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jsr166 |
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/* |
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* Written by Doug Lea with assistance from members of JCP JSR-166 |
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* Expert Group and released to the public domain, as explained at |
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* http://creativecommons.org/publicdomain/zero/1.0/ |
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*/ |
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package java.util.concurrent; |
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/** |
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* Interrelated interfaces and static methods for establishing |
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* flow-controlled components in which {@link Publisher Publishers} |
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* produce items consumed by one or more {@link Subscriber |
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* Subscribers}, each managed by a {@link Subscription |
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* Subscription}. |
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* |
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* <p>These interfaces correspond to the <a |
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* href="http://www.reactive-streams.org/"> reactive-streams</a> |
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* specification. They apply in both concurrent and distributed |
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* asynchronous settings: All (seven) methods are defined in {@code |
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* void} "one-way" message style. Communication relies on a simple form |
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* of flow control (method {@link Subscription#request}) that can be |
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* used to avoid resource management problems that may otherwise occur |
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* in "push" based systems. |
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* |
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* <p><b>Examples.</b> A {@link Publisher} usually defines its own |
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* {@link Subscription} implementation; constructing one in method |
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* {@code subscribe} and issuing it to the calling {@link |
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* Subscriber}. It publishes items to the subscriber asynchronously, |
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* normally using an {@link Executor}. For example, here is a very |
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* simple publisher that only issues (when requested) a single {@code |
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* TRUE} item to a single subscriber. Because the subscriber receives |
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* only a single item, this class does not use buffering and ordering |
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* control required in most implementations (for example {@link |
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* SubmissionPublisher}). |
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* |
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* <pre> {@code |
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* class OneShotPublisher implements Publisher<Boolean> { |
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* private final ExecutorService executor = ForkJoinPool.commonPool(); // daemon-based |
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* private boolean subscribed; // true after first subscribe |
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* public synchronized void subscribe(Subscriber<? super Boolean> subscriber) { |
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* if (subscribed) |
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* subscriber.onError(new IllegalStateException()); // only one allowed |
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* else { |
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* subscribed = true; |
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* subscriber.onSubscribe(new OneShotSubscription(subscriber, executor)); |
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* } |
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* } |
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* static class OneShotSubscription implements Subscription { |
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* private final Subscriber<? super Boolean> subscriber; |
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* private final ExecutorService executor; |
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* private Future<?> future; // to allow cancellation |
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* private boolean completed; |
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* OneShotSubscription(Subscriber<? super Boolean> subscriber, |
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* ExecutorService executor) { |
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* this.subscriber = subscriber; |
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* this.executor = executor; |
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* } |
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* public synchronized void request(long n) { |
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* if (n != 0 && !completed) { |
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* completed = true; |
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* if (n < 0) { |
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* IllegalArgumentException ex = new IllegalArgumentException(); |
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* executor.execute(() -> subscriber.onError(ex)); |
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* } else { |
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* future = executor.submit(() -> { |
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* subscriber.onNext(Boolean.TRUE); |
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* subscriber.onComplete(); |
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* }); |
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* } |
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* } |
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* } |
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* public synchronized void cancel() { |
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* completed = true; |
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* if (future != null) future.cancel(false); |
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* } |
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* } |
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* }}</pre> |
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* |
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* <p>A {@link Subscriber} arranges that items be requested and |
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* processed. Items (invocations of {@link Subscriber#onNext}) are |
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* not issued unless requested, but multiple items may be requested. |
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* Many Subscriber implementations can arrange this in the style of |
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* the following example, where a buffer size of 1 single-steps, and |
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* larger sizes usually allow for more efficient overlapped processing |
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* with less communication; for example with a value of 64, this keeps |
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* total outstanding requests between 32 and 64. |
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* Because Subscriber method invocations for a given {@link |
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* Subscription} are strictly ordered, there is no need for these |
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* methods to use locks or volatiles unless a Subscriber maintains |
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* multiple Subscriptions (in which case it is better to instead |
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* define multiple Subscribers, each with its own Subscription). |
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* |
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* <pre> {@code |
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* class SampleSubscriber<T> implements Subscriber<T> { |
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* final Consumer<? super T> consumer; |
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* Subscription subscription; |
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* final long bufferSize; |
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* long count; |
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* SampleSubscriber(long bufferSize, Consumer<? super T> consumer) { |
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* this.bufferSize = bufferSize; |
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* this.consumer = consumer; |
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* } |
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* public void onSubscribe(Subscription subscription) { |
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* long initialRequestSize = bufferSize; |
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* count = bufferSize - bufferSize / 2; // re-request when half consumed |
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* (this.subscription = subscription).request(initialRequestSize); |
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* } |
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* public void onNext(T item) { |
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* if (--count <= 0) |
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* subscription.request(count = bufferSize - bufferSize / 2); |
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* consumer.accept(item); |
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* } |
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* public void onError(Throwable ex) { ex.printStackTrace(); } |
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* public void onComplete() {} |
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* }}</pre> |
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* |
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* <p>The default value of {@link #defaultBufferSize} may provide a |
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* useful starting point for choosing request sizes and capacities in |
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* Flow components based on expected rates, resources, and usages. |
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* Or, when flow control is never needed, a subscriber may initially |
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* request an effectively unbounded number of items, as in: |
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* |
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* <pre> {@code |
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* class UnboundedSubscriber<T> implements Subscriber<T> { |
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* public void onSubscribe(Subscription subscription) { |
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* subscription.request(Long.MAX_VALUE); // effectively unbounded |
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* } |
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* public void onNext(T item) { use(item); } |
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* public void onError(Throwable ex) { ex.printStackTrace(); } |
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* public void onComplete() {} |
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* void use(T item) { ... } |
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* }}</pre> |
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* |
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* @author Doug Lea |
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* @since 9 |
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*/ |
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public final class Flow { |
