/*
* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
*
* This code is free software; you can redistribute it and/or modify it
* under the terms of the GNU General Public License version 2 only, as
* published by the Free Software Foundation. Oracle designates this
* particular file as subject to the "Classpath" exception as provided
* by Oracle in the LICENSE file that accompanied this code.
*
* This code is distributed in the hope that it will be useful, but WITHOUT
* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
* version 2 for more details (a copy is included in the LICENSE file that
* accompanied this code).
*
* You should have received a copy of the GNU General Public License version
* 2 along with this work; if not, write to the Free Software Foundation,
* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
*
* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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/*
* This file is available under and governed by the GNU General Public
* License version 2 only, as published by the Free Software Foundation.
* However, the following notice accompanied the original version of this
* file:
*
* Written by Doug Lea with assistance from members of JCP JSR-166
* Expert Group and released to the public domain, as explained at
* http://creativecommons.org/publicdomain/zero/1.0/
*/
package java.util.concurrent;
/**
* Interrelated interfaces and static methods for establishing
* flow-controlled components in which {@link Publisher Publishers}
* produce items consumed by one or more {@link Subscriber
* Subscribers}, each managed by a {@link Subscription
* Subscription}.
*
* <p>These interfaces correspond to the <a
* href="http://www.reactive-streams.org/"> reactive-streams</a>
* specification. They apply in both concurrent and distributed
* asynchronous settings: All (seven) methods are defined in {@code
* void} "one-way" 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.
*
* <p><b>Examples.</b> 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}).
*
* <pre> {@code
* 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 (n != 0 && !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);
* }
* }
* }}</pre>
*
* <p>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.
* 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).
*
* <pre> {@code
* 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() {}
* }}</pre>
*
* <p>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 never needed, a subscriber may initially
* request an effectively unbounded number of items, as in:
*
* <pre> {@code
* 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) { ... }
* }}</pre>
*
* @author Doug Lea
* @since 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 <a
* href="package-summary.html#MemoryVisibility"><i>happens-before</i></a>
* order.
*
* <p>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 <T> the published item type
*/
@FunctionalInterface
public static interface Publisher<T> {
/**
* Adds the given Subscriber if possible. If already
* subscribed, or the attempt to subscribe fails due to policy
* violations or errors, the Subscriber's {@code onError}
* method is invoked with an {@link IllegalStateException}.
* Otherwise, the Subscriber's {@code onSubscribe} method is
* invoked with a new {@link Subscription}. Subscribers may
* enable receiving items by invoking the {@code request}
* method of this Subscription, and may unsubscribe by
* invoking its {@code cancel} method.
*
* @param subscriber the subscriber
* @throws NullPointerException if subscriber is null
*/
public void subscribe(Subscriber<? super T> subscriber);
}
/**
* A receiver of messages. The methods in this interface are
* invoked in strict sequential order for each {@link
* Subscription}.
*
* @param <T> the subscribed item type
*/
public static interface Subscriber<T> {
/**
* Method invoked prior to invoking any other Subscriber
* methods for the given Subscription. If this method throws
* an exception, resulting behavior is not guaranteed, but may
* cause the Subscription not to be established or to be cancelled.
*
* <p>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} or {@code onError} signal.
*/
public void cancel();
}
/**
* A component that acts as both a Subscriber and Publisher.
*
* @param <T> the subscribed item type
* @param <R> the published item type
*/
public static interface Processor<T,R> extends Subscriber<T>, Publisher<R> {
}
static final int DEFAULT_BUFFER_SIZE = 256;
/**
* Returns a default value for Publisher or Subscriber buffering,
* that may be used in the absence of other constraints.
*
* @implNote
* The current value returned is 256.
*
* @return the buffer size value
*/
public static int defaultBufferSize() {
return DEFAULT_BUFFER_SIZE;
}
}