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> {
 *   private final ExecutorService executor = ForkJoinPool.commonPool(); // daemon-based
 *   private boolean subscribed; // true after first subscribe
 *   public synchronized void subscribe(Subscriber<? super Boolean> subscriber) {
 *     if (subscribed)
 *        subscriber.onError(new IllegalStateException()); // only one allowed
 *     else {
 *       subscribed = true;
 *       subscriber.onSubscribe(new OneShotSubscription(subscriber, executor));
 *     }
 *   }
 *   static class OneShotSubscription implements Subscription {
 *     private final Subscriber<? super Boolean> subscriber;
 *     private final ExecutorService executor;
 *     private Future<?> future; // to allow cancellation
 *     private boolean completed;
 *     OneShotSubscription(Subscriber<? super Boolean> subscriber,
 *                         ExecutorService executor) {
 *       this.subscriber = subscriber;
 *       this.executor = executor;
 *     }
 *     public synchronized void request(long n) {
 *       if (n != 0 && !completed) {
 *         completed = true;
 *         if (n < 0) {
 *           IllegalArgumentException ex = new IllegalArgumentException();
 *           executor.execute(() -> subscriber.onError(ex));
 *         } else {
 *           future = executor.submit(() -> {
 *             subscriber.onNext(Boolean.TRUE);
 *             subscriber.onComplete();
 *           });
 *         }
 *       }
 *     }
 *     public synchronized void cancel() {
 *       completed = true;
 *       if (future != null) future.cancel(false);
 *     }
 *   }
 * }}</pre>
 *
 * <p>A {@link Subscriber} arranges that items be requested and
 * processed.  Items (invocations of {@link Subscriber#onNext}) are
 * not issued unless requested, but multiple items may be requested.
 * Many Subscriber implementations can arrange this in the style of
 * the following example, where a buffer size of 1 single-steps, and
 * larger sizes usually allow for more efficient overlapped processing
 * with less communication; for example with a value of 64, this keeps
 * total outstanding requests between 32 and 64.
 * Because Subscriber method invocations for a given {@link
 * Subscription} are strictly ordered, there is no need for these
 * methods to use locks or volatiles unless a Subscriber maintains
 * multiple Subscriptions (in which case it is better to instead
 * define multiple Subscribers, each with its own Subscription).
 *
 * <pre> {@code
 * class SampleSubscriber<T> implements Subscriber<T> {
 *   final Consumer<? super T> consumer;
 *   Subscription subscription;
 *   final long bufferSize;
 *   long count;
 *   SampleSubscriber(long bufferSize, Consumer<? super T> consumer) {
 *     this.bufferSize = bufferSize;
 *     this.consumer = consumer;
 *   }
 *   public void onSubscribe(Subscription subscription) {
 *     long initialRequestSize = bufferSize;
 *     count = bufferSize - bufferSize / 2; // re-request when half consumed
 *     (this.subscription = subscription).request(initialRequestSize);
 *   }
 *   public void onNext(T item) {
 *     if (--count <= 0)
 *       subscription.request(count = bufferSize - bufferSize / 2);
 *     consumer.accept(item);
 *   }
 *   public void onError(Throwable ex) { ex.printStackTrace(); }
 *   public void onComplete() {}
 * }}</pre>
 *
 * <p>The default value of {@link #defaultBufferSize} may provide a
 * useful starting point for choosing request sizes and capacities in
 * Flow components based on expected rates, resources, and usages.
 * Or, when flow control is never needed, a subscriber may initially
 * request an effectively unbounded number of items, as in:
 *
 * <pre> {@code
 * class UnboundedSubscriber<T> implements Subscriber<T> {
 *   public void onSubscribe(Subscription subscription) {
 *     subscription.request(Long.MAX_VALUE); // effectively unbounded
 *   }
 *   public void onNext(T item) { use(item); }
 *   public void onError(Throwable ex) { ex.printStackTrace(); }
 *   public void onComplete() {}
 *   void use(T item) { ... }
 * }}</pre>
 *
 * @author Doug Lea
 * @since 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 not to be established or to be cancelled.
         *
         * <p>Typically, implementations of this method invoke {@code
         * subscription.request} to enable receiving items.
