1/*
2 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
3 *
4 * This code is free software; you can redistribute it and/or modify it
5 * under the terms of the GNU General Public License version 2 only, as
6 * published by the Free Software Foundation.  Oracle designates this
7 * particular file as subject to the "Classpath" exception as provided
8 * by Oracle in the LICENSE file that accompanied this code.
9 *
10 * This code is distributed in the hope that it will be useful, but WITHOUT
11 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
12 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
13 * version 2 for more details (a copy is included in the LICENSE file that
14 * accompanied this code).
15 *
16 * You should have received a copy of the GNU General Public License version
17 * 2 along with this work; if not, write to the Free Software Foundation,
18 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
19 *
20 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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24
25/*
26 * This file is available under and governed by the GNU General Public
27 * License version 2 only, as published by the Free Software Foundation.
28 * However, the following notice accompanied the original version of this
29 * file:
30 *
31 * Written by Doug Lea with assistance from members of JCP JSR-166
32 * Expert Group and released to the public domain, as explained at
33 * http://creativecommons.org/publicdomain/zero/1.0/
34 */
35
36package java.util.concurrent;
37
38/**
39 * Interrelated interfaces and static methods for establishing
40 * flow-controlled components in which {@link Publisher Publishers}
41 * produce items consumed by one or more {@link Subscriber
42 * Subscribers}, each managed by a {@link Subscription
43 * Subscription}.
44 *
45 * <p>These interfaces correspond to the <a
46 * href="http://www.reactive-streams.org/"> reactive-streams</a>
47 * specification.  They apply in both concurrent and distributed
48 * asynchronous settings: All (seven) methods are defined in {@code
49 * void} "one-way" message style. Communication relies on a simple form
50 * of flow control (method {@link Subscription#request}) that can be
51 * used to avoid resource management problems that may otherwise occur
52 * in "push" based systems.
53 *
54 * <p><b>Examples.</b> A {@link Publisher} usually defines its own
55 * {@link Subscription} implementation; constructing one in method
56 * {@code subscribe} and issuing it to the calling {@link
57 * Subscriber}. It publishes items to the subscriber asynchronously,
58 * normally using an {@link Executor}.  For example, here is a very
59 * simple publisher that only issues (when requested) a single {@code
60 * TRUE} item to a single subscriber.  Because the subscriber receives
61 * only a single item, this class does not use buffering and ordering
62 * control required in most implementations (for example {@link
63 * SubmissionPublisher}).
64 *
65 * <pre> {@code
66 * class OneShotPublisher implements Publisher<Boolean> {
67 *   private final ExecutorService executor = ForkJoinPool.commonPool(); // daemon-based
68 *   private boolean subscribed; // true after first subscribe
69 *   public synchronized void subscribe(Subscriber<? super Boolean> subscriber) {
70 *     if (subscribed)
71 *       subscriber.onError(new IllegalStateException()); // only one allowed
72 *     else {
73 *       subscribed = true;
74 *       subscriber.onSubscribe(new OneShotSubscription(subscriber, executor));
75 *     }
76 *   }
77 *   static class OneShotSubscription implements Subscription {
78 *     private final Subscriber<? super Boolean> subscriber;
79 *     private final ExecutorService executor;
80 *     private Future<?> future; // to allow cancellation
81 *     private boolean completed;
82 *     OneShotSubscription(Subscriber<? super Boolean> subscriber,
83 *                         ExecutorService executor) {
84 *       this.subscriber = subscriber;
85 *       this.executor = executor;
86 *     }
87 *     public synchronized void request(long n) {
88 *       if (!completed) {
89 *         completed = true;
90 *         if (n <= 0) {
91 *           IllegalArgumentException ex = new IllegalArgumentException();
92 *           executor.execute(() -> subscriber.onError(ex));
93 *         } else {
94 *           future = executor.submit(() -> {
95 *             subscriber.onNext(Boolean.TRUE);
96 *             subscriber.onComplete();
97 *           });
98 *         }
99 *       }
100 *     }
101 *     public synchronized void cancel() {
102 *       completed = true;
103 *       if (future != null) future.cancel(false);
104 *     }
105 *   }
106 * }}</pre>
107 *
108 * <p>A {@link Subscriber} arranges that items be requested and
109 * processed.  Items (invocations of {@link Subscriber#onNext}) are
110 * not issued unless requested, but multiple items may be requested.
