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:	1.27
Committed:	Mon Sep 14 06:17:32 2015 UTC (8 years, 8 months ago) by jsr166
Branch:	MAIN
Changes since 1.26:	+3 -3 lines
Log Message:	whitespace
#	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; // 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;
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	* IllegalArgumentException ex = new IllegalArgumentException();
68	* executor.execute(() -> subscriber.onError(ex));
69	* } else {
70	* future = executor.submit(() -> {
71	* subscriber.onNext(Boolean.TRUE);
72	* subscriber.onComplete();
73	* });
74	* }
75	* }
76	* }
77	* public synchronized void cancel() {
78	* completed = true;
79	* if (future != null) future.cancel(false);
80	* }
81	* }
82	* }}</pre>
83	*
84	* <p>A {@link Subscriber} arranges that items be requested and
85	* 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	* total outstanding requests between 32 and 64.
92	* 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	*
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	* long initialRequestSize = bufferSize;
110	* count = bufferSize - bufferSize / 2; // re-request when half consumed
111	* (this.subscription = subscription).request(initialRequestSize);
112	* }
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	*
122	* <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	* Or, when flow control is never needed, a subscriber may initially
126	* request an effectively unbounded number of items, as in:
127	*
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	*
139	* @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	* items (via method {@code onNext}) in the same order, unless
150	* drops or errors are encountered. If a Publisher encounters an
151	* 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	* invocations for each subscription are strictly ordered in <a
157	* href="package-summary.html#MemoryVisibility"><i>happens-before</i></a>
158	* order.
159	*
160	* <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	*
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	* 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	*
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	* A receiver of messages. The methods in this interface are
189	* invoked in strict sequential order for each {@link
190	* Subscription}.
191	*
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	* cause the Subscription not to be established or to be cancelled.
200	*
201	* <p>Typically, implementations of this method invoke {@code
202	* subscription.request} to enable receiving items.
203	*
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	* 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	* undefined.
235	*/
236	public void onComplete();
237	}
238
239	/**
240	* 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	*/
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	* negative, the Subscriber will receive an {@code onError}
251	* 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	*
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	* Causes the Subscriber to (eventually) stop receiving
263	* messages. Implementation is best-effort -- additional
264	* messages may be received after invoking this method.
265	* A cancelled subscription need not ever receive an
266	* {@code onComplete} or {@code onError} signal.
267	*/
268	public void cancel();
269	}
270
271	/**
272	* A component that acts as both a Subscriber and Publisher.
273	*
274	* @param <T> the subscribed item type
275	* @param <R> the published item type
276	*/
277	public static interface Processor<T,R> extends Subscriber<T>, Publisher<R> {
278	}
279
280	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
295	}