A Flow.Publisher that asynchronously issues submitted (non-null) items to current subscribers until it is closed. Each current subscriber receives newly submitted items in the same order unless drops or exceptions are encountered. Using a SubmissionPublisher allows item generators to act as compliant reactive-streams Publishers relying on drop handling and/or blocking for flow control.

A SubmissionPublisher uses the Executor supplied in its constructor for delivery to subscribers. The best choice of Executor depends on expected usage. If the generator(s) of submitted items run in separate threads, and the number of subscribers can be estimated, consider using a Executors#newFixedThreadPool. Otherwise consider using the default, normally the ForkJoinPool#commonPool.

Buffering allows producers and consumers to transiently operate at different rates. Each subscriber uses an independent buffer. Buffers are created upon first use and expanded as needed up to the given maximum. (The enforced capacity may be rounded up to the nearest power of two and/or bounded by the largest value supported by this implementation.) Invocations of request do not directly result in buffer expansion, but risk saturation if unfilled requests exceed the maximum capacity. The default value of Flow#defaultBufferSize() may provide a useful starting point for choosing a capacity based on expected rates, resources, and usages.

A single SubmissionPublisher may be shared among multiple sources. Actions in a source thread prior to publishing an item or issuing a signal happen-before actions subsequent to the corresponding access by each subscriber. But reported estimates of lag and demand are designed for use in monitoring, not for synchronization control, and may reflect stale or inaccurate views of progress.

Publication methods support different policies about what to do when buffers are saturated. Method submit blocks until resources are available. This is simplest, but least responsive. The offer methods may drop items (either immediately or with bounded timeout), but provide an opportunity to interpose a handler and then retry.

If any Subscriber method throws an exception, its subscription is cancelled. If a handler is supplied as a constructor argument, it is invoked before cancellation upon an exception in method onNext, but exceptions in methods onSubscribe, onError and onComplete are not recorded or handled before cancellation. If the supplied Executor throws RejectedExecutionException (or any other RuntimeException or Error) when attempting to execute a task, or a drop handler throws an exception when processing a dropped item, then the exception is rethrown. In these cases, not all subscribers will have been issued the published item. It is usually good practice to closeExceptionally in these cases.

Method consume(Consumer) simplifies support for a common case in which the only action of a subscriber is to request and process all items using a supplied function.

This class may also serve as a convenient base for subclasses that generate items, and use the methods in this class to publish them. For example here is a class that periodically publishes the items generated from a supplier. (In practice you might add methods to independently start and stop generation, to share Executors among publishers, and so on, or use a SubmissionPublisher as a component rather than a superclass.)

 class PeriodicPublisher<T> extends SubmissionPublisher<T> {
  final ScheduledFuture<?> periodicTask;
  final ScheduledExecutorService scheduler;
  PeriodicPublisher(Executor executor, int maxBufferCapacity,
                    Supplier<? extends T> supplier,
                    long period, TimeUnit unit) {
    super(executor, maxBufferCapacity);
    scheduler = new ScheduledThreadPoolExecutor(1);
    periodicTask = scheduler.scheduleAtFixedRate(
      () -> submit(supplier.get()), 0, period, unit);
  }
  public void close() {
    periodicTask.cancel(false);
    scheduler.shutdown();
    super.close();
  }
}

Here is an example of a Flow.Processor implementation. It uses single-step requests to its publisher for simplicity of illustration. A more adaptive version could monitor flow using the lag estimate returned from submit, along with other utility methods.

 class TransformProcessor<S,T> extends SubmissionPublisher<T>
  implements Flow.Processor<S,T> {
  final Function<? super S, ? extends T> function;
  Flow.Subscription subscription;
  TransformProcessor(Executor executor, int maxBufferCapacity,
                     Function<? super S, ? extends T> function) {
    super(executor, maxBufferCapacity);
    this.function = function;
  }
  public void onSubscribe(Flow.Subscription subscription) {
    (this.subscription = subscription).request(1);
  }
  public void onNext(S item) {
    subscription.request(1);
    submit(function.apply(item));
  }
  public void onError(Throwable ex) { closeExceptionally(ex); }
  public void onComplete() { close(); }
}