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
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 |
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