util/concurrent/Flow.java

/*
 * 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;

import java.util.ArrayList;
import java.util.function.Consumer;
import java.util.function.Function;
import java.util.stream.Stream;

/**
 * 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> {
 *   final ExecutorService executor = ForkJoinPool.commonPool(); // daemon-based
 *   boolean subscribed = false; // 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 {
 *     final Subscriber<? super Boolean> subscriber;
 *     final ExecutorService executor;
 *     Future<?> future; // to allow cancellation
 *     boolean completed = false;
 *     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) {
 *           IllegalStateException ex = new IllegalStateException();
 *           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.  (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).
 *
 * <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) {
 *     (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() {}
 * }}</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 known to be always inapplicable, 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 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 <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 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} 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> {
    }

    // Support for static methods

    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;
    }

    abstract static class CompletableSubscriber<T,U>
        implements Subscriber<T>, Consumer<T>
    {
        final CompletableFuture<U> status;
        Subscription subscription;
        long requestSize;
        long count;
        CompletableSubscriber(long bufferSize, CompletableFuture<U> status) {
            this.status = status;
            this.requestSize = bufferSize;
        }
        public final void onSubscribe(Subscription subscription) {
            (this.subscription = subscription).request(requestSize);
            count = requestSize -= (requestSize >>> 1);
        }
        public final void onError(Throwable ex) {
            status.completeExceptionally(ex);
        }
        public void onNext(T item) {
            try {
                if (--count <= 0)
                    subscription.request(count = requestSize);
                accept(item);
            } catch (Throwable ex) {
                status.completeExceptionally(ex);
            }
        }
    }

    static final class ConsumeSubscriber<T> extends CompletableSubscriber<T,Void> {
        final Consumer<? super T> consumer;
        ConsumeSubscriber(long bufferSize,
                          CompletableFuture<Void> status,
                          Consumer<? super T> consumer) {
            super(bufferSize, status);
            this.consumer = consumer;
        }
        public void accept(T item) { consumer.accept(item); }
        public void onComplete() { status.complete(null); }
    }

    /**
     * Creates and subscribes a Subscriber that consumes all items
     * from the given publisher using the given Consumer function, and
     * using the given bufferSize for buffering. Returns a
     * CompletableFuture that is completed normally when the publisher
     * signals {@code onComplete}, or completed exceptionally upon any
     * error, including an exception thrown by the Consumer (in which
     * case the subscription is cancelled if not already terminated).
     *
     * @param <T> the published item type
     * @param bufferSize the request size for subscriptions
     * @param publisher the publisher
     * @param consumer the function applied to each onNext item
     * @return a CompletableFuture that is completed normally
     * when the publisher signals onComplete, and exceptionally
     * upon any error
     * @throws NullPointerException if publisher or consumer are null
     * @throws IllegalArgumentException if bufferSize not positive
     */
    public static <T> CompletableFuture<Void> consume(
        long bufferSize, Publisher<T> publisher, Consumer<? super T> consumer) {
        if (bufferSize <= 0L)
            throw new IllegalArgumentException("bufferSize must be positive");
        if (publisher == null || consumer == null)
            throw new NullPointerException();
        CompletableFuture<Void> status = new CompletableFuture<>();
        publisher.subscribe(new ConsumeSubscriber<T>(
                                bufferSize, status, consumer));
        return status;
    }

    /**
     * Equivalent to {@link #consume(long, Publisher, Consumer)}
     * with {@link #defaultBufferSize}.
     *
     * @param <T> the published item type
     * @param publisher the publisher
     * @param consumer the function applied to each onNext item
     * @return a CompletableFuture that is completed normally
     * when the publisher signals onComplete, and exceptionally
     * upon any error
     * @throws NullPointerException if publisher or consumer are null
     */
    public static <T> CompletableFuture<Void> consume(
        Publisher<T> publisher, Consumer<? super T> consumer) {
        return consume(defaultBufferSize(), publisher, consumer);
    }

    /**
     * Temporary implementation for Stream, collecting all items
     * and then applying stream operation.
