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} "oneway" 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>When flow control is 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 long DEFAULT_BUFFER_SIZE = 64L;

    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 a buffer size of 64.
     *
     * @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(DEFAULT_BUFFER_SIZE, 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 a buffer size of 64.
     *
     * @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(DEFAULT_BUFFER_SIZE, publisher, streamFunction);
    }
}
Revision:	1.17
Committed:	Mon Jan 19 18:15:09 2015 UTC (9 years, 4 months ago) by dl
Branch:	MAIN
Changes since 1.16:	+7 -5 lines
Log Message:	Improve example
#	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} "oneway" 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>When flow control is inapplicable, a subscriber may initially
121	* request an effectively unbounded number of items, as in:
122	*
123	* <pre> {@code
124	* class UnboundedSubscriber<T> implements Subscriber<T> {
125	* public void onSubscribe(Subscription subscription) {
126	* subscription.request(Long.MAX_VALUE); // effectively unbounded
127	* }
128	* public void onNext(T item) { use(item); }
129	* public void onError(Throwable ex) { ex.printStackTrace(); }
130	* public void onComplete() {}
131	* void use(T item) { ... }
132	* }}</pre>
133	*
134	* @author Doug Lea
135	* @since 1.9
136	*/
137	public final class Flow {
138
139	private Flow() {} // uninstantiable
140
141	/**
142	* A producer of items (and related control messages) received by
143	* Subscribers. Each current {@link Subscriber} receives the same
144	* items (via method {@code onNext}) in the same order, unless
145	* drops or errors are encountered. If a Publisher encounters an
146	* error that does not allow items to be issued to a Subscriber,
147	* that Subscriber receives {@code onError}, and then receives no
148	* further messages. Otherwise, when it is known that no further
149	* messages will be issued to it, a subscriber receives {@code
150	* onComplete}. Publishers ensure that Subscriber method
151	* invocations for each subscription are strictly ordered in <a
152	* href="package-summary.html#MemoryVisibility"><i>happens-before</i></a>
153	* order.
154	*
155	* <p>Publishers may vary in policy about whether drops (failures
156	* to issue an item because of resource limitations) are treated
157	* as unrecoverable errors. Publishers may also vary about
158	* whether Subscribers receive items that were produced or
159	* available before they subscribed.
160	*
161	* @param <T> the published item type
162	*/
163	@FunctionalInterface
164	public static interface Publisher<T> {
165	/**
166	* Adds the given Subscriber if possible. If already
167	* subscribed, or the attempt to subscribe fails due to policy
168	* violations or errors, the Subscriber's {@code onError}
169	* method is invoked with an {@link IllegalStateException}.
170	* Otherwise, the Subscriber's {@code onSubscribe} method is
171	* invoked with a new {@link Subscription}. Subscribers may
172	* enable receiving items by invoking the {@code request}
173	* method of this Subscription, and may unsubscribe by
174	* invoking its {@code cancel} method.
175	*
176	* @param subscriber the subscriber
177	* @throws NullPointerException if subscriber is null
178	*/
179	public void subscribe(Subscriber<? super T> subscriber);
180	}
181
182	/**
183	* A receiver of messages. The methods in this interface are
184	* invoked in strict sequential order for each {@link
185	* Subscription}.
186	*
187	* @param <T> the subscribed item type
188	*/
189	public static interface Subscriber<T> {
190	/**
191	* Method invoked prior to invoking any other Subscriber
192	* methods for the given Subscription. If this method throws
193	* an exception, resulting behavior is not guaranteed, but may
194	* cause the Subscription to be cancelled.
195	*
196	* <p>Typically, implementations of this method invoke {@code
197	* subscription.request} to enable receiving items.
198	*
199	* @param subscription a new subscription
200	*/
201	public void onSubscribe(Subscription subscription);
202
203	/**
204	* Method invoked with a Subscription's next item. If this
205	* method throws an exception, resulting behavior is not
206	* guaranteed, but may cause the Subscription to be cancelled.
207	*
208	* @param item the item
209	*/
210	public void onNext(T item);
211
212	/**
213	* Method invoked upon an unrecoverable error encountered by a
214	* Publisher or Subscription, after which no other Subscriber
215	* methods are invoked by the Subscription. If this method
216	* itself throws an exception, resulting behavior is
217	* undefined.
