Understanding CompletableFuture::runAsync

Question

I just read the documentation about CompletableFuture::runAsync and was pretty confused by the explanation. Here is what's written there:

Returns a new CompletableFuture that is asynchronously completed by a task running in the given executor after it runs the given action.

As far as I understand it, CompletableFuture looks like Future with that it can "register" some sort of callbacks and invoke them implicitly once a given action is finished.

Taking that into account, let's consider the following code:

ExecutorService threadsPool;
Runnable r;
//...
CompletableFuture.runAsync(r, threadsPool);

In this code we register the Runnable to be executed asynchronously in the given ThreadPool.

But what does it mean CompletableFuture that is asynchronously completed by a task. How can the task make the CompletableFuture become completed... ? It doesn't make much sense to me.

Answer 1

Inside CompletableFuture there is the following code called by runAsync.

static CompletableFuture<Void> asyncRunStage(Executor e, Runnable f) {
    if (f == null) throw new NullPointerException();
    CompletableFuture<Void> d = new CompletableFuture<Void>();
    e.execute(new AsyncRun(d, f));
    return d;
}

AsyncRun is the asynchronously executed task that will, after running the Runnable f, complete the CompletableFuture d asynchronously. I won't bother with the code here because it's not very informative, and it just performs the completion of d by calling its postComplete() method (the package-private one).