     */
    static final class StreamSubscriber<T,R> extends CompletableSubscriber<T,R> {
        final Function<? super Stream<T>, ? extends R> fn;
        final ArrayList<T> items;
        StreamSubscriber(long bufferSize,
                         CompletableFuture<R> status,
                         Function<? super Stream<T>, ? extends R> fn) {
            super(bufferSize, status);
            this.fn = fn;
            this.items = new ArrayList<T>();
        }
        public void accept(T item) { items.add(item); }
        public void onComplete() { status.complete(fn.apply(items.stream())); }
    }

    /**
     * Creates and subscribes a Subscriber that applies the given
     * stream operation to items, and uses the given bufferSize for
     * buffering. Returns a CompletableFuture that is completed
     * normally with the result of this function when the publisher
     * signals {@code onComplete}, or is completed exceptionally upon
     * any error.
     *
     * <p><b>Preliminary release note:</b> Currently, this method
     * collects all items before executing the stream
     * computation. Improvements are pending Stream integration.
     *
     * @param <T> the published item type
     * @param <R> the result type of the stream function
     * @param bufferSize the request size for subscriptions
     * @param publisher the publisher
     * @param streamFunction the operation on elements
     * @return a CompletableFuture that is completed normally with the
     * result of the given function as result when the publisher signals
     * onComplete, and exceptionally upon any error
     * @throws NullPointerException if publisher or function are null
     * @throws IllegalArgumentException if bufferSize not positive
     */
    public static <T,R> CompletableFuture<R> stream(
        long bufferSize, Publisher<T> publisher,
        Function<? super Stream<T>, ? extends R> streamFunction) {
        if (bufferSize <= 0L)
            throw new IllegalArgumentException("bufferSize must be positive");
        if (publisher == null || streamFunction == null)
            throw new NullPointerException();
        CompletableFuture<R> status = new CompletableFuture<>();
        publisher.subscribe(new StreamSubscriber<T,R>(
                                bufferSize, status, streamFunction));
        return status;
    }

    /**
     * Equivalent to {@link #stream(long, Publisher, Function)}
     * with {@link #defaultBufferSize}.
     *
     * @param <T> the published item type
     * @param <R> the result type of the stream function
     * @param publisher the publisher
     * @param streamFunction the operation on elements
     * @return a CompletableFuture that is completed normally with the
     * result of the given function as result when the publisher signals
     * onComplete, and exceptionally upon any error
     * @throws NullPointerException if publisher or function are null
     */
    public static <T,R> CompletableFuture<R> stream(
        Publisher<T> publisher,
        Function<? super Stream<T>,? extends R> streamFunction) {
        return stream(defaultBufferSize(), publisher, streamFunction);
    }
}
Revision:	1.22
Committed:	Sun Jan 25 15:19:40 2015 UTC (9 years, 4 months ago) by dl
Branch:	MAIN
Changes since 1.21:	+1 -1 lines
Log Message:	Minor improvements
#	User	Rev	Content
1	dl	1.1	/*
2			* Written by Doug Lea with assistance from members of JCP JSR-166
3			* Expert Group and released to the public domain, as explained at
4			* http://creativecommons.org/publicdomain/zero/1.0/
5			*/
6
7			package java.util.concurrent;
8	jsr166	1.9
9			import java.util.ArrayList;
10	dl	1.1	import java.util.function.Consumer;
11			import java.util.function.Function;
12			import java.util.stream.Stream;
13
14			/**
15			* Interrelated interfaces and static methods for establishing
16			* flow-controlled components in which {@link Publisher Publishers}
17			* produce items consumed by one or more {@link Subscriber
18			* Subscribers}, each managed by a {@link Subscription
19	dl	1.14	* Subscription}.
