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;

/**
 * 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 (!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 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
         * less than or equal to zero, 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:	1.32
Committed:	Fri Aug 4 23:05:47 2017 UTC (6 years, 9 months ago) by dl
Branch:	MAIN
CVS Tags:	HEAD
Changes since 1.31:	+2 -2 lines
Log Message:	make javadoc sample code conform to request rule
#	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	dl	1.1	/**
10			* Interrelated interfaces and static methods for establishing
11			* flow-controlled components in which {@link Publisher Publishers}
12			* produce items consumed by one or more {@link Subscriber
13			* Subscribers}, each managed by a {@link Subscription
14	dl	1.14	* Subscription}.
15	jsr166	1.2	*
16	dl	1.1	* <p>These interfaces correspond to the <a
17			* href="http://www.reactive-streams.org/"> reactive-streams</a>
18	dl	1.14	* specification. They apply in both concurrent and distributed
19			* asynchronous settings: All (seven) methods are defined in {@code
20	jsr166	1.20	* void} "one-way" message style. Communication relies on a simple form
21	dl	1.14	* of flow control (method {@link Subscription#request}) that can be
22			* used to avoid resource management problems that may otherwise occur
23			* in "push" based systems.
24	dl	1.1	*
25	dl	1.10	* <p><b>Examples.</b> A {@link Publisher} usually defines its own
26			* {@link Subscription} implementation; constructing one in method
27			* {@code subscribe} and issuing it to the calling {@link
28			* Subscriber}. It publishes items to the subscriber asynchronously,
29			* normally using an {@link Executor}. For example, here is a very
30			* simple publisher that only issues (when requested) a single {@code
31	dl	1.16	* TRUE} item to a single subscriber. Because the subscriber receives
32			* only a single item, this class does not use buffering and ordering
33			* control required in most implementations (for example {@link
34			* SubmissionPublisher}).
35	dl	1.10	*
36			* <pre> {@code
37			* class OneShotPublisher implements Publisher<Boolean> {
38	dl	1.23	* private final ExecutorService executor = ForkJoinPool.commonPool(); // daemon-based
39	dl	1.26	* private boolean subscribed; // true after first subscribe
40	dl	1.16	* public synchronized void subscribe(Subscriber<? super Boolean> subscriber) {
41			* if (subscribed)
42	jsr166	1.30	* subscriber.onError(new IllegalStateException()); // only one allowed
43	dl	1.16	* else {
44			* subscribed = true;
45	dl	1.11	* subscriber.onSubscribe(new OneShotSubscription(subscriber, executor));
46	dl	1.16	* }
47	dl	1.10	* }
48			* static class OneShotSubscription implements Subscription {
49	dl	1.23	* private final Subscriber<? super Boolean> subscriber;
50			* private final ExecutorService executor;
51			* private Future<?> future; // to allow cancellation
52	dl	1.26	* private boolean completed;
53	dl	1.10	* OneShotSubscription(Subscriber<? super Boolean> subscriber,
54	dl	1.17	* ExecutorService executor) {
55	dl	1.10	* this.subscriber = subscriber;
56			* this.executor = executor;
57			* }
58			* public synchronized void request(long n) {
59	dl	1.32	* if (!completed) {
60	dl	1.10	* completed = true;
61	dl	1.32	* if (n <= 0) {
62	dl	1.26	* IllegalArgumentException ex = new IllegalArgumentException();
63	dl	1.22	* executor.execute(() -> subscriber.onError(ex));
64	dl	1.26	* } else {
65	dl	1.21	* future = executor.submit(() -> {
66	dl	1.16	* subscriber.onNext(Boolean.TRUE);
67			* subscriber.onComplete();
68	dl	1.21	* });
69			* }
70	dl	1.10	* }
71			* }
72	dl	1.15	* public synchronized void cancel() {
73	dl	1.17	* completed = true;
74			* if (future != null) future.cancel(false);
75	dl	1.15	* }
76	dl	1.10	* }
77			* }}</pre>
78			*
79	jsr166	1.13	* <p>A {@link Subscriber} arranges that items be requested and
80	dl	1.10	* processed. Items (invocations of {@link Subscriber#onNext}) are
81			* not issued unless requested, but multiple items may be requested.
82			* Many Subscriber implementations can arrange this in the style of
83			* the following example, where a buffer size of 1 single-steps, and
84			* larger sizes usually allow for more efficient overlapped processing
85			* with less communication; for example with a value of 64, this keeps
86	jsr166	1.25	* total outstanding requests between 32 and 64.