111 * Many Subscriber implementations can arrange this in the style of
112 * the following example, where a buffer size of 1 single-steps, and
113 * larger sizes usually allow for more efficient overlapped processing
114 * with less communication; for example with a value of 64, this keeps
115 * total outstanding requests between 32 and 64.
116 * Because Subscriber method invocations for a given {@link
117 * Subscription} are strictly ordered, there is no need for these
118 * methods to use locks or volatiles unless a Subscriber maintains
119 * multiple Subscriptions (in which case it is better to instead
120 * define multiple Subscribers, each with its own Subscription).
121 *
122 * <pre> {@code
123 * class SampleSubscriber<T> implements Subscriber<T> {
124 *   final Consumer<? super T> consumer;
125 *   Subscription subscription;
126 *   final long bufferSize;
127 *   long count;
128 *   SampleSubscriber(long bufferSize, Consumer<? super T> consumer) {
129 *     this.bufferSize = bufferSize;
130 *     this.consumer = consumer;
131 *   }
132 *   public void onSubscribe(Subscription subscription) {
133 *     long initialRequestSize = bufferSize;
134 *     count = bufferSize - bufferSize / 2; // re-request when half consumed
135 *     (this.subscription = subscription).request(initialRequestSize);
136 *   }
137 *   public void onNext(T item) {
138 *     if (--count <= 0)
139 *       subscription.request(count = bufferSize - bufferSize / 2);
140 *     consumer.accept(item);
141 *   }
142 *   public void onError(Throwable ex) { ex.printStackTrace(); }
143 *   public void onComplete() {}
144 * }}</pre>
145 *
146 * <p>The default value of {@link #defaultBufferSize} may provide a
147 * useful starting point for choosing request sizes and capacities in
148 * Flow components based on expected rates, resources, and usages.
149 * Or, when flow control is never needed, a subscriber may initially
150 * request an effectively unbounded number of items, as in:
151 *
152 * <pre> {@code
153 * class UnboundedSubscriber<T> implements Subscriber<T> {
154 *   public void onSubscribe(Subscription subscription) {
155 *     subscription.request(Long.MAX_VALUE); // effectively unbounded
156 *   }
157 *   public void onNext(T item) { use(item); }
158 *   public void onError(Throwable ex) { ex.printStackTrace(); }
159 *   public void onComplete() {}
160 *   void use(T item) { ... }
161 * }}</pre>
162 *
163 * @author Doug Lea
164 * @since 9
165 */
166public final class Flow {
167
168    private Flow() {} // uninstantiable
169
170    /**
171     * A producer of items (and related control messages) received by
172     * Subscribers.  Each current {@link Subscriber} receives the same
173     * items (via method {@code onNext}) in the same order, unless
174     * drops or errors are encountered. If a Publisher encounters an
175     * error that does not allow items to be issued to a Subscriber,
176     * that Subscriber receives {@code onError}, and then receives no
177     * further messages.  Otherwise, when it is known that no further
178     * messages will be issued to it, a subscriber receives {@code
179     * onComplete}.  Publishers ensure that Subscriber method
180     * invocations for each subscription are strictly ordered in <a
181     * href="package-summary.html#MemoryVisibility"><i>happens-before</i></a>
182     * order.
183     *
184     * <p>Publishers may vary in policy about whether drops (failures
185     * to issue an item because of resource limitations) are treated
186     * as unrecoverable errors.  Publishers may also vary about
187     * whether Subscribers receive items that were produced or
188     * available before they subscribed.
189     *
190     * @param <T> the published item type
191     */
192    @FunctionalInterface
193    public static interface Publisher<T> {
194        /**
195         * Adds the given Subscriber if possible.  If already
196         * subscribed, or the attempt to subscribe fails due to policy
197         * violations or errors, the Subscriber's {@code onError}
198         * method is invoked with an {@link IllegalStateException}.
