[concurrency-interest] Soliciting suggestions for CompletableFuture utilities

[Doug Lea]

[Changing subject header]

On 08/31/2014 03:14 AM, Kasper Nielsen wrote:
> Sorry for hijacking this thread. But if additional functionality for
> CompletableFuture is on the table. I have a couple of requests, some of them
> just syntactically sugar.

Yes, you were one step ahead of me! As mentioned, there are a few
clear candidates (delays, IO), but it is a good time to solicit
further suggestions and discussion. I'll wait for these to settle
before proposing concrete additions.

-Doug


>
> 1)
> A simple way to create an immutable CompletionStage from a CompletionFuture.
> Right now if I have something like like this
> class ProvidedToUsers {
>      CompletionFuture f;
>      CompletionStage doAsyncStuff() {
>          return f;
>      }
> }
> Any client can just do:
> xx.doAsyncStuff().toCompletionFuture().cancel();
>
> Right now I'm using this to make an "immutable" future:
>
> CompletableFuture<Void> cf = new CompletableFuture<>();
> f.handle((a, b) -> {
>      if (b == null) {
>          cf.complete(null);
>      } else {
>          cf.completeExceptionally(b);
>      }
>      return null;
> });
> return cf;
>
>
> 2)
> An easy way to create a CompletableFuture that is exceptionally completed:
>
> return CompletableFuture.completedExceptionally(throwable);
>
> is better than:
>
> CompletableFuture<T> cf = new CompletableFuture<>();
> cf.completeExceptionally(throwable);
> return cf;
>
> 3)
> An easy way to get the exceptional cause from a CompletableFuture, right now I'm
> using this
>
> if (cf.isCompletedExceptionally()) {
>      try {
>          cf.getNow(null);
>      } catch (CancellationException e) {
>          return e;
>      } catch (CompletionException e) {
>          return e.getCause();
>      }
> }
> return null;
>
> 4)
> With regards to async-io completions I'm using the following piece of code to
> make completable futures that timeout at some point.
>
> public static <T> CompletableFuture<T> timeout(CompletableFuture<T> delegate,
> ScheduledExecutorService ses,
>          long timeout, TimeUnit unit) {
>      CompletableFuture<T> cf = new CompletableFuture<>();
>      Future<?> f;
>      try {
>          f = ses.schedule(new Runnable() {
>              public void run() {
>                  if (!delegate.isDone()) {
>                      cf.completeExceptionally(new TimeoutException("Timed out
> after " + timeout + " "
>                              + unit.toString().toLowerCase()));
>                  }
>              }
>          }, timeout, unit);
>      } catch (RejectedExecutionException e) {
>          // Unfortunately TimeoutException does not allow exceptions in its
> constructor
>          cf.completeExceptionally(new RuntimeException("Could not schedule
> timeout task, ", e));
>          return cf;
>      }
>      delegate.handle(new BiFunction<T, Throwable, Void>() {
>          public Void apply(T t, Throwable throwable) {
>              // Users must manually purge if many outstanding tasks
>              f.cancel(false);
>              if (throwable != null) {
>                  cf.completeExceptionally(throwable);
>              } else {
>                  cf.complete(t);
>              }
>              return null;
>          }
>      });
>      return cf;
> }
>
> Something similar might be nice to add to CompletableFuture.
>
> 5)
> Not related to CompletableFuture, but could CancellationException and
> TimeoutException get constructors that also takes a
> Throwable.
>
> Cheers
>    Kasper
>
> On Sat, Aug 30, 2014 at 4:51 PM, Doug Lea <dl at cs.oswego.edu
> <mailto:dl at cs.oswego.edu>> wrote:
>
>
>     It is easy to define utilities automating common use cases, including
>     simple periodic suppliers (via a common ScheduledExecutor), and
>     async-io completions (via adaptors from nio CompletionHandler).
>
>     -Doug
>
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