         *
         * @param subscription a new subscription
         */
        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} or {@code onError} 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> {
    }

    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;
    }

}
Revision:	1.26
Committed:	Sun Sep 13 11:41:23 2015 UTC (8 years, 8 months ago) by dl
Branch:	MAIN
Changes since 1.25:	+8 -10 lines
Log Message:	incorporate suggested doc improvements
#	User	Rev	Content
1	dl	1.1	/*
2			* Written by Doug Lea with assistance from members of JCP JSR-166
3			* Expert Group and released to the public domain, as explained at
4			* http://creativecommons.org/publicdomain/zero/1.0/
5			*/
6
7			package java.util.concurrent;
8	jsr166	1.9
9			import java.util.ArrayList;
10	dl	1.1	import java.util.function.Consumer;
11			import java.util.function.Function;
12			import java.util.stream.Stream;
13
14			/**
15			* Interrelated interfaces and static methods for establishing
16			* flow-controlled components in which {@link Publisher Publishers}
17			* produce items consumed by one or more {@link Subscriber
18			* Subscribers}, each managed by a {@link Subscription
19	dl	1.14	* Subscription}.
20	jsr166	1.2	*
21	dl	1.1	* <p>These interfaces correspond to the <a
22			* href="http://www.reactive-streams.org/"> reactive-streams</a>
23	dl	1.14	* specification. They apply in both concurrent and distributed
24			* asynchronous settings: All (seven) methods are defined in {@code
25	jsr166	1.20	* void} "one-way" message style. Communication relies on a simple form
26	dl	1.14	* of flow control (method {@link Subscription#request}) that can be
27			* used to avoid resource management problems that may otherwise occur
28			* in "push" based systems.
29	dl	1.1	*
30	dl	1.10	* <p><b>Examples.</b> A {@link Publisher} usually defines its own
31			* {@link Subscription} implementation; constructing one in method
32			* {@code subscribe} and issuing it to the calling {@link
33			* Subscriber}. It publishes items to the subscriber asynchronously,
34			* normally using an {@link Executor}. For example, here is a very
35			* simple publisher that only issues (when requested) a single {@code
36	dl	1.16	* TRUE} item to a single subscriber. Because the subscriber receives
37			* only a single item, this class does not use buffering and ordering
38			* control required in most implementations (for example {@link
39			* SubmissionPublisher}).
40	dl	1.10	*
41			* <pre> {@code
42			* class OneShotPublisher implements Publisher<Boolean> {
43	dl	1.23	* private final ExecutorService executor = ForkJoinPool.commonPool(); // daemon-based
44	dl	1.26	* private boolean subscribed; // true after first subscribe
45	dl	1.16	* 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	dl	1.23	* private final Subscriber<? super Boolean> subscriber;
55			* private final ExecutorService executor;
56			* private Future<?> future; // to allow cancellation
57	dl	1.26	* private boolean completed;
58	dl	1.10	* OneShotSubscription(Subscriber<? super Boolean> subscriber,
59	dl	1.17	* ExecutorService executor) {
60	dl	1.10	* this.subscriber = subscriber;
61			* this.executor = executor;
62			* }
63			* public synchronized void request(long n) {
64	dl	1.16	* if (n != 0 && !completed) {
65	dl	1.10	* completed = true;
66	dl	1.21	* if (n < 0) {
67	dl	1.26	* IllegalArgumentException ex = new IllegalArgumentException();
68	dl	1.22	* executor.execute(() -> subscriber.onError(ex));
69	dl	1.26	* } else {
70	dl	1.21	* future = executor.submit(() -> {
71	dl	1.16	* subscriber.onNext(Boolean.TRUE);
72			* subscriber.onComplete();
73	dl	1.21	* });
74			* }
75	dl	1.10	* }
76			* }
77	dl	1.15	* public synchronized void cancel() {
78	dl	1.17	* completed = true;
79			* if (future != null) future.cancel(false);
80	dl	1.15	* }
81	dl	1.10	* }
82			* }}</pre>
83			*
84	jsr166	1.13	* <p>A {@link Subscriber} arranges that items be requested and
85	dl	1.10	* processed. Items (invocations of {@link Subscriber#onNext}) are
86			* not issued unless requested, but multiple items may be requested.
