How to cancel Java 8 completable future?
Apparently, it's intentional. The Javadoc for the method CompletableFuture::cancel states:
[Parameters:] mayInterruptIfRunning - this value has no effect in this implementation because interrupts are not used to control processing.
Interestingly, the method ForkJoinTask::cancel uses almost the same wording for the parameter mayInterruptIfRunning.
I have a guess on this issue:
- interruption is intended to be used with blocking operations, like sleep, wait or I/O operations,
- but neither CompletableFuture nor ForkJoinTask are intended to be used with blocking operations.
Instead of blocking, a CompletableFuture should create a new CompletionStage, and cpu-bound tasks are a prerequisite for the fork-join model. So, using interruption with either of them would defeat their purpose. And on the other hand, it might increase complexity, that's not required if used as intended.
When you call CompletableFuture#cancel
, you only stop the downstream part of the chain. Upstream part, i. e. something that will eventually call complete(...)
or completeExceptionally(...)
, doesn't get any signal that the result is no more needed.
What are those 'upstream' and 'downstream' things?
Let's consider the following code:
CompletableFuture
.supplyAsync(() -> "hello") //1
.thenApply(s -> s + " world!") //2
.thenAccept(s -> System.out.println(s)); //3
Here, the data flows from top to bottom - from being created by supplier, through being modified by function, to being consumed by println
. The part above particular step is called upstream, and the part below is downstream. E. g. steps 1 and 2 are upstream for step 3.
Here's what happens behind the scenes. This is not precise, rather it's a convenient mind model of what's going on.
- Supplier (step 1) is being executed (inside the JVM's common
ForkJoinPool
). - The result of the supplier is then being passed by
complete(...)
to the nextCompletableFuture
downstream. - Upon receiving the result, that
CompletableFuture
invokes next step - a function (step 2) which takes in previous step result and returns something that will be passed further, to the downstreamCompletableFuture
'scomplete(...)
. - Upon receiving the step 2 result, step 3
CompletableFuture
invokes the consumer,System.out.println(s)
. After consumer is finished, the downstreamCompletableFuture
will receive it's value,(Void) null
As we can see, each CompletableFuture
in this chain has to know who are there downstream waiting for the value to be passed to their's complete(...)
(or completeExceptionally(...)
). But the CompletableFuture
don't have to know anything about it's upstream (or upstreams - there might be several).
Thus, calling cancel()
upon step 3 doesn't abort steps 1 and 2, because there's no link from step 3 to step 2.
It is supposed that if you're using CompletableFuture
then your steps are small enough so that there's no harm if a couple of extra steps will get executed.
If you want cancellation to be propagated upstream, you have two options:
- Implement this yourself - create a dedicated
CompletableFuture
(name it likecancelled
) which is checked after every step (something likestep.applyToEither(cancelled, Function.identity())
) - Use reactive stack like RxJava 2, ProjectReactor/Flux or Akka Streams
You need an alternative implementation of CompletionStage to accomplish true thread interruption. I've just released a small library that serves exactly this purpose - https://github.com/vsilaev/tascalate-concurrent
If you actually want to be able to cancel a task, then you have to use Future
itself (e.g. as returned by ExecutorService.submit(Callable<T>)
, not CompletableFuture
. As pointed out in the answer by nosid, CompletableFuture
completely ignores any call to cancel(true)
.
My suspicion is that the JDK team did not implement interruption because:
- Interruption was always hacky, difficult for people to understand, and difficult to work with. The Java I/O system is not even interruptible, despite calls to
InputStream.read()
being blocking calls! (And the JDK team have no plans to make the standard I/O system interruptible again, like it was in the very early Java days.) - The JDK team have been trying very hard to phase out old broken APIs from the early Java days, such as
Object.finalize()
,Object.wait()
,Thread.stop()
, etc. I believeThread.interrupt()
is considered to be in the category of things that must be eventually deprecated and replaced. Therefore, newer APIs (likeForkJoinPool
andCompletableFuture
) are already not supporting it. CompletableFuture
was designed for building DAG-structured pipelines of operations, similar to the JavaStream
API. It's very dificult to succinctly describe how interruption of one node of a dataflow DAG should affect execution in the rest of the DAG. (Should all concurrent tasks be canceled immediately, when any node is interrupted?)- I suspect the JDK team just didn't want to deal with getting interruption right, given the levels of internal complexity that the JDK and libraries have reached these days. (The internals of the lambda system -- ugh.)
One very hacky way around this would be to have each CompletableFuture
export a reference to itself to an externally-visible AtomicReference
, then the Thread
reference could be interrupted directly when needed from another external thread. Or if you start all the tasks using your own ExecutorService
, in your own ThreadPool
, you can manually interrupt any or all the threads that were started, even if CompletableFuture
refuses to trigger interruption via cancel(true)
. (Note though that CompletableFuture
lambdas cannot throw checked exceptions, so if you have an interruptible wait in a CompletableFuture
, you'll have to re-throw as an unchecked exception.)
More simply, you could just declare an AtomicReference<Boolean> cancel = new AtomicReference<>()
in an external scope, and periodically check this flag from inside each CompletableFuture
task's lambda.
You could also try setting up a DAG of Future
instances rather than a DAG of CompletableFuture
instances, that way you can exactly specify how exceptions and interruption/cancellation in any one task should affect the other currently-running tasks. I show how to do this in my example code in my question here, and it works well, but it's a lot of boilerplate.