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
21 * or visit www.oracle.com if you need additional information or have any
22 * questions.
23 */
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
36 package 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 (n != 0 && !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 1.9
165 */
166 public 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 * negative, the Subscriber will receive an {@code onError}
275 * signal with an {@link IllegalArgumentException} argument.
276 * Otherwise, the Subscriber will receive up to {@code n}
277 * additional {@code onNext} invocations (or fewer if
278 * 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 }