218	*
219	* @param throwable the exception
220	*/
221	public void onError(Throwable throwable);
222
223	/**
224	* Method invoked when it is known that no additional
225	* Subscriber method invocations will occur for a Subscription
226	* that is not already terminated by error, after which no
227	* other Subscriber methods are invoked by the Subscription.
228	* If this method throws an exception, resulting behavior is
229	* undefined.
230	*/
231	public void onComplete();
232	}
233
234	/**
235	* Message control linking a {@link Publisher} and {@link
236	* Subscriber}. Subscribers receive items only when requested,
237	* and may cancel at any time. The methods in this interface are
238	* intended to be invoked only by their Subscribers; usages in
239	* other contexts have undefined effects.
240	*/
241	public static interface Subscription {
242	/**
243	* Adds the given number {@code n} of items to the current
244	* unfulfilled demand for this subscription. If {@code n} is
245	* negative, the Subscriber will receive an {@code onError}
246	* signal with an {@link IllegalArgumentException}
247	* argument. Otherwise, the Subscriber will receive up to
248	* {@code n} additional {@code onNext} invocations (or fewer
249	* if terminated).
250	*
251	* @param n the increment of demand; a value of {@code
252	* Long.MAX_VALUE} may be considered as effectively unbounded
253	*/
254	public void request(long n);
255
256	/**
257	* Causes the Subscriber to (eventually) stop receiving
258	* messages. Implementation is best-effort -- additional
259	* messages may be received after invoking this method. A
260	* cancelled subscription need not ever receive an {@code
261	* onComplete} signal.
262	*/
263	public void cancel();
264	}
265
266	/**
267	* A component that acts as both a Subscriber and Publisher.
268	*
269	* @param <T> the subscribed item type
270	* @param <R> the published item type
271	*/
272	public static interface Processor<T,R> extends Subscriber<T>, Publisher<R> {
273	}
274
275	// Support for static methods
276
277	static final long DEFAULT_BUFFER_SIZE = 64L;
278
279	abstract static class CompletableSubscriber<T,U> implements Subscriber<T>,
280	Consumer<T> {
281	final CompletableFuture<U> status;
282	Subscription subscription;
283	long requestSize;
284	long count;
285	CompletableSubscriber(long bufferSize, CompletableFuture<U> status) {
286	this.status = status;
287	this.requestSize = bufferSize;
288	}
289	public final void onSubscribe(Subscription subscription) {
290	(this.subscription = subscription).request(requestSize);
291	count = requestSize -= (requestSize >>> 1);
292	}
293	public final void onError(Throwable ex) {
294	status.completeExceptionally(ex);
295	}
296	public void onNext(T item) {
297	try {
298	if (--count <= 0)
299	subscription.request(count = requestSize);
300	accept(item);
301	} catch (Throwable ex) {
302	status.completeExceptionally(ex);
303	}
304	}
305	}
306
307	static final class ConsumeSubscriber<T> extends CompletableSubscriber<T,Void> {
308	final Consumer<? super T> consumer;
309	ConsumeSubscriber(long bufferSize,
310	CompletableFuture<Void> status,
311	Consumer<? super T> consumer) {
312	super(bufferSize, status);
313	this.consumer = consumer;
314	}
315	public void accept(T item) { consumer.accept(item); }
316	public void onComplete() { status.complete(null); }
317	}
318
319	/**
320	* Creates and subscribes a Subscriber that consumes all items
321	* from the given publisher using the given Consumer function, and
322	* using the given bufferSize for buffering. Returns a
323	* CompletableFuture that is completed normally when the publisher
324	* signals {@code onComplete}, or completed exceptionally upon any
325	* error, including an exception thrown by the Consumer (in which
326	* case the subscription is cancelled if not already terminated).
