ...")
            delay(500L)
        }
    }
    delay(1300L) // delay a bit
    println("main: I'm tired of waiting!")
    job.cancel() // cancels the job
    job.join() // waits for job's completion 
    println("main: Now I can quit.")
}

You can get full code here

It produces the following output:

I'm sleeping 0 ...
I'm sleeping 1 ...
I'm sleeping 2 ...
main: I'm tired of waiting!
main: Now I can quit.

As soon as main invokes job.cancel, we don't see any output from the other coroutine because it was cancelled. There is also a Job extension function cancelAndJoin that combines cancel and join invocations.

Cancellation is cooperative

Coroutine cancellation is cooperative. A coroutine code has to cooperate to be cancellable. All the suspending functions in kotlinx.coroutines are cancellable. They check for cancellation of coroutine and throw CancellationException when cancelled. However, if a coroutine is working in a computation and does not check for cancellation, then it cannot be cancelled, like the following example shows:

fun main(args: Array<String>) = runBlocking {
    val startTime = System.currentTimeMillis()
    val job = launch(Dispatchers.Default) {
        var nextPrintTime = startTime
        var i = 0
        while (i < 5) { // computation loop, just wastes CPU
            // print a message twice a second
            if (System.currentTimeMillis() >= nextPrintTime) {
                println("I'm sleeping ${i++} ...")
                nextPrintTime += 500L
            }
        }
    }
    delay(1300L) // delay a bit
    println("main: I'm tired of waiting!")
    job.cancelAndJoin() // cancels the job and waits for its completion
    println("main: Now I can quit.")
}

You can get full code here

Run it to see that it continues to print "I'm sleeping" even after cancellation until the job completes by itself after five iterations.

Making computation code cancellable

There are two approaches to making computation code cancellable. The first one is to periodically invoke a suspending function that checks for cancellation. There is a yield function that is a good choice for that purpose. The other one is to explicitly check the cancellation status. Let us try the later approach.

Replace while (i < 5) in the previous example with while (isActive) and rerun it.

fun main(args: Array<String>) = runBlocking {
    val startTime = System.currentTimeMillis()
    val job = launch(Dispatchers.Default) {
        var nextPrintTime = startTime
        var i = 0
        while (isActive) { // cancellable computation loop
            // print a message twice a second
            if (System.currentTimeMillis() >= nextPrintTime) {
                println("I'm sleeping ${i++} ...")
                nextPrintTime += 500L
            }
        }
    }
    delay(1300L) // delay a bit
    println("main: I'm tired of waiting!")
    job.cancelAndJoin() // cancels the job and waits for its completion
    println("main: Now I can quit.")
}

You can get full code here

As you can see, now this loop is cancelled. isActive is an extension property that is available inside the code of coroutine via CoroutineScope object.

Closing resources with finally

Cancellable suspending functions throw CancellationException on cancellation which can be handled in a usual way. For example, try {...} finally {...} expression and Kotlin use function execute their finalization actions normally when coroutine is cancelled:

fun main(args: Array<String>) = runBlocking {
    val job = launch {
        try {
            repeat(1000) { i ->
                println("I'm sleeping $i ...")
                delay(500L)
            }
        } finally {
            println("I'm running finally")
        }
    }
    delay(1300L) // delay a bit
    println("main: I'm tired of waiting!")
    job.cancelAndJoin() // cancels the job and waits for its completion
    println("main: Now I can quit.")
}

You can get full code here

Both join and cancelAndJoin wait for all the finalization actions to complete, so the example above produces the following output:

I'm sleeping 0 ...
I'm sleeping 1 ...
I'm sleeping 2 ...
main: I'm tired of waiting!
I'm running finally
main: Now I can quit.

Run non-cancellable block

Any attempt to use a suspending function in the finally block of the previous example causes CancellationException, because the coroutine running this code is cancelled. Usually, this is not a problem, since all well-behaving closing operations (closing a file, cancelling a job, or closing any kind of a communication channel) are usually non-blocking and do not involve any suspending functions. However, in the rare case when you need to suspend in the cancelled coroutine you can wrap the corresponding code in withContext(NonCancellable) {...} using withContext function and NonCancellable context as the following example shows:

fun main(args: Array<String>) = runBlocking {
    val job = launch {
        try {
            repeat(1000) { i ->
                println("I'm sleeping $i ...")
                delay(500L)
            }
        } finally {
            withContext(NonCancellable) {
                println("I'm running finally")
                delay(1000L)
                println("And I've just delayed for 1 sec because I'm non-cancellable")
            }
        }
    }
    delay(1300L) // delay a bit
    println("main: I'm tired of waiting!")
    job.cancelAndJoin() // cancels the job and waits for its completion
    println("main: Now I can quit.")
}

You can get full code here

Timeout

The most obvious reason to cancel coroutine execution in practice is because its execution time has exceeded some timeout. While you can manually track the reference to the corresponding Job and launch a separate coroutine to cancel the tracked one after delay, there is a ready to use withTimeout function that does it. Look at the following example:

fun main(args: Array<String>) = runBlocking {
    withTimeout(1300L) {
        repeat(1000) { i ->
            println("I'm sleeping $i ...")
            delay(500L)
        }
    }
}

You can get full code here

It produces the following output:

I'm sleeping 0 ...
I'm sleeping 1 ...
I'm sleeping 2 ...
Exception in thread "main" kotlinx.coroutines.experimental.TimeoutCancellationException: Timed out waiting for 1300 ms

The TimeoutCancellationException that is thrown by withTimeout is a subclass of CancellationException. We have not seen its stack trace printed on the console before. That is because inside a cancelled coroutine CancellationException is considered to be a normal reason for coroutine completion. However, in this example we have used withTimeout right inside the main function.

Because cancellation is just an exception, all the resources are closed in a usual way. You can wrap the code with timeout in try {...} catch (e: TimeoutCancellationException) {...} block if you need to do some additional action specifically on any kind of timeout or use withTimeoutOrNull function that is similar to withTimeout, but returns null on timeout instead of throwing an exception:

fun main(args: Array<String>) = runBlocking {
    val result = withTimeoutOrNull(1300L) {
        repeat(1000) { i ->
            println("I'm sleeping $i ...")
            delay(500L)
        }
        "Done" // will get cancelled before it produces this result
    }
    println("Result is $result")
}

You can get full code here

There is no longer an exception when running this code:

I'm sleeping 0 ...
I'm sleeping 1 ...
I'm sleeping 2 ...
Result is null