Module java.base
Package java.util.concurrent

## Class Flow

- java.lang.Object
- - java.util.concurrent.Flow

-     public final class Flow
      extends Object

  Interrelated interfaces and static methods for establishing flow-controlled components in which `Publishers` produce items consumed by one or more `Subscribers`, each managed by a `Subscription`.

  These interfaces correspond to the reactive-streams specification. They apply in both concurrent and distributed asynchronous settings: All (seven) methods are defined in `void` "one-way" message style. Communication relies on a simple form of flow control (method `Flow.Subscription.request(long)`) that can be used to avoid resource management problems that may otherwise occur in "push" based systems.

  Examples. A `Flow.Publisher` usually defines its own `Flow.Subscription` implementation; constructing one in method `subscribe` and issuing it to the calling `Flow.Subscriber`. It publishes items to the subscriber asynchronously, normally using an `Executor`. For example, here is a very simple publisher that only issues (when requested) a single `TRUE` item to a single subscriber. Because the subscriber receives only a single item, this class does not use buffering and ordering control required in most implementations (for example `SubmissionPublisher`).

       class OneShotPublisher implements Publisher<Boolean> {
         private final ExecutorService executor = ForkJoinPool.commonPool(); // daemon-based
         private boolean subscribed; // true after first subscribe
         public synchronized void subscribe(Subscriber<? super Boolean> subscriber) {
           if (subscribed)
             subscriber.onError(new IllegalStateException()); // only one allowed
           else {
             subscribed = true;
             subscriber.onSubscribe(new OneShotSubscription(subscriber, executor));
           }
         }
         static class OneShotSubscription implements Subscription {
           private final Subscriber<? super Boolean> subscriber;
           private final ExecutorService executor;
           private Future<?> future; // to allow cancellation
           private boolean completed;
           OneShotSubscription(Subscriber<? super Boolean> subscriber,
                               ExecutorService executor) {
             this.subscriber = subscriber;
             this.executor = executor;
           }
           public synchronized void request(long n) {
             if (!completed) {
               completed = true;
               if (n <= 0) {
                 IllegalArgumentException ex = new IllegalArgumentException();
                 executor.execute(() -> subscriber.onError(ex));
               } else {
                 future = executor.submit(() -> {
                   subscriber.onNext(Boolean.TRUE);
                   subscriber.onComplete();
                 });
               }
             }
           }
           public synchronized void cancel() {
             completed = true;
             if (future != null) future.cancel(false);
           }
         }
       }

  A `Flow.Subscriber` arranges that items be requested and processed. Items (invocations of `Flow.Subscriber.onNext(T)`) are not issued unless requested, but multiple items may be requested. Many Subscriber implementations can arrange this in the style of the following example, where a buffer size of 1 single-steps, and larger sizes usually allow for more efficient overlapped processing with less communication; for example with a value of 64, this keeps total outstanding requests between 32 and 64. Because Subscriber method invocations for a given `Flow.Subscription` are strictly ordered, there is no need for these methods to use locks or volatiles unless a Subscriber maintains multiple Subscriptions (in which case it is better to instead define multiple Subscribers, each with its own Subscription).

       class SampleSubscriber<T> implements Subscriber<T> {
         final Consumer<? super T> consumer;
         Subscription subscription;
         final long bufferSize;
         long count;
         SampleSubscriber(long bufferSize, Consumer<? super T> consumer) {
           this.bufferSize = bufferSize;
           this.consumer = consumer;
         }
         public void onSubscribe(Subscription subscription) {
           long initialRequestSize = bufferSize;
           count = bufferSize - bufferSize / 2; // re-request when half consumed
           (this.subscription = subscription).request(initialRequestSize);
         }
         public void onNext(T item) {
           if (--count <= 0)
             subscription.request(count = bufferSize - bufferSize / 2);
           consumer.accept(item);
         }
         public void onError(Throwable ex) { ex.printStackTrace(); }
         public void onComplete() {}
       }

  The default value of `defaultBufferSize()` may provide a useful starting point for choosing request sizes and capacities in Flow components based on expected rates, resources, and usages. Or, when flow control is never needed, a subscriber may initially request an effectively unbounded number of items, as in:

       class UnboundedSubscriber<T> implements Subscriber<T> {
         public void onSubscribe(Subscription subscription) {
           subscription.request(Long.MAX_VALUE); // effectively unbounded
         }
         public void onNext(T item) { use(item); }
         public void onError(Throwable ex) { ex.printStackTrace(); }
         public void onComplete() {}
         void use(T item) { ... }
       }

  Since:
    9

- - ### Nested Class Summary

    Nested Classes
    Modifier and Type | Class | Description
    --- | --- | ---
    `static interface` | `Flow.Processor<T,​R>` | A component that acts as both a Subscriber and Publisher.
    `static interface` | `Flow.Publisher<T>` | A producer of items (and related control messages) received by Subscribers.
    `static interface` | `Flow.Subscriber<T>` | A receiver of messages.
    `static interface` | `Flow.Subscription` | Message control linking a `Flow.Publisher` and `Flow.Subscriber`.

  - ### Method Summary

    Modifier and Type | Method | Description
    --- | --- | ---
    `static int` | `defaultBufferSize()` | Returns a default value for Publisher or Subscriber buffering, that may be used in the absence of other constraints.

    - ### Methods declared in class java.lang.Object

      `clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

- - ### Method Detail

    - #### defaultBufferSize

          public static int defaultBufferSize()

      Returns a default value for Publisher or Subscriber buffering, that may be used in the absence of other constraints.
      Implementation Note:
        The current value returned is 256.
      Returns:
        the buffer size value