327	*
328	* @param <T> the published item type
329	* @param bufferSize the request size for subscriptions
330	* @param publisher the publisher
331	* @param consumer the function applied to each onNext item
332	* @return a CompletableFuture that is completed normally
333	* when the publisher signals onComplete, and exceptionally
334	* upon any error
335	* @throws NullPointerException if publisher or consumer are null
336	* @throws IllegalArgumentException if bufferSize not positive
337	*/
338	public static <T> CompletableFuture<Void> consume(
339	long bufferSize, Publisher<T> publisher, Consumer<? super T> consumer) {
340	if (bufferSize <= 0L)
341	throw new IllegalArgumentException("bufferSize must be positive");
342	if (publisher == null \|\| consumer == null)
343	throw new NullPointerException();
344	CompletableFuture<Void> status = new CompletableFuture<>();
345	publisher.subscribe(new ConsumeSubscriber<T>(
346	bufferSize, status, consumer));
347	return status;
348	}
349
350	/**
351	* Equivalent to {@link #consume(long, Publisher, Consumer)}
352	* with a buffer size of 64.
353	*
354	* @param <T> the published item type
355	* @param publisher the publisher
356	* @param consumer the function applied to each onNext item
357	* @return a CompletableFuture that is completed normally
358	* when the publisher signals onComplete, and exceptionally
359	* upon any error
360	* @throws NullPointerException if publisher or consumer are null
361	*/
362	public static <T> CompletableFuture<Void> consume(
363	Publisher<T> publisher, Consumer<? super T> consumer) {
364	return consume(DEFAULT_BUFFER_SIZE, publisher, consumer);
365	}
366
367	/**
368	* Temporary implementation for Stream, collecting all items
369	* and then applying stream operation.
370	*/
371	static final class StreamSubscriber<T,R> extends CompletableSubscriber<T,R> {
372	final Function<? super Stream<T>, ? extends R> fn;
373	final ArrayList<T> items;
374	StreamSubscriber(long bufferSize,
375	CompletableFuture<R> status,
376	Function<? super Stream<T>, ? extends R> fn) {
377	super(bufferSize, status);
378	this.fn = fn;
379	this.items = new ArrayList<T>();
380	}
381	public void accept(T item) { items.add(item); }
382	public void onComplete() { status.complete(fn.apply(items.stream())); }
383	}
384
385	/**
386	* Creates and subscribes a Subscriber that applies the given
387	* stream operation to items, and uses the given bufferSize for
388	* buffering. Returns a CompletableFuture that is completed
389	* normally with the result of this function when the publisher
390	* signals {@code onComplete}, or is completed exceptionally upon
391	* any error.
392	*
393	* <p><b>Preliminary release note:</b> Currently, this method
394	* collects all items before executing the stream
395	* computation. Improvements are pending Stream integration.
396	*
397	* @param <T> the published item type
398	* @param <R> the result type of the stream function
399	* @param bufferSize the request size for subscriptions
400	* @param publisher the publisher
401	* @param streamFunction the operation on elements
402	* @return a CompletableFuture that is completed normally with the
403	* result of the given function as result when the publisher signals
404	* onComplete, and exceptionally upon any error
405	* @throws NullPointerException if publisher or function are null
406	* @throws IllegalArgumentException if bufferSize not positive
407	*/
408	public static <T,R> CompletableFuture<R> stream(
409	long bufferSize, Publisher<T> publisher,
410	Function<? super Stream<T>, ? extends R> streamFunction) {
411	if (bufferSize <= 0L)
412	throw new IllegalArgumentException("bufferSize must be positive");
413	if (publisher == null \|\| streamFunction == null)
414	throw new NullPointerException();
415	CompletableFuture<R> status = new CompletableFuture<>();
416	publisher.subscribe(new StreamSubscriber<T,R>(
417	bufferSize, status, streamFunction));
418	return status;
419	}
420
421	/**
422	* Equivalent to {@link #stream(long, Publisher, Function)}
423	* with a buffer size of 64.
424	*
425	* @param <T> the published item type
426	* @param <R> the result type of the stream function
427	* @param publisher the publisher
428	* @param streamFunction the operation on elements
429	* @return a CompletableFuture that is completed normally with the
430	* result of the given function as result when the publisher signals
431	* onComplete, and exceptionally upon any error
432	* @throws NullPointerException if publisher or function are null
433	*/
434	public static <T,R> CompletableFuture<R> stream(
435	Publisher<T> publisher,
436	Function<? super Stream<T>,? extends R> streamFunction) {
437	return stream(DEFAULT_BUFFER_SIZE, publisher, streamFunction);
438	}
439	}