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 = 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 {
 *     private final Subscriber<? super Boolean> subscriber;
 *     private final ExecutorService executor;
 *     private Future<?> future; // to allow cancellation
 *     private 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;
 *         if (n < 0) {
 *           IllegalStateException ex = new IllegalStateException();
 *           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 known to be always inapplicable, a
 * subscriber may initially request an effectively unbounded number of
 * items, as in:
 *
 * <pre> {@code
 * class UnboundedSubscriber<T> implements Subscriber<T> {
 *   public void onSubscribe(Subscription subscription) {
 *     subscription.request(Long.MAX_VALUE); // effectively unbounded
 *   }
 *   public void onNext(T item) { use(item); }
 *   public void onError(Throwable ex) { ex.printStackTrace(); }
 *   public void onComplete() {}
 *   void use(T item) { ... }
 * }}</pre>
 *
 * @author Doug Lea
 * @since 1.9
 */
public final class Flow {

    private Flow() {} // uninstantiable

    /**
     * A producer of items (and related control messages) received by
     * Subscribers.  Each current {@link Subscriber} receives the same
     * items (via method {@code onNext}) in the same order, unless
     * drops or errors are encountered. If a Publisher encounters an
     * error that does not allow items to be issued to a Subscriber,
     * that Subscriber receives {@code onError}, and then receives no
     * further messages.  Otherwise, when it is known that no further
     * messages will be issued to it, a subscriber receives {@code
     * onComplete}.  Publishers ensure that Subscriber method
     * invocations for each subscription are strictly ordered in <a
     * href="package-summary.html#MemoryVisibility"><i>happens-before</i></a>
     * order.
     *
     * <p>Publishers may vary in policy about whether drops (failures
     * to issue an item because of resource limitations) are treated
     * as unrecoverable errors.  Publishers may also vary about
     * whether Subscribers receive items that were produced or
     * available before they subscribed.
     *
     * @param <T> the published item type
     */
    @FunctionalInterface
    public static interface Publisher<T> {
        /**
         * Adds the given Subscriber if possible.  If already
         * subscribed, or the attempt to subscribe fails due to policy
         * violations or errors, the Subscriber's {@code onError}
         * method is invoked with an {@link IllegalStateException}.
         * Otherwise, the Subscriber's {@code onSubscribe} method is
         * invoked with a new {@link Subscription}.  Subscribers may
         * enable receiving items by invoking the {@code request}
         * method of this Subscription, and may unsubscribe by
         * invoking its {@code cancel} method.
         *
         * @param subscriber the subscriber
         * @throws NullPointerException if subscriber is null
         */
        public void subscribe(Subscriber<? super T> subscriber);
    }

    /**
     * A receiver of messages.  The methods in this interface are
     * invoked in strict sequential order for each {@link
     * Subscription}.
     *
     * @param <T> the subscribed item type
     */
    public static interface Subscriber<T> {
        /**
         * Method invoked prior to invoking any other Subscriber
         * methods for the given Subscription. If this method throws
         * an exception, resulting behavior is not guaranteed, but may
         * cause the Subscription to be cancelled.
         *
         * <p>Typically, implementations of this method invoke {@code
         * subscription.request} to enable receiving items.
         *
         * @param subscription a new subscription
         */
        public void onSubscribe(Subscription subscription);

        /**
         * Method invoked with a Subscription's next item.  If this
         * method throws an exception, resulting behavior is not
         * guaranteed, but may cause the Subscription to be cancelled.
         *
         * @param item the item
         */
        public void onNext(T item);

        /**
         * Method invoked upon an unrecoverable error encountered by a
         * Publisher or Subscription, after which no other Subscriber
         * methods are invoked by the Subscription.  If this method
         * itself throws an exception, resulting behavior is
         * undefined.
         *
         * @param throwable the exception
         */
        public void onError(Throwable throwable);

        /**
         * Method invoked when it is known that no additional
         * Subscriber method invocations will occur for a Subscription
         * that is not already terminated by error, after which no
         * other Subscriber methods are invoked by the Subscription.
         * If this method throws an exception, resulting behavior is
         * undefined.
         */
        public void onComplete();
    }

