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