199         * Otherwise, the Subscriber's {@code onSubscribe} method is
200         * invoked with a new {@link Subscription}.  Subscribers may
201         * enable receiving items by invoking the {@code request}
202         * method of this Subscription, and may unsubscribe by
203         * invoking its {@code cancel} method.
204         *
205         * @param subscriber the subscriber
206         * @throws NullPointerException if subscriber is null
207         */
208        public void subscribe(Subscriber<? super T> subscriber);
209    }
210
211    /**
212     * A receiver of messages.  The methods in this interface are
213     * invoked in strict sequential order for each {@link
214     * Subscription}.
215     *
216     * @param <T> the subscribed item type
217     */
218    public static interface Subscriber<T> {
219        /**
220         * Method invoked prior to invoking any other Subscriber
221         * methods for the given Subscription. If this method throws
222         * an exception, resulting behavior is not guaranteed, but may
223         * cause the Subscription not to be established or to be cancelled.
224         *
225         * <p>Typically, implementations of this method invoke {@code
226         * subscription.request} to enable receiving items.
227         *
228         * @param subscription a new subscription
229         */
230        public void onSubscribe(Subscription subscription);
231
232        /**
233         * Method invoked with a Subscription's next item.  If this
234         * method throws an exception, resulting behavior is not
235         * guaranteed, but may cause the Subscription to be cancelled.
236         *
237         * @param item the item
238         */
239        public void onNext(T item);
240
241        /**
242         * Method invoked upon an unrecoverable error encountered by a
243         * Publisher or Subscription, after which no other Subscriber
244         * methods are invoked by the Subscription.  If this method
245         * itself throws an exception, resulting behavior is
246         * undefined.
247         *
248         * @param throwable the exception
249         */
250        public void onError(Throwable throwable);
251
252        /**
253         * Method invoked when it is known that no additional
254         * Subscriber method invocations will occur for a Subscription
255         * that is not already terminated by error, after which no
256         * other Subscriber methods are invoked by the Subscription.
257         * If this method throws an exception, resulting behavior is
258         * undefined.
259         */
260        public void onComplete();
261    }
262
263    /**
264     * Message control linking a {@link Publisher} and {@link
265     * Subscriber}.  Subscribers receive items only when requested,
266     * and may cancel at any time. The methods in this interface are
267     * intended to be invoked only by their Subscribers; usages in
268     * other contexts have undefined effects.
269     */
270    public static interface Subscription {
271        /**
272         * Adds the given number {@code n} of items to the current
273         * unfulfilled demand for this subscription.  If {@code n} is
274         * less than or equal to zero, the Subscriber will receive an
275         * {@code onError} signal with an {@link
276         * IllegalArgumentException} argument.  Otherwise, the
277         * Subscriber will receive up to {@code n} additional {@code
278         * onNext} invocations (or fewer if terminated).
279         *
280         * @param n the increment of demand; a value of {@code
281         * Long.MAX_VALUE} may be considered as effectively unbounded
282         */
283        public void request(long n);
284
285        /**
286         * Causes the Subscriber to (eventually) stop receiving
287         * messages.  Implementation is best-effort -- additional
288         * messages may be received after invoking this method.
289         * A cancelled subscription need not ever receive an
290         * {@code onComplete} or {@code onError} signal.
291         */
292        public void cancel();
293    }
294
295    /**
296     * A component that acts as both a Subscriber and Publisher.
297     *
298     * @param <T> the subscribed item type
299     * @param <R> the published item type
300     */
301    public static interface Processor<T,R> extends Subscriber<T>, Publisher<R> {
302    }
303
304    static final int DEFAULT_BUFFER_SIZE = 256;
305
306    /**
307     * Returns a default value for Publisher or Subscriber buffering,
308     * that may be used in the absence of other constraints.
309     *
310     * @implNote
311     * The current value returned is 256.
312     *
313     * @return the buffer size value
314     */
315    public static int defaultBufferSize() {
316        return DEFAULT_BUFFER_SIZE;
317    }
318
319}
320