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 (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 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:	1.2
Committed:	Sat Mar 11 00:17:04 2017 UTC (7 years, 2 months ago) by jsr166
Branch:	MAIN
CVS Tags:	HEAD
Changes since 1.1:	+6 -6 lines
Log Message:	sync with src/main
#	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	/**
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	* Subscription}.
15	*
16	* <p>These interfaces correspond to the <a
17	* href="http://www.reactive-streams.org/"> reactive-streams</a>
18	* specification. They apply in both concurrent and distributed
19	* asynchronous settings: All (seven) methods are defined in {@code
20	* void} "one-way" message style. Communication relies on a simple form
21	* 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	*
25	* <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	* 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	*
36	* <pre> {@code
37	* class OneShotPublisher implements Publisher<Boolean> {
38	* private final ExecutorService executor = ForkJoinPool.commonPool(); // daemon-based
39	* private boolean subscribed; // true after first subscribe
40	* public synchronized void subscribe(Subscriber<? super Boolean> subscriber) {
41	* if (subscribed)
42	* subscriber.onError(new IllegalStateException()); // only one allowed
43	* else {
44	* subscribed = true;
45	* subscriber.onSubscribe(new OneShotSubscription(subscriber, executor));
46	* }
47	* }
48	* static class OneShotSubscription implements Subscription {
49	* private final Subscriber<? super Boolean> subscriber;
50	* private final ExecutorService executor;
51	* private Future<?> future; // to allow cancellation
52	* private boolean completed;
53	* OneShotSubscription(Subscriber<? super Boolean> subscriber,
54	* ExecutorService executor) {
55	* this.subscriber = subscriber;
56	* this.executor = executor;
57	* }
58	* public synchronized void request(long n) {
59	* if (n != 0 && !completed) {
60	* completed = true;
61	* if (n < 0) {
62	* IllegalArgumentException ex = new IllegalArgumentException();
63	* executor.execute(() -> subscriber.onError(ex));
64	* } else {
65	* future = executor.submit(() -> {
66	* subscriber.onNext(Boolean.TRUE);
67	* subscriber.onComplete();
68	* });
69	* }
70	* }
71	* }
72	* public synchronized void cancel() {
73	* completed = true;
74	* if (future != null) future.cancel(false);
75	* }
76	* }
77	* }}</pre>
78	*
79	* <p>A {@link Subscriber} arranges that items be requested and
80	* 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	* total outstanding requests between 32 and 64.
87	* 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	*
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	* long initialRequestSize = bufferSize;
105	* count = bufferSize - bufferSize / 2; // re-request when half consumed
106	* (this.subscription = subscription).request(initialRequestSize);
107	* }
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	*
117	* <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	* Or, when flow control is never needed, 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 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 not to be established or 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	* 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	*
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.
260	* A cancelled subscription need not ever receive an
261	* {@code onComplete} or {@code onError} 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	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
290	}