20	jsr166	1.2	*
21	dl	1.1	* <p>These interfaces correspond to the <a
22			* href="http://www.reactive-streams.org/"> reactive-streams</a>
23	dl	1.14	* specification. They apply in both concurrent and distributed
24			* asynchronous settings: All (seven) methods are defined in {@code
25	jsr166	1.20	* void} "one-way" message style. Communication relies on a simple form
26	dl	1.14	* of flow control (method {@link Subscription#request}) that can be
27			* used to avoid resource management problems that may otherwise occur
28			* in "push" based systems.
29	dl	1.1	*
30	dl	1.10	* <p><b>Examples.</b> A {@link Publisher} usually defines its own
31			* {@link Subscription} implementation; constructing one in method
32			* {@code subscribe} and issuing it to the calling {@link
33			* Subscriber}. It publishes items to the subscriber asynchronously,
34			* normally using an {@link Executor}. For example, here is a very
35			* simple publisher that only issues (when requested) a single {@code
36	dl	1.16	* TRUE} item to a single subscriber. Because the subscriber receives
37			* only a single item, this class does not use buffering and ordering
38			* control required in most implementations (for example {@link
39			* SubmissionPublisher}).
40	dl	1.10	*
41			* <pre> {@code
42			* class OneShotPublisher implements Publisher<Boolean> {
43	dl	1.17	* final ExecutorService executor = ForkJoinPool.commonPool(); // daemon-based
44	dl	1.16	* boolean subscribed = false; // true after first subscribe
45			* public synchronized void subscribe(Subscriber<? super Boolean> subscriber) {
46			* if (subscribed)
47			* subscriber.onError(new IllegalStateException()); // only one allowed
48			* else {
49			* subscribed = true;
50	dl	1.11	* subscriber.onSubscribe(new OneShotSubscription(subscriber, executor));
51	dl	1.16	* }
52	dl	1.10	* }
53			* static class OneShotSubscription implements Subscription {
54			* final Subscriber<? super Boolean> subscriber;
55	dl	1.17	* final ExecutorService executor;
56			* Future<?> future; // to allow cancellation
57	dl	1.16	* boolean completed = false;
58	dl	1.10	* OneShotSubscription(Subscriber<? super Boolean> subscriber,
59	dl	1.17	* ExecutorService executor) {
60	dl	1.10	* this.subscriber = subscriber;
61			* this.executor = executor;
62			* }
63			* public synchronized void request(long n) {
64	dl	1.16	* if (n != 0 && !completed) {
65	dl	1.10	* completed = true;
66	dl	1.21	* if (n < 0) {
67			* IllegalStateException ex = new IllegalStateException();
68	dl	1.22	* executor.execute(() -> subscriber.onError(ex));
69	dl	1.21	* }
70			* else {
71			* future = executor.submit(() -> {
72	dl	1.16	* subscriber.onNext(Boolean.TRUE);
73			* subscriber.onComplete();
74	dl	1.21	* });
75			* }
76	dl	1.10	* }
77			* }
78	dl	1.15	* public synchronized void cancel() {
79	dl	1.17	* completed = true;
80			* if (future != null) future.cancel(false);
81	dl	1.15	* }
82	dl	1.10	* }
83			* }}</pre>
84			*
85	jsr166	1.13	* <p>A {@link Subscriber} arranges that items be requested and
86	dl	1.10	* processed. Items (invocations of {@link Subscriber#onNext}) are
87			* not issued unless requested, but multiple items may be requested.
88			* Many Subscriber implementations can arrange this in the style of
89			* the following example, where a buffer size of 1 single-steps, and
90			* larger sizes usually allow for more efficient overlapped processing
91			* with less communication; for example with a value of 64, this keeps
92			* total outstanding requests between 32 and 64. (See also {@link
93			* #consume(long, Publisher, Consumer)} that automates a common case.)
94	dl	1.14	* Because Subscriber method invocations for a given {@link
95			* Subscription} are strictly ordered, there is no need for these
96			* methods to use locks or volatiles unless a Subscriber maintains
97			* multiple Subscriptions (in which case it is better to instead
98			* define multiple Subscribers, each with its own Subscription).
