These interfaces correspond to the reactive-streams * specification. They apply in both concurrent and distributed * asynchronous settings: All (seven) methods are defined in {@code * void} "oneway" message style. Communication relies on a simple form * of flow control (method {@link Subscription#request}) that can be * used to avoid resource management problems that may otherwise occur * in "push" based systems. * *
Examples. A {@link Publisher} usually defines its own * {@link Subscription} implementation; constructing one in method * {@code subscribe} and issuing it to the calling {@link * Subscriber}. It publishes items to the subscriber asynchronously, * normally using an {@link Executor}. For example, here is a very * simple publisher that only issues (when requested) a single {@code * 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 {@link * SubmissionPublisher}). * *
{@code
* class OneShotPublisher implements Publisher {
* final ExecutorService executor = ForkJoinPool.commonPool(); // daemon-based
* boolean subscribed = false; // true after first subscribe
* public synchronized void subscribe(Subscriber subscriber) {
* if (subscribed)
* subscriber.onError(new IllegalStateException()); // only one allowed
* else {
* subscribed = true;
* subscriber.onSubscribe(new OneShotSubscription(subscriber, executor));
* }
* }
* static class OneShotSubscription implements Subscription {
* final Subscriber subscriber;
* final ExecutorService executor;
* Future future; // to allow cancellation
* boolean completed = false;
* OneShotSubscription(Subscriber subscriber,
* ExecutorService executor) {
* this.subscriber = subscriber;
* this.executor = executor;
* }
* public synchronized void request(long n) {
* if (n != 0 && !completed) {
* completed = true;
* future = executor.submit(() -> {
* if (n < 0)
* subscriber.onError(new IllegalArgumentException());
* else {
* subscriber.onNext(Boolean.TRUE);
* subscriber.onComplete();
* }});
* }
* }
* public synchronized void cancel() {
* completed = true;
* if (future != null) future.cancel(false);
* }
* }
* }}
*
* A {@link Subscriber} arranges that items be requested and * processed. Items (invocations of {@link Subscriber#onNext}) 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. (See also {@link * #consume(long, Publisher, Consumer)} that automates a common case.) * Because Subscriber method invocations for a given {@link * 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). * *
{@code
* class SampleSubscriber implements Subscriber {
* final Consumer consumer;
* Subscription subscription;
* final long bufferSize;
* long count;
* SampleSubscriber(long bufferSize, Consumer consumer) {
* this.bufferSize = bufferSize;
* this.consumer = consumer;
* }
* public void onSubscribe(Subscription subscription) {
* (this.subscription = subscription).request(bufferSize);
* count = bufferSize - bufferSize / 2; // re-request when half consumed
* }
* 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 {@link #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 known to be always inapplicable, a * subscriber may initially request an effectively unbounded number of * items, as in: * *
{@code
* class UnboundedSubscriber implements Subscriber {
* 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) { ... }
* }}
*
* @author Doug Lea
* @since 1.9
*/
public final class Flow {
private Flow() {} // uninstantiable
/**
* A producer of items (and related control messages) received by
* Subscribers. Each current {@link Subscriber} receives the same
* items (via method {@code onNext}) in the same order, unless
* drops or errors are encountered. If a Publisher encounters an
* error that does not allow items to be issued to a Subscriber,
* that Subscriber receives {@code onError}, and then receives no
* further messages. Otherwise, when it is known that no further
* messages will be issued to it, a subscriber receives {@code
* onComplete}. Publishers ensure that Subscriber method
* invocations for each subscription are strictly ordered in happens-before
* order.
*
* Publishers may vary in policy about whether drops (failures
* to issue an item because of resource limitations) are treated
* as unrecoverable errors. Publishers may also vary about
* whether Subscribers receive items that were produced or
* available before they subscribed.
*
* @param Typically, implementations of this method invoke {@code
* subscription.request} to enable receiving items.
*
* @param subscription a new subscription
*/
public void onSubscribe(Subscription subscription);
/**
* Method invoked with a Subscription's next item. If this
* method throws an exception, resulting behavior is not
* guaranteed, but may cause the Subscription to be cancelled.
*
* @param item the item
*/
public void onNext(T item);
/**
* Method invoked upon an unrecoverable error encountered by a
* Publisher or Subscription, after which no other Subscriber
* methods are invoked by the Subscription. If this method
* itself throws an exception, resulting behavior is
* undefined.
*
* @param throwable the exception
*/
public void onError(Throwable throwable);
/**
* Method invoked when it is known that no additional
* Subscriber method invocations will occur for a Subscription
* that is not already terminated by error, after which no
* other Subscriber methods are invoked by the Subscription.
* If this method throws an exception, resulting behavior is
* undefined.
*/
public void onComplete();
}
/**
* Message control linking a {@link Publisher} and {@link
* Subscriber}. Subscribers receive items only when requested,
* and may cancel at any time. The methods in this interface are
* intended to be invoked only by their Subscribers; usages in
* other contexts have undefined effects.
*/
public static interface Subscription {
/**
* Adds the given number {@code n} of items to the current
* unfulfilled demand for this subscription. If {@code n} is
* negative, the Subscriber will receive an {@code onError}
* signal with an {@link IllegalArgumentException} argument.
* Otherwise, the Subscriber will receive up to {@code n}
* additional {@code onNext} invocations (or fewer if
* terminated).
*
* @param n the increment of demand; a value of {@code
* Long.MAX_VALUE} may be considered as effectively unbounded
*/
public void request(long n);
/**
* Causes the Subscriber to (eventually) stop receiving
* messages. Implementation is best-effort -- additional
* messages may be received after invoking this method. A
* cancelled subscription need not ever receive an {@code
* onComplete} signal.
*/
public void cancel();
}
/**
* A component that acts as both a Subscriber and Publisher.
*
* @param Preliminary release note: Currently, this method
* collects all items before executing the stream
* computation. Improvements are pending Stream integration.
*
* @param