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private Flow() {} // uninstantiable |
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/** |
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* A producer of items (and related control messages) received by |
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* Subscribers. Each current {@link Subscriber} receives the same |
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* items (via method {@code onNext}) in the same order, unless |
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* drops or errors are encountered. If a Publisher encounters an |
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* error that does not allow items to be issued to a Subscriber, |
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* that Subscriber receives {@code onError}, and then receives no |
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* further messages. Otherwise, when it is known that no further |
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* messages will be issued to it, a subscriber receives {@code |
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* onComplete}. Publishers ensure that Subscriber method |
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* invocations for each subscription are strictly ordered in <a |
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* href="package-summary.html#MemoryVisibility"><i>happens-before</i></a> |
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* order. |
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* |
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* <p>Publishers may vary in policy about whether drops (failures |
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* to issue an item because of resource limitations) are treated |
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* as unrecoverable errors. Publishers may also vary about |
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* whether Subscribers receive items that were produced or |
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* available before they subscribed. |
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* |
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* @param <T> the published item type |
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*/ |
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@FunctionalInterface |
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public static interface Publisher<T> { |
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/** |
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* Adds the given Subscriber if possible. If already |
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* subscribed, or the attempt to subscribe fails due to policy |
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* violations or errors, the Subscriber's {@code onError} |
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* method is invoked with an {@link IllegalStateException}. |
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* Otherwise, the Subscriber's {@code onSubscribe} method is |
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* invoked with a new {@link Subscription}. Subscribers may |
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* enable receiving items by invoking the {@code request} |
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* method of this Subscription, and may unsubscribe by |
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* invoking its {@code cancel} method. |
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* |
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* @param subscriber the subscriber |
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* @throws NullPointerException if subscriber is null |
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*/ |
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public void subscribe(Subscriber<? super T> subscriber); |
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} |
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/** |
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* A receiver of messages. The methods in this interface are |
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* invoked in strict sequential order for each {@link |
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* Subscription}. |
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* |
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* @param <T> the subscribed item type |
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*/ |
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public static interface Subscriber<T> { |
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/** |
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* Method invoked prior to invoking any other Subscriber |
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* methods for the given Subscription. If this method throws |
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* an exception, resulting behavior is not guaranteed, but may |
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* cause the Subscription not to be established or to be cancelled. |
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* |
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* <p>Typically, implementations of this method invoke {@code |
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* subscription.request} to enable receiving items. |
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* |
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* @param subscription a new subscription |
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*/ |
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public void onSubscribe(Subscription subscription); |
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/** |
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* Method invoked with a Subscription's next item. If this |
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* method throws an exception, resulting behavior is not |
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* guaranteed, but may cause the Subscription to be cancelled. |
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* |
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* @param item the item |
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*/ |
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public void onNext(T item); |
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/** |
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* Method invoked upon an unrecoverable error encountered by a |
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* Publisher or Subscription, after which no other Subscriber |
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* methods are invoked by the Subscription. If this method |
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* itself throws an exception, resulting behavior is |
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* undefined. |
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* |
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* @param throwable the exception |
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*/ |
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public void onError(Throwable throwable); |
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/** |
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* Method invoked when it is known that no additional |
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* Subscriber method invocations will occur for a Subscription |
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* that is not already terminated by error, after which no |
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* other Subscriber methods are invoked by the Subscription. |
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* If this method throws an exception, resulting behavior is |
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* undefined. |
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*/ |
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public void onComplete(); |
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} |
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/** |
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* Message control linking a {@link Publisher} and {@link |
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* Subscriber}. Subscribers receive items only when requested, |
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* and may cancel at any time. The methods in this interface are |
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* intended to be invoked only by their Subscribers; usages in |
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* other contexts have undefined effects. |
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*/ |
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public static interface Subscription { |
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/** |
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* Adds the given number {@code n} of items to the current |
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* unfulfilled demand for this subscription. If {@code n} is |
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* negative, the Subscriber will receive an {@code onError} |
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* signal with an {@link IllegalArgumentException} argument. |
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* Otherwise, the Subscriber will receive up to {@code n} |
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* additional {@code onNext} invocations (or fewer if |
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* terminated). |
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* |
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* @param n the increment of demand; a value of {@code |
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* Long.MAX_VALUE} may be considered as effectively unbounded |
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*/ |
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public void request(long n); |
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/** |
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* Causes the Subscriber to (eventually) stop receiving |
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* messages. Implementation is best-effort -- additional |
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* messages may be received after invoking this method. |
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* A cancelled subscription need not ever receive an |
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* {@code onComplete} or {@code onError} signal. |
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*/ |
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public void cancel(); |
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} |
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/** |
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* A component that acts as both a Subscriber and Publisher. |
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* |
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* @param <T> the subscribed item type |
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* @param <R> the published item type |
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*/ |
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public static interface Processor<T,R> extends Subscriber<T>, Publisher<R> { |
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} |
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static final int DEFAULT_BUFFER_SIZE = 256; |
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/** |
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* Returns a default value for Publisher or Subscriber buffering, |
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* that may be used in the absence of other constraints. |
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* |
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* @implNote |
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* The current value returned is 256. |
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* |
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* @return the buffer size value |
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*/ |
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public static int defaultBufferSize() { |
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return DEFAULT_BUFFER_SIZE; |
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} |
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} |