99	dl	1.10	*
100			* <pre> {@code
101			* class SampleSubscriber<T> implements Subscriber<T> {
102			* final Consumer<? super T> consumer;
103			* Subscription subscription;
104			* final long bufferSize;
105			* long count;
106			* SampleSubscriber(long bufferSize, Consumer<? super T> consumer) {
107			* this.bufferSize = bufferSize;
108			* this.consumer = consumer;
109			* }
110			* public void onSubscribe(Subscription subscription) {
111			* (this.subscription = subscription).request(bufferSize);
112			* count = bufferSize - bufferSize / 2; // re-request when half consumed
113			* }
114			* public void onNext(T item) {
115			* if (--count <= 0)
116			* subscription.request(count = bufferSize - bufferSize / 2);
117			* consumer.accept(item);
118			* }
119			* public void onError(Throwable ex) { ex.printStackTrace(); }
120			* public void onComplete() {}
121			* }}</pre>
122	dl	1.1	*
123	dl	1.18	* <p>The default value of {@link #defaultBufferSize} may provide a
124			* useful starting point for choosing request sizes and capacities in
125			* Flow components based on expected rates, resources, and usages.
126			* Or, when flow control is known to be always inapplicable, a
127			* subscriber may initially request an effectively unbounded number of
128			* items, as in:
129	dl	1.12	*
130			* <pre> {@code
131			* class UnboundedSubscriber<T> implements Subscriber<T> {
132			* public void onSubscribe(Subscription subscription) {
133			* subscription.request(Long.MAX_VALUE); // effectively unbounded
134			* }
135			* public void onNext(T item) { use(item); }
136			* public void onError(Throwable ex) { ex.printStackTrace(); }
137			* public void onComplete() {}
138			* void use(T item) { ... }
139			* }}</pre>
140	jsr166	1.13	*
141	dl	1.1	* @author Doug Lea
142			* @since 1.9
143			*/
144			public final class Flow {
145
146			private Flow() {} // uninstantiable
147
148			/**
149			* A producer of items (and related control messages) received by
150			* Subscribers. Each current {@link Subscriber} receives the same
151	dl	1.14	* items (via method {@code onNext}) in the same order, unless
152			* drops or errors are encountered. If a Publisher encounters an
153	dl	1.15	* error that does not allow items to be issued to a Subscriber,
154			* that Subscriber receives {@code onError}, and then receives no
155			* further messages. Otherwise, when it is known that no further
156			* messages will be issued to it, a subscriber receives {@code
157			* onComplete}. Publishers ensure that Subscriber method
158	dl	1.14	* invocations for each subscription are strictly ordered in <a
159			* href="package-summary.html#MemoryVisibility"><i>happens-before</i></a>
160			* order.
161			*
162	dl	1.15	* <p>Publishers may vary in policy about whether drops (failures
163			* to issue an item because of resource limitations) are treated
164			* as unrecoverable errors. Publishers may also vary about
165			* whether Subscribers receive items that were produced or
166			* available before they subscribed.
167	dl	1.1	*
168			* @param <T> the published item type
169			*/
170			@FunctionalInterface
171			public static interface Publisher<T> {
172			/**
173			* Adds the given Subscriber if possible. If already
174			* subscribed, or the attempt to subscribe fails due to policy
175	dl	1.14	* violations or errors, the Subscriber's {@code onError}
176			* method is invoked with an {@link IllegalStateException}.
177			* Otherwise, the Subscriber's {@code onSubscribe} method is
178			* invoked with a new {@link Subscription}. Subscribers may
179			* enable receiving items by invoking the {@code request}
180			* method of this Subscription, and may unsubscribe by
181			* invoking its {@code cancel} method.