87	dl	1.14	* Because Subscriber method invocations for a given {@link
88			* Subscription} are strictly ordered, there is no need for these
89			* methods to use locks or volatiles unless a Subscriber maintains
90			* multiple Subscriptions (in which case it is better to instead
91			* define multiple Subscribers, each with its own Subscription).
92	dl	1.10	*
93			* <pre> {@code
94			* class SampleSubscriber<T> implements Subscriber<T> {
95			* final Consumer<? super T> consumer;
96			* Subscription subscription;
97			* final long bufferSize;
98			* long count;
99			* SampleSubscriber(long bufferSize, Consumer<? super T> consumer) {
100			* this.bufferSize = bufferSize;
101			* this.consumer = consumer;
102			* }
103			* public void onSubscribe(Subscription subscription) {
104	dl	1.23	* long initialRequestSize = bufferSize;
105	dl	1.10	* count = bufferSize - bufferSize / 2; // re-request when half consumed
106	dl	1.23	* (this.subscription = subscription).request(initialRequestSize);
107	dl	1.10	* }
108			* public void onNext(T item) {
109			* if (--count <= 0)
110			* subscription.request(count = bufferSize - bufferSize / 2);
111			* consumer.accept(item);
112			* }
113			* public void onError(Throwable ex) { ex.printStackTrace(); }
114			* public void onComplete() {}
115			* }}</pre>
116	dl	1.1	*
117	dl	1.18	* <p>The default value of {@link #defaultBufferSize} may provide a
118			* useful starting point for choosing request sizes and capacities in
119			* Flow components based on expected rates, resources, and usages.
120	dl	1.26	* Or, when flow control is never needed, a subscriber may initially
121			* request an effectively unbounded number of items, as in:
122	dl	1.12	*
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	jsr166	1.13	*
134	dl	1.1	* @author Doug Lea
135	jsr166	1.29	* @since 9
136	dl	1.1	*/
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	dl	1.14	* items (via method {@code onNext}) in the same order, unless
145			* drops or errors are encountered. If a Publisher encounters an
146	dl	1.15	* 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	dl	1.14	* invocations for each subscription are strictly ordered in <a
152			* href="package-summary.html#MemoryVisibility"><i>happens-before</i></a>
153			* order.
154			*
155	dl	1.15	* <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	dl	1.1	*
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	dl	1.14	* 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	dl	1.1	*
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	dl	1.14	* A receiver of messages. The methods in this interface are
184			* invoked in strict sequential order for each {@link
185			* Subscription}.
186	dl	1.1	*
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	dl	1.26	* cause the Subscription not to be established or to be cancelled.
195	dl	1.1	*
196	dl	1.15	* <p>Typically, implementations of this method invoke {@code
197			* subscription.request} to enable receiving items.
198	dl	1.1	*
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	dl	1.15	* 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	dl	1.14	* undefined.
230	dl	1.1	*/
231			public void onComplete();
232			}
233
234			/**
235	dl	1.14	* 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	dl	1.1	*/
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	dl	1.31	* less than or equal to zero, the Subscriber will receive an
246			* {@code onError} signal with an {@link
247			* IllegalArgumentException} argument. Otherwise, the
248			* Subscriber will receive up to {@code n} additional {@code
249			* onNext} invocations (or fewer if terminated).
250	dl	1.1	*
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	dl	1.14	* Causes the Subscriber to (eventually) stop receiving
258	dl	1.15	* messages. Implementation is best-effort -- additional
259	jsr166	1.27	* messages may be received after invoking this method.
260			* A cancelled subscription need not ever receive an
261			* {@code onComplete} or {@code onError} signal.
262	dl	1.1	*/
263			public void cancel();
264			}
265
266			/**
267	jsr166	1.2	* A component that acts as both a Subscriber and Publisher.
268	dl	1.1	*
269	jsr166	1.2	* @param <T> the subscribed item type
270			* @param <R> the published item type
271			*/
272	jsr166	1.5	public static interface Processor<T,R> extends Subscriber<T>, Publisher<R> {
273	dl	1.1	}
274
275	dl	1.18	static final int DEFAULT_BUFFER_SIZE = 256;
276
277			/**
278			* Returns a default value for Publisher or Subscriber buffering,
279			* that may be used in the absence of other constraints.
280			*
281			* @implNote
282			* The current value returned is 256.
283			*
284			* @return the buffer size value
285			*/
286			public static int defaultBufferSize() {
287			return DEFAULT_BUFFER_SIZE;
288			}
289	dl	1.1
290			}