Understanding CompletableFuture::runAsync

completable-future, java, java-8, multithreading

Solution

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).

Problem

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.

Original source