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
#	Content
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
9	import java.util.ArrayList;
10	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	* Subscription}.
20	*
21	* <p>These interfaces correspond to the <a
22	* href="http://www.reactive-streams.org/"> reactive-streams</a>
23	* specification. They apply in both concurrent and distributed
24	* asynchronous settings: All (seven) methods are defined in {@code
25	* void} "one-way" message style. Communication relies on a simple form
26	* 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	*
30	* <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	* 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	*
41	* <pre> {@code
42	* class OneShotPublisher implements Publisher<Boolean> {
43	* final ExecutorService executor = ForkJoinPool.commonPool(); // daemon-based
44	* 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	* subscriber.onSubscribe(new OneShotSubscription(subscriber, executor));
51	* }
52	* }
53	* static class OneShotSubscription implements Subscription {
54	* final Subscriber<? super Boolean> subscriber;
55	* final ExecutorService executor;
56	* Future<?> future; // to allow cancellation
57	* boolean completed = false;
58	* OneShotSubscription(Subscriber<? super Boolean> subscriber,
59	* ExecutorService executor) {
60	* this.subscriber = subscriber;
61	* this.executor = executor;
62	* }
63	* public synchronized void request(long n) {
64	* if (n != 0 && !completed) {
65	* completed = true;
66	* future = executor.submit(() -> {
67	* if (n < 0)
68	* subscriber.onError(new IllegalArgumentException());
69	* else {
70	* subscriber.onNext(Boolean.TRUE);
71	* subscriber.onComplete();
72	* }});
73	* }
74	* }
75	* public synchronized void cancel() {
76	* completed = true;
77	* if (future != null) future.cancel(false);
78	* }
79	* }
80	* }}</pre>
81	*
82	* <p>A {@link Subscriber} arranges that items be requested and
83	* 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	* 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	*
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	*
120	* <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	*
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	*
138	* @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	* items (via method {@code onNext}) in the same order, unless
149	* drops or errors are encountered. If a Publisher encounters an
150	* 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	* invocations for each subscription are strictly ordered in <a
156	* href="package-summary.html#MemoryVisibility"><i>happens-before</i></a>
157	* order.
158	*
159	* <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	*
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	* 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	*
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	* A receiver of messages. The methods in this interface are
188	* invoked in strict sequential order for each {@link
189	* Subscription}.
190	*
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	* <p>Typically, implementations of this method invoke {@code
201	* subscription.request} to enable receiving items.
202	*
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	* 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	* undefined.
234	*/
235	public void onComplete();
236	}
237
238	/**
239	* 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	*/
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	* negative, the Subscriber will receive an {@code onError}
250	* 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	*
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	* Causes the Subscriber to (eventually) stop receiving
262	* 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	*/
267	public void cancel();
268	}
269
270	/**
271	* A component that acts as both a Subscriber and Publisher.
272	*
273	* @param <T> the subscribed item type
274	* @param <R> the published item type
275	*/
276	public static interface Processor<T,R> extends Subscriber<T>, Publisher<R> {
277	}
278
279	// Support for static methods
280
281	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
296	abstract static class CompletableSubscriber<T,U>
297	implements Subscriber<T>, Consumer<T>
298	{
299	final CompletableFuture<U> status;
300	Subscription subscription;
301	long requestSize;
302	long count;
303	CompletableSubscriber(long bufferSize, CompletableFuture<U> status) {
304	this.status = status;
305	this.requestSize = bufferSize;
306	}
307	public final void onSubscribe(Subscription subscription) {
308	(this.subscription = subscription).request(requestSize);
309	count = requestSize -= (requestSize >>> 1);
310	}
311	public final void onError(Throwable ex) {
312	status.completeExceptionally(ex);
313	}
314	public void onNext(T item) {
315	try {
316	if (--count <= 0)
317	subscription.request(count = requestSize);
318	accept(item);
319	} catch (Throwable ex) {
320	status.completeExceptionally(ex);
321	}
322	}
323	}
324
325	static final class ConsumeSubscriber<T> extends CompletableSubscriber<T,Void> {
326	final Consumer<? super T> consumer;
327	ConsumeSubscriber(long bufferSize,
328	CompletableFuture<Void> status,
329	Consumer<? super T> consumer) {
330	super(bufferSize, status);
331	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	* using the given bufferSize for buffering. Returns a
341	* CompletableFuture that is completed normally when the publisher
342	* 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	*
346	* @param <T> the published item type
347	* @param bufferSize the request size for subscriptions
348	* @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	* upon any error
353	* @throws NullPointerException if publisher or consumer are null
354	* @throws IllegalArgumentException if bufferSize not positive
355	*/
356	public static <T> CompletableFuture<Void> consume(
357	long bufferSize, Publisher<T> publisher, Consumer<? super T> consumer) {
358	if (bufferSize <= 0L)
359	throw new IllegalArgumentException("bufferSize must be positive");
360	if (publisher == null \|\| consumer == null)
361	throw new NullPointerException();
362	CompletableFuture<Void> status = new CompletableFuture<>();
363	publisher.subscribe(new ConsumeSubscriber<T>(
364	bufferSize, status, consumer));
365	return status;
366	}
367
368	/**
369	* Equivalent to {@link #consume(long, Publisher, Consumer)}
370	* with {@link #defaultBufferSize}.
371	*
372	* @param <T> the published item type
373	* @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	* upon any error
378	* @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	return consume(defaultBufferSize(), publisher, consumer);
383	}
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	StreamSubscriber(long bufferSize,
393	CompletableFuture<R> status,
394	Function<? super Stream<T>, ? extends R> fn) {
395	super(bufferSize, status);
396	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	* stream operation to items, and uses the given bufferSize for
406	* buffering. Returns a CompletableFuture that is completed
407	* normally with the result of this function when the publisher
408	* signals {@code onComplete}, or is completed exceptionally upon
409	* any error.
410	*
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	* @param <T> the published item type
416	* @param <R> the result type of the stream function
417	* @param bufferSize the request size for subscriptions
418	* @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	* onComplete, and exceptionally upon any error
423	* @throws NullPointerException if publisher or function are null
424	* @throws IllegalArgumentException if bufferSize not positive
425	*/
426	public static <T,R> CompletableFuture<R> stream(
427	long bufferSize, Publisher<T> publisher,
428	Function<? super Stream<T>, ? extends R> streamFunction) {
429	if (bufferSize <= 0L)
430	throw new IllegalArgumentException("bufferSize must be positive");
431	if (publisher == null \|\| streamFunction == null)
432	throw new NullPointerException();
433	CompletableFuture<R> status = new CompletableFuture<>();
434	publisher.subscribe(new StreamSubscriber<T,R>(
435	bufferSize, status, streamFunction));
436	return status;
437	}
438
439	/**
440	* Equivalent to {@link #stream(long, Publisher, Function)}
441	* with {@link #defaultBufferSize}.
442	*
443	* @param <T> the published item type
444	* @param <R> the result type of the stream function
445	* @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	* onComplete, and exceptionally upon any error
450	* @throws NullPointerException if publisher or function are null
451	*/
452	public static <T,R> CompletableFuture<R> stream(
453	Publisher<T> publisher,
454	Function<? super Stream<T>,? extends R> streamFunction) {
455	return stream(defaultBufferSize(), publisher, streamFunction);
456	}
457	}