182	dl	1.1	*
183			* @param subscriber the subscriber
184			* @throws NullPointerException if subscriber is null
185			*/
186			public void subscribe(Subscriber<? super T> subscriber);
187			}
188
189			/**
190	dl	1.14	* A receiver of messages. The methods in this interface are
191			* invoked in strict sequential order for each {@link
192			* Subscription}.
193	dl	1.1	*
194			* @param <T> the subscribed item type
195			*/
196			public static interface Subscriber<T> {
197			/**
198			* Method invoked prior to invoking any other Subscriber
199			* methods for the given Subscription. If this method throws
200			* an exception, resulting behavior is not guaranteed, but may
201			* cause the Subscription to be cancelled.
202			*
203	dl	1.15	* <p>Typically, implementations of this method invoke {@code
204			* subscription.request} to enable receiving items.
205	dl	1.1	*
206			* @param subscription a new subscription
207			*/
208			public void onSubscribe(Subscription subscription);
209
210			/**
211			* Method invoked with a Subscription's next item. If this
212			* method throws an exception, resulting behavior is not
213			* guaranteed, but may cause the Subscription to be cancelled.
214			*
215			* @param item the item
216			*/
217			public void onNext(T item);
218
219			/**
220			* Method invoked upon an unrecoverable error encountered by a
221			* Publisher or Subscription, after which no other Subscriber
222			* methods are invoked by the Subscription. If this method
223			* itself throws an exception, resulting behavior is
224			* undefined.
225			*
226			* @param throwable the exception
227			*/
228			public void onError(Throwable throwable);
229
230			/**
231	dl	1.15	* Method invoked when it is known that no additional
232			* Subscriber method invocations will occur for a Subscription
233			* that is not already terminated by error, after which no
234			* other Subscriber methods are invoked by the Subscription.
235			* If this method throws an exception, resulting behavior is
236	dl	1.14	* undefined.
237	dl	1.1	*/
238			public void onComplete();
239			}
240
241			/**
242	dl	1.14	* Message control linking a {@link Publisher} and {@link
243			* Subscriber}. Subscribers receive items only when requested,
244			* and may cancel at any time. The methods in this interface are
245			* intended to be invoked only by their Subscribers; usages in
246			* other contexts have undefined effects.
247	dl	1.1	*/
248			public static interface Subscription {
249			/**
250			* Adds the given number {@code n} of items to the current
251			* unfulfilled demand for this subscription. If {@code n} is
252	dl	1.14	* negative, the Subscriber will receive an {@code onError}
253	jsr166	1.19	* signal with an {@link IllegalArgumentException} argument.
254			* Otherwise, the Subscriber will receive up to {@code n}
255			* additional {@code onNext} invocations (or fewer if
256			* terminated).
257	dl	1.1	*
258			* @param n the increment of demand; a value of {@code
259			* Long.MAX_VALUE} may be considered as effectively unbounded
260			*/
261			public void request(long n);
262
263			/**
264	dl	1.14	* Causes the Subscriber to (eventually) stop receiving
265	dl	1.15	* messages. Implementation is best-effort -- additional
266			* messages may be received after invoking this method. A
267			* cancelled subscription need not ever receive an {@code
268			* onComplete} signal.
269	dl	1.1	*/
270			public void cancel();
271			}
272
273			/**
274	jsr166	1.2	* A component that acts as both a Subscriber and Publisher.
275	dl	1.1	*
276	jsr166	1.2	* @param <T> the subscribed item type
277			* @param <R> the published item type
278			*/
279	jsr166	1.5	public static interface Processor<T,R> extends Subscriber<T>, Publisher<R> {
280	dl	1.1	}
281
282			// Support for static methods
283
284	dl	1.18	static final int DEFAULT_BUFFER_SIZE = 256;
285
286			/**
287			* Returns a default value for Publisher or Subscriber buffering,
288			* that may be used in the absence of other constraints.
289			*
290			* @implNote
291			* The current value returned is 256.