    /**
     * Message control linking a {@link Publisher} and {@link
     * Subscriber}.  Subscribers receive items only when requested,
     * and may cancel at any time. The methods in this interface are
     * intended to be invoked only by their Subscribers; usages in
     * other contexts have undefined effects.
     */
    public static interface Subscription {
        /**
         * Adds the given number {@code n} of items to the current
         * unfulfilled demand for this subscription.  If {@code n} is
         * negative, the Subscriber will receive an {@code onError}
         * signal with an {@link IllegalArgumentException} argument.
         * Otherwise, the Subscriber will receive up to {@code n}
         * additional {@code onNext} invocations (or fewer if
         * terminated).
         *
         * @param n the increment of demand; a value of {@code
         * Long.MAX_VALUE} may be considered as effectively unbounded
         */
        public void request(long n);

        /**
         * Causes the Subscriber to (eventually) stop receiving
         * messages.  Implementation is best-effort -- additional
         * messages may be received after invoking this method.  A
         * cancelled subscription need not ever receive an {@code
         * onComplete} signal.
         */
        public void cancel();
    }

    /**
     * A component that acts as both a Subscriber and Publisher.
     *
     * @param <T> the subscribed item type
     * @param <R> the published item type
     */
    public static interface Processor<T,R> extends Subscriber<T>, Publisher<R> {
    }