87			* Many Subscriber implementations can arrange this in the style of
88			* the following example, where a buffer size of 1 single-steps, and
89			* larger sizes usually allow for more efficient overlapped processing
90			* with less communication; for example with a value of 64, this keeps
91	jsr166	1.25	* total outstanding requests between 32 and 64.
92	dl	1.14	* Because Subscriber method invocations for a given {@link
93			* Subscription} are strictly ordered, there is no need for these
94			* methods to use locks or volatiles unless a Subscriber maintains
95			* multiple Subscriptions (in which case it is better to instead
96			* define multiple Subscribers, each with its own Subscription).
97	dl	1.10	*
98			* <pre> {@code
99			* class SampleSubscriber<T> implements Subscriber<T> {
100			* final Consumer<? super T> consumer;
101			* Subscription subscription;
102			* final long bufferSize;
103			* long count;
104			* SampleSubscriber(long bufferSize, Consumer<? super T> consumer) {
105			* this.bufferSize = bufferSize;
106			* this.consumer = consumer;
107			* }
108			* public void onSubscribe(Subscription subscription) {
109	dl	1.23	* long initialRequestSize = bufferSize;
110	dl	1.10	* count = bufferSize - bufferSize / 2; // re-request when half consumed
111	dl	1.23	* (this.subscription = subscription).request(initialRequestSize);
112	dl	1.10	* }
113			* public void onNext(T item) {
114			* if (--count <= 0)
115			* subscription.request(count = bufferSize - bufferSize / 2);
116			* consumer.accept(item);
117			* }
118			* public void onError(Throwable ex) { ex.printStackTrace(); }
119			* public void onComplete() {}
120			* }}</pre>
121	dl	1.1	*
122	dl	1.18	* <p>The default value of {@link #defaultBufferSize} may provide a
123			* useful starting point for choosing request sizes and capacities in
124			* Flow components based on expected rates, resources, and usages.
125	dl	1.26	* Or, when flow control is never needed, a subscriber may initially
126			* request an effectively unbounded number of items, as in:
127	dl	1.12	*
128			* <pre> {@code
129			* class UnboundedSubscriber<T> implements Subscriber<T> {
130			* public void onSubscribe(Subscription subscription) {
131			* subscription.request(Long.MAX_VALUE); // effectively unbounded
132			* }
133			* public void onNext(T item) { use(item); }
134			* public void onError(Throwable ex) { ex.printStackTrace(); }
135			* public void onComplete() {}
136			* void use(T item) { ... }
137			* }}</pre>
138	jsr166	1.13	*
139	dl	1.1	* @author Doug Lea
140			* @since 1.9
141			*/
142			public final class Flow {
143
144			private Flow() {} // uninstantiable
145
146			/**
147			* A producer of items (and related control messages) received by
148			* Subscribers. Each current {@link Subscriber} receives the same
149	dl	1.14	* items (via method {@code onNext}) in the same order, unless
150			* drops or errors are encountered. If a Publisher encounters an
151	dl	1.15	* error that does not allow items to be issued to a Subscriber,
152			* that Subscriber receives {@code onError}, and then receives no
153			* further messages. Otherwise, when it is known that no further
154			* messages will be issued to it, a subscriber receives {@code
155			* onComplete}. Publishers ensure that Subscriber method
156	dl	1.14	* invocations for each subscription are strictly ordered in <a
157			* href="package-summary.html#MemoryVisibility"><i>happens-before</i></a>
158			* order.
159			*
160	dl	1.15	* <p>Publishers may vary in policy about whether drops (failures
161			* to issue an item because of resource limitations) are treated
162			* as unrecoverable errors. Publishers may also vary about
163			* whether Subscribers receive items that were produced or
164			* available before they subscribed.
165	dl	1.1	*
166			* @param <T> the published item type
167			*/
168			@FunctionalInterface
169			public static interface Publisher<T> {
170			/**
171			* Adds the given Subscriber if possible. If already
172			* subscribed, or the attempt to subscribe fails due to policy
173	dl	1.14	* violations or errors, the Subscriber's {@code onError}
174			* method is invoked with an {@link IllegalStateException}.
175			* Otherwise, the Subscriber's {@code onSubscribe} method is
176			* invoked with a new {@link Subscription}. Subscribers may
177			* enable receiving items by invoking the {@code request}
178			* method of this Subscription, and may unsubscribe by
179			* invoking its {@code cancel} method.