292			*
293			* @return the buffer size value
294			*/
295			public static int defaultBufferSize() {
296			return DEFAULT_BUFFER_SIZE;
297			}
298	dl	1.1
299	jsr166	1.19	abstract static class CompletableSubscriber<T,U>
300			implements Subscriber<T>, Consumer<T>
301			{
302	dl	1.1	final CompletableFuture<U> status;
303			Subscription subscription;
304	dl	1.10	long requestSize;
305	jsr166	1.2	long count;
306	dl	1.10	CompletableSubscriber(long bufferSize, CompletableFuture<U> status) {
307	dl	1.1	this.status = status;
308	dl	1.10	this.requestSize = bufferSize;
309	dl	1.1	}
310			public final void onSubscribe(Subscription subscription) {
311			(this.subscription = subscription).request(requestSize);
312	dl	1.10	count = requestSize -= (requestSize >>> 1);
313	dl	1.1	}
314	jsr166	1.2	public final void onError(Throwable ex) {
315	dl	1.1	status.completeExceptionally(ex);
316			}
317			public void onNext(T item) {
318	dl	1.10	try {
319			if (--count <= 0)
320			subscription.request(count = requestSize);
321			accept(item);
322			} catch (Throwable ex) {
323			status.completeExceptionally(ex);
324	dl	1.1	}
325			}
326			}
327	jsr166	1.2
328	dl	1.1	static final class ConsumeSubscriber<T> extends CompletableSubscriber<T,Void> {
329			final Consumer<? super T> consumer;
330	dl	1.10	ConsumeSubscriber(long bufferSize,
331	dl	1.1	CompletableFuture<Void> status,
332			Consumer<? super T> consumer) {
333	dl	1.10	super(bufferSize, status);
334	dl	1.1	this.consumer = consumer;
335			}
336			public void accept(T item) { consumer.accept(item); }
337			public void onComplete() { status.complete(null); }
338			}
339
340			/**
341			* Creates and subscribes a Subscriber that consumes all items
342			* from the given publisher using the given Consumer function, and
343	dl	1.10	* using the given bufferSize for buffering. Returns a
344	dl	1.1	* CompletableFuture that is completed normally when the publisher
345	dl	1.14	* signals {@code onComplete}, or completed exceptionally upon any
346			* error, including an exception thrown by the Consumer (in which
347			* case the subscription is cancelled if not already terminated).
348	dl	1.1	*
349	jsr166	1.4	* @param <T> the published item type
350	dl	1.10	* @param bufferSize the request size for subscriptions
351	dl	1.1	* @param publisher the publisher
352			* @param consumer the function applied to each onNext item
353			* @return a CompletableFuture that is completed normally
354			* when the publisher signals onComplete, and exceptionally
355	jsr166	1.7	* upon any error
356	dl	1.1	* @throws NullPointerException if publisher or consumer are null
357	dl	1.10	* @throws IllegalArgumentException if bufferSize not positive
358	dl	1.1	*/
359			public static <T> CompletableFuture<Void> consume(
360	dl	1.10	long bufferSize, Publisher<T> publisher, Consumer<? super T> consumer) {
361			if (bufferSize <= 0L)
362			throw new IllegalArgumentException("bufferSize must be positive");
363	dl	1.1	if (publisher == null \|\| consumer == null)
364			throw new NullPointerException();
365			CompletableFuture<Void> status = new CompletableFuture<>();
366			publisher.subscribe(new ConsumeSubscriber<T>(
367	dl	1.10	bufferSize, status, consumer));
368	dl	1.1	return status;
369			}
370
371			/**
372			* Equivalent to {@link #consume(long, Publisher, Consumer)}
373	dl	1.18	* with {@link #defaultBufferSize}.