    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:	1.24
Committed:	Sat Sep 12 11:25:10 2015 UTC (8 years, 8 months ago) by dl
Branch:	MAIN
Changes since 1.23:	+1 -170 lines
Log Message:	Remove Flow.stream for now; move consume to SubmissionPublisher
#	Content
1	/*
2	* Written by Doug Lea with assistance from members of JCP JSR-166
3	* Expert Group and released to the public domain, as explained at
4	* http://creativecommons.org/publicdomain/zero/1.0/
5	*/
6
7	package java.util.concurrent;
8
9	import java.util.ArrayList;
10	import java.util.function.Consumer;
11	import java.util.function.Function;
12	import java.util.stream.Stream;
13
14	/**
15	* Interrelated interfaces and static methods for establishing
16	* flow-controlled components in which {@link Publisher Publishers}
17	* produce items consumed by one or more {@link Subscriber
18	* Subscribers}, each managed by a {@link Subscription
19	* Subscription}.
20	*
21	* <p>These interfaces correspond to the <a
22	* href="http://www.reactive-streams.org/"> reactive-streams</a>
23	* specification. They apply in both concurrent and distributed
24	* asynchronous settings: All (seven) methods are defined in {@code
25	* void} "one-way" message style. Communication relies on a simple form
26	* of flow control (method {@link Subscription#request}) that can be
27	* used to avoid resource management problems that may otherwise occur
28	* in "push" based systems.
29	*
30	* <p><b>Examples.</b> A {@link Publisher} usually defines its own
31	* {@link Subscription} implementation; constructing one in method
32	* {@code subscribe} and issuing it to the calling {@link
33	* Subscriber}. It publishes items to the subscriber asynchronously,
34	* normally using an {@link Executor}. For example, here is a very
35	* simple publisher that only issues (when requested) a single {@code
36	* TRUE} item to a single subscriber. Because the subscriber receives
37	* only a single item, this class does not use buffering and ordering
38	* control required in most implementations (for example {@link
39	* SubmissionPublisher}).
40	*
41	* <pre> {@code
42	* class OneShotPublisher implements Publisher<Boolean> {
43	* private final ExecutorService executor = ForkJoinPool.commonPool(); // daemon-based
44	* private 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	* private final Subscriber<? super Boolean> subscriber;
55	* private final ExecutorService executor;
56	* private Future<?> future; // to allow cancellation
57	* private 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	* if (n < 0) {
67	* IllegalStateException ex = new IllegalStateException();
68	* executor.execute(() -> subscriber.onError(ex));
69	* }
70	* else {
71	* future = executor.submit(() -> {
72	* subscriber.onNext(Boolean.TRUE);
73	* subscriber.onComplete();
74	* });
75	* }
76	* }
77	* }
78	* public synchronized void cancel() {
79	* completed = true;
80	* if (future != null) future.cancel(false);
81	* }
82	* }
83	* }}</pre>
84	*
85	* <p>A {@link Subscriber} arranges that items be requested and
86	* processed. Items (invocations of {@link Subscriber#onNext}) are
87	* not issued unless requested, but multiple items may be requested.
88	* Many Subscriber implementations can arrange this in the style of
89	* the following example, where a buffer size of 1 single-steps, and
90	* larger sizes usually allow for more efficient overlapped processing
91	* with less communication; for example with a value of 64, this keeps
92	* total outstanding requests between 32 and 64.
93	* Because Subscriber method invocations for a given {@link
94	* Subscription} are strictly ordered, there is no need for these
95	* methods to use locks or volatiles unless a Subscriber maintains
96	* multiple Subscriptions (in which case it is better to instead
97	* define multiple Subscribers, each with its own Subscription).
98	*
99	* <pre> {@code
100	* class SampleSubscriber<T> implements Subscriber<T> {
101	* final Consumer<? super T> consumer;
102	* Subscription subscription;
103	* final long bufferSize;
104	* long count;
105	* SampleSubscriber(long bufferSize, Consumer<? super T> consumer) {
106	* this.bufferSize = bufferSize;
107	* this.consumer = consumer;
108	* }
109	* public void onSubscribe(Subscription subscription) {
110	* long initialRequestSize = bufferSize;
111	* count = bufferSize - bufferSize / 2; // re-request when half consumed
112	* (this.subscription = subscription).request(initialRequestSize);
113	* }
114	* public void onNext(T item) {
115	* if (--count <= 0)
116	* subscription.request(count = bufferSize - bufferSize / 2);
117	* consumer.accept(item);
118	* }
119	* public void onError(Throwable ex) { ex.printStackTrace(); }
120	* public void onComplete() {}
121	* }}</pre>
122	*
123	* <p>The default value of {@link #defaultBufferSize} may provide a
124	* useful starting point for choosing request sizes and capacities in
125	* Flow components based on expected rates, resources, and usages.
126	* Or, when flow control is known to be always inapplicable, a
127	* subscriber may initially request an effectively unbounded number of
128	* items, as in:
129	*
130	* <pre> {@code
131	* class UnboundedSubscriber<T> implements Subscriber<T> {
132	* public void onSubscribe(Subscription subscription) {
133	* subscription.request(Long.MAX_VALUE); // effectively unbounded
134	* }
135	* public void onNext(T item) { use(item); }
136	* public void onError(Throwable ex) { ex.printStackTrace(); }
137	* public void onComplete() {}
138	* void use(T item) { ... }
139	* }}</pre>
140	*
141	* @author Doug Lea
142	* @since 1.9
143	*/
144	public final class Flow {
145