180	dl	1.1	*
181			* @param subscriber the subscriber
182			* @throws NullPointerException if subscriber is null
183			*/
184			public void subscribe(Subscriber<? super T> subscriber);
185			}
186
187			/**
188	dl	1.14	* A receiver of messages. The methods in this interface are
189			* invoked in strict sequential order for each {@link
190			* Subscription}.
191	dl	1.1	*
192			* @param <T> the subscribed item type
193			*/
194			public static interface Subscriber<T> {
195			/**
196			* Method invoked prior to invoking any other Subscriber
197			* methods for the given Subscription. If this method throws
198			* an exception, resulting behavior is not guaranteed, but may
199	dl	1.26	* cause the Subscription not to be established or to be cancelled.
200	dl	1.1	*
201	dl	1.15	* <p>Typically, implementations of this method invoke {@code
202			* subscription.request} to enable receiving items.
203	dl	1.1	*
204			* @param subscription a new subscription
205			*/
206			public void onSubscribe(Subscription subscription);
207
208			/**
209			* Method invoked with a Subscription's next item. If this
210			* method throws an exception, resulting behavior is not
211			* guaranteed, but may cause the Subscription to be cancelled.
212			*
213			* @param item the item
214			*/
215			public void onNext(T item);
216
217			/**
218			* Method invoked upon an unrecoverable error encountered by a
219			* Publisher or Subscription, after which no other Subscriber
220			* methods are invoked by the Subscription. If this method
221			* itself throws an exception, resulting behavior is
222			* undefined.
223			*
224			* @param throwable the exception
225			*/
226			public void onError(Throwable throwable);
227
228			/**
229	dl	1.15	* Method invoked when it is known that no additional
230			* Subscriber method invocations will occur for a Subscription
231			* that is not already terminated by error, after which no
232			* other Subscriber methods are invoked by the Subscription.
233			* If this method throws an exception, resulting behavior is
234	dl	1.14	* undefined.
235	dl	1.1	*/
236			public void onComplete();
237			}
238
239			/**
240	dl	1.14	* Message control linking a {@link Publisher} and {@link
241			* Subscriber}. Subscribers receive items only when requested,
242			* and may cancel at any time. The methods in this interface are
243			* intended to be invoked only by their Subscribers; usages in
244			* other contexts have undefined effects.
245	dl	1.1	*/
246			public static interface Subscription {
247			/**
248			* Adds the given number {@code n} of items to the current
249			* unfulfilled demand for this subscription. If {@code n} is
250	dl	1.14	* negative, the Subscriber will receive an {@code onError}
251	jsr166	1.19	* signal with an {@link IllegalArgumentException} argument.
252			* Otherwise, the Subscriber will receive up to {@code n}
253			* additional {@code onNext} invocations (or fewer if
254			* terminated).
255	dl	1.1	*
256			* @param n the increment of demand; a value of {@code
257			* Long.MAX_VALUE} may be considered as effectively unbounded
258			*/
259			public void request(long n);
260
261			/**
262	dl	1.14	* Causes the Subscriber to (eventually) stop receiving
263	dl	1.15	* messages. Implementation is best-effort -- additional
264			* messages may be received after invoking this method. A
265			* cancelled subscription need not ever receive an {@code
266	dl	1.26	* onComplete} or {@code onError} signal.
267	dl	1.1	*/
268			public void cancel();
269			}
270
271			/**
272	jsr166	1.2	* A component that acts as both a Subscriber and Publisher.
273	dl	1.1	*
274	jsr166	1.2	* @param <T> the subscribed item type
275			* @param <R> the published item type
276			*/
277	jsr166	1.5	public static interface Processor<T,R> extends Subscriber<T>, Publisher<R> {
278	dl	1.1	}
279
280	dl	1.18	static final int DEFAULT_BUFFER_SIZE = 256;
281
282			/**
283			* Returns a default value for Publisher or Subscriber buffering,
284			* that may be used in the absence of other constraints.
285			*
286			* @implNote
287			* The current value returned is 256.
288			*
289			* @return the buffer size value
290			*/
291			public static int defaultBufferSize() {
292			return DEFAULT_BUFFER_SIZE;
293			}
294	dl	1.1
295			}