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 
36 package java.util.concurrent;
37 
38 // Android-changed: Remove reference to SubmissionPublisher class (not present on Android).
39 /**
40  * Interrelated interfaces and static methods for establishing
41  * flow-controlled components in which {@link Publisher Publishers}
42  * produce items consumed by one or more {@link Subscriber
43  * Subscribers}, each managed by a {@link Subscription
44  * Subscription}.
45  *
46  * <p>These interfaces correspond to the <a
47  * href="http://www.reactive-streams.org/"> reactive-streams</a>
48  * specification.  They apply in both concurrent and distributed
49  * asynchronous settings: All (seven) methods are defined in {@code
50  * void} "one-way" message style. Communication relies on a simple form
51  * of flow control (method {@link Subscription#request}) that can be
52  * used to avoid resource management problems that may otherwise occur
53  * in "push" based systems.
54  *
55  * <p><b>Examples.</b> A {@link Publisher} usually defines its own
56  * {@link Subscription} implementation; constructing one in method
57  * {@code subscribe} and issuing it to the calling {@link
58  * Subscriber}. It publishes items to the subscriber asynchronously,
59  * normally using an {@link Executor}.  For example, here is a very
60  * simple publisher that only issues (when requested) a single {@code
61  * TRUE} item to a single subscriber.  Because the subscriber receives
62  * only a single item, this class does not use buffering and ordering
63  * control required in most implementations.
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 9
165  */
166 public final class Flow {
167 
Flow()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          */
subscribe(Subscriber<? super T> subscriber)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          */
onSubscribe(Subscription subscription)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          */
onNext(T item)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          */
onError(Throwable throwable)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          */
onComplete()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          */
request(long n)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          */
cancel()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      */
defaultBufferSize()315     public static int defaultBufferSize() {
316         return DEFAULT_BUFFER_SIZE;
317     }
318 
319 }
320