Flow.java

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 * 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.
 *
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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 (!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
 */
/*
 * 响应式编程接口，完成【生产-消费】/【发布-订阅】/【推送-接收】模式的编程
 *
 * 注：以下的发布统称为生产，订阅统称为消费
 *
 * Publisher    -【生产】消息和【控制】信号，以下简称【生产者】
 * Subscriber   -【消费】消息和信号，以下简称【消费者】
 * Subscription -【链接】Publisher和Subscriber，以下简称【中介】
 * Processor    - 执行一些高级操作，如【转换】Publisher到Subscriber
 */
public final class Flow {
    static final int DEFAULT_BUFFER_SIZE = 256;
    
    private Flow() {
        // uninstantiable
    }
    
    /**
     * Returns a default value for Publisher or Subscriber buffering,
     * that may be used in the absence of other constraints.
     *
     * @return the buffer size value
     *
     * @implNote The current value returned is 256.
     */
    public static int defaultBufferSize() {
        return DEFAULT_BUFFER_SIZE;
    }
    
    
    
    /**
     * 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.
     *
     * 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
     */
    // 生产者，【生产】消息和【控制】信号
    @FunctionalInterface
    public 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
         */
        // 注册消费者
        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
     */
    // 消费者，【消费】消息和信号
    public 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
         */
        /*
         * 中介已准备就绪时的回调（中介刚刚设置了OPEN标记）
         * 一般在这里需要拿到中介的引用，进而向生产者发出消费请求
         */
        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
         */
        /*
         * 消费者开始消费指定的消息(item)
         * 一般在这里消费完成后，需要继续向生产者发出消费请求
         */
        void onNext(T item);
        
        /**
         * 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.
         */
        // （从中介内部调用）如果消费者在消费的时候，消息队列已经消费完了，且中介已被关闭时，回调此方法
        void onComplete();
        
        /**
         * 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
         */
        // （从中介内部调用）如果消费者在注册时（onSubscribe()内部）发生了异常，或者在消费中（onNext()内部）发生了异常，回调此方法
        void onError(Throwable throwable);
    }
    
    /**
     * 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.
     */
    // 中介，内部包含注册的消费者，自身注册在生产者内。用于连接生产者与消费者，为它们提供通讯渠道
    public interface Subscription {
        /**
         * Adds the given number {@code n} of items to the current
         * unfulfilled demand for this subscription.  If {@code n} is
         * less than or equal to zero, 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
         */
        // （消费者通过中介发出）消费请求
        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.
         */
        // （尽力）使消费者停止接收消息
        void cancel();
    }
    
    /**
     * A component that acts as both a Subscriber and Publisher.
     *
     * @param <T> the subscribed item type
     * @param <R> the published item type
     */
    // 执行一些高级操作，如【转换】Publisher到Subscriber
    public interface Processor<T, R> extends Subscriber<T>, Publisher<R> {
    }
}