146	private Flow() {} // uninstantiable
147
148	/**
149	* A producer of items (and related control messages) received by
150	* Subscribers. Each current {@link Subscriber} receives the same
151	* items (via method {@code onNext}) in the same order, unless
152	* drops or errors are encountered. If a Publisher encounters an
153	* error that does not allow items to be issued to a Subscriber,
154	* that Subscriber receives {@code onError}, and then receives no
155	* further messages. Otherwise, when it is known that no further
156	* messages will be issued to it, a subscriber receives {@code
157	* onComplete}. Publishers ensure that Subscriber method
158	* invocations for each subscription are strictly ordered in <a
159	* href="package-summary.html#MemoryVisibility"><i>happens-before</i></a>
160	* order.
161	*
162	* <p>Publishers may vary in policy about whether drops (failures
163	* to issue an item because of resource limitations) are treated
164	* as unrecoverable errors. Publishers may also vary about
165	* whether Subscribers receive items that were produced or
166	* available before they subscribed.
167	*
168	* @param <T> the published item type
169	*/
170	@FunctionalInterface
171	public static interface Publisher<T> {
172	/**
173	* Adds the given Subscriber if possible. If already
174	* subscribed, or the attempt to subscribe fails due to policy
175	* violations or errors, the Subscriber's {@code onError}
176	* method is invoked with an {@link IllegalStateException}.
177	* Otherwise, the Subscriber's {@code onSubscribe} method is
178	* invoked with a new {@link Subscription}. Subscribers may
179	* enable receiving items by invoking the {@code request}
180	* method of this Subscription, and may unsubscribe by
181	* invoking its {@code cancel} method.
182	*
183	* @param subscriber the subscriber
184	* @throws NullPointerException if subscriber is null
185	*/
186	public void subscribe(Subscriber<? super T> subscriber);
187	}
188
189	/**
190	* A receiver of messages. The methods in this interface are
191	* invoked in strict sequential order for each {@link
192	* Subscription}.
193	*
194	* @param <T> the subscribed item type
195	*/
196	public static interface Subscriber<T> {
197	/**
198	* Method invoked prior to invoking any other Subscriber
199	* methods for the given Subscription. If this method throws
200	* an exception, resulting behavior is not guaranteed, but may
201	* cause the Subscription to be cancelled.
202	*
203	* <p>Typically, implementations of this method invoke {@code
204	* subscription.request} to enable receiving items.
205	*
206	* @param subscription a new subscription
207	*/
208	public void onSubscribe(Subscription subscription);
209
210	/**
211	* Method invoked with a Subscription's next item. If this
212	* method throws an exception, resulting behavior is not
213	* guaranteed, but may cause the Subscription to be cancelled.
214	*
215	* @param item the item
216	*/
217	public void onNext(T item);
218
219	/**
220	* Method invoked upon an unrecoverable error encountered by a
221	* Publisher or Subscription, after which no other Subscriber
222	* methods are invoked by the Subscription. If this method
223	* itself throws an exception, resulting behavior is
224	* undefined.
225	*
226	* @param throwable the exception
227	*/
228	public void onError(Throwable throwable);
229
230	/**
231	* Method invoked when it is known that no additional
232	* Subscriber method invocations will occur for a Subscription
233	* that is not already terminated by error, after which no
234	* other Subscriber methods are invoked by the Subscription.
235	* If this method throws an exception, resulting behavior is
236	* undefined.
237	*/
238	public void onComplete();
239	}
240
241	/**
242	* Message control linking a {@link Publisher} and {@link
243	* Subscriber}. Subscribers receive items only when requested,
244	* and may cancel at any time. The methods in this interface are
245	* intended to be invoked only by their Subscribers; usages in
246	* other contexts have undefined effects.
247	*/
248	public static interface Subscription {
249	/**
250	* Adds the given number {@code n} of items to the current
251	* unfulfilled demand for this subscription. If {@code n} is
252	* negative, the Subscriber will receive an {@code onError}
253	* signal with an {@link IllegalArgumentException} argument.
254	* Otherwise, the Subscriber will receive up to {@code n}
255	* additional {@code onNext} invocations (or fewer if
256	* terminated).
257	*
258	* @param n the increment of demand; a value of {@code
259	* Long.MAX_VALUE} may be considered as effectively unbounded
260	*/
261	public void request(long n);
262
263	/**
264	* Causes the Subscriber to (eventually) stop receiving
265	* messages. Implementation is best-effort -- additional
266	* messages may be received after invoking this method. A
267	* cancelled subscription need not ever receive an {@code
268	* onComplete} signal.
269	*/
270	public void cancel();
271	}
272
273	/**
274	* A component that acts as both a Subscriber and Publisher.
275	*
276	* @param <T> the subscribed item type
277	* @param <R> the published item type
278	*/
279	public static interface Processor<T,R> extends Subscriber<T>, Publisher<R> {
280	}
281
282	static final int DEFAULT_BUFFER_SIZE = 256;
283
284	/**
285	* Returns a default value for Publisher or Subscriber buffering,
286	* that may be used in the absence of other constraints.
287	*
288	* @implNote
289	* The current value returned is 256.
290	*
291	* @return the buffer size value
292	*/
293	public static int defaultBufferSize() {
294	return DEFAULT_BUFFER_SIZE;
295	}
296
297	}