374	dl	1.1	*
375	jsr166	1.4	* @param <T> the published item type
376	dl	1.1	* @param publisher the publisher
377			* @param consumer the function applied to each onNext item
378			* @return a CompletableFuture that is completed normally
379			* when the publisher signals onComplete, and exceptionally
380	jsr166	1.7	* upon any error
381	dl	1.1	* @throws NullPointerException if publisher or consumer are null
382			*/
383			public static <T> CompletableFuture<Void> consume(
384			Publisher<T> publisher, Consumer<? super T> consumer) {
385	dl	1.18	return consume(defaultBufferSize(), publisher, consumer);
386	dl	1.1	}
387
388			/**
389			* Temporary implementation for Stream, collecting all items
390			* and then applying stream operation.
391			*/
392			static final class StreamSubscriber<T,R> extends CompletableSubscriber<T,R> {
393			final Function<? super Stream<T>, ? extends R> fn;
394			final ArrayList<T> items;
395	dl	1.10	StreamSubscriber(long bufferSize,
396	dl	1.1	CompletableFuture<R> status,
397			Function<? super Stream<T>, ? extends R> fn) {
398	dl	1.10	super(bufferSize, status);
399	dl	1.1	this.fn = fn;
400			this.items = new ArrayList<T>();
401			}
402			public void accept(T item) { items.add(item); }
403			public void onComplete() { status.complete(fn.apply(items.stream())); }
404			}
405
406			/**
407			* Creates and subscribes a Subscriber that applies the given
408	dl	1.10	* stream operation to items, and uses the given bufferSize for
409	dl	1.1	* buffering. Returns a CompletableFuture that is completed
410			* normally with the result of this function when the publisher
411	dl	1.14	* signals {@code onComplete}, or is completed exceptionally upon
412			* any error.
413	dl	1.1	*
414			* <p><b>Preliminary release note:</b> Currently, this method
415			* collects all items before executing the stream
416			* computation. Improvements are pending Stream integration.
417			*
418	jsr166	1.4	* @param <T> the published item type
419			* @param <R> the result type of the stream function
420	dl	1.10	* @param bufferSize the request size for subscriptions
421	dl	1.1	* @param publisher the publisher
422			* @param streamFunction the operation on elements
423			* @return a CompletableFuture that is completed normally with the
424			* result of the given function as result when the publisher signals
425	jsr166	1.7	* onComplete, and exceptionally upon any error
426	dl	1.1	* @throws NullPointerException if publisher or function are null
427	dl	1.10	* @throws IllegalArgumentException if bufferSize not positive
428	dl	1.1	*/
429			public static <T,R> CompletableFuture<R> stream(
430	dl	1.10	long bufferSize, Publisher<T> publisher,
431	dl	1.1	Function<? super Stream<T>, ? extends R> streamFunction) {
432	dl	1.10	if (bufferSize <= 0L)
433			throw new IllegalArgumentException("bufferSize must be positive");
434	dl	1.1	if (publisher == null \|\| streamFunction == null)
435			throw new NullPointerException();
436			CompletableFuture<R> status = new CompletableFuture<>();
437			publisher.subscribe(new StreamSubscriber<T,R>(
438	dl	1.10	bufferSize, status, streamFunction));
439	dl	1.1	return status;
440			}
441	jsr166	1.2
442	dl	1.1	/**
443			* Equivalent to {@link #stream(long, Publisher, Function)}
444	dl	1.18	* with {@link #defaultBufferSize}.
445	dl	1.1	*
446	jsr166	1.4	* @param <T> the published item type
447			* @param <R> the result type of the stream function
448	dl	1.1	* @param publisher the publisher
449			* @param streamFunction the operation on elements
450			* @return a CompletableFuture that is completed normally with the
451			* result of the given function as result when the publisher signals
452	jsr166	1.7	* onComplete, and exceptionally upon any error
453	dl	1.1	* @throws NullPointerException if publisher or function are null
454			*/
455			public static <T,R> CompletableFuture<R> stream(
456	jsr166	1.2	Publisher<T> publisher,
457	dl	1.1	Function<? super Stream<T>,? extends R> streamFunction) {
458	dl	1.18	return stream(defaultBufferSize(), publisher, streamFunction);
459	dl	1.1	}
460			}