How to cancel Java 8 completable future?

Question

I am playing with Java 8 completable futures. I have the following code:

CountDownLatch waitLatch = new CountDownLatch(1);

CompletableFuture<?> future = CompletableFuture.runAsync(() -> {
    try {
        System.out.println("Wait");
        waitLatch.await(); //cancel should interrupt
        System.out.println("Done");
    } catch (InterruptedException e) {
        System.out.println("Interrupted");
        throw new RuntimeException(e);
    }
});

sleep(10); //give it some time to start (ugly, but works)
future.cancel(true);
System.out.println("Cancel called");

assertTrue(future.isCancelled());

assertTrue(future.isDone());
sleep(100); //give it some time to finish

Using runAsync I schedule execution of a code that waits on a latch. Next I cancel the future, expecting an interrupted exception to be thrown inside. But it seems that the thread remains blocked on the await call and the InterruptedException is never thrown even though the future is canceled (assertions pass). An equivalent code using ExecutorService works as expected. Is it a bug in the CompletableFuture or in my example?

Answer 1

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.

1. Supplier (step 1) is being executed (inside the JVM's common ForkJoinPool).
2. The result of the supplier is then being passed by complete(...) to the next CompletableFuture downstream.
3. 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 downstream CompletableFuture's complete(...).
4. Upon receiving the step 2 result, step 3 CompletableFuture invokes the consumer, System.out.println(s). After consumer is finished, the downstream CompletableFuture 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 like cancelled) which is checked after every step (something like step.applyToEither(cancelled, Function.identity()))
• Use reactive stack like RxJava 2, ProjectReactor/Flux or Akka Streams

Answer 2

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.

Answer 3

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:

1. 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.)
2. 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 believe Thread.interrupt() is considered to be in the category of things that must be eventually deprecated and replaced. Therefore, newer APIs (like ForkJoinPool and CompletableFuture) are already not supporting it.
3. CompletableFuture was designed for building DAG-structured pipelines of operations, similar to the Java Stream 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?)
4. 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.