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| 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 org.glassfish.jersey.internal.jsr166; |
| 8 | + |
| 9 | +/** |
| 10 | + * Interrelated interfaces and static methods for establishing |
| 11 | + * flow-controlled components in which {@link Publisher Publishers} |
| 12 | + * produce items consumed by one or more {@link Subscriber |
| 13 | + * Subscribers}, each managed by a {@link Subscription |
| 14 | + * Subscription}. |
| 15 | + * |
| 16 | + * <p>These interfaces correspond to the <a |
| 17 | + * href="http://www.reactive-streams.org/"> reactive-streams</a> |
| 18 | + * specification. They apply in both concurrent and distributed |
| 19 | + * asynchronous settings: All (seven) methods are defined in {@code |
| 20 | + * void} "one-way" message style. Communication relies on a simple form |
| 21 | + * of flow control (method {@link Subscription#request}) that can be |
| 22 | + * used to avoid resource management problems that may otherwise occur |
| 23 | + * in "push" based systems. |
| 24 | + * |
| 25 | + * <p><b>Examples.</b> A {@link Publisher} usually defines its own |
| 26 | + * {@link Subscription} implementation; constructing one in method |
| 27 | + * {@code subscribe} and issuing it to the calling {@link |
| 28 | + * Subscriber}. It publishes items to the subscriber asynchronously, |
| 29 | + * normally using an {@link Executor}. For example, here is a very |
| 30 | + * simple publisher that only issues (when requested) a single {@code |
| 31 | + * TRUE} item to a single subscriber. Because the subscriber receives |
| 32 | + * only a single item, this class does not use buffering and ordering |
| 33 | + * control required in most implementations (for example {@link |
| 34 | + * SubmissionPublisher}). |
| 35 | + * |
| 36 | + * <pre> {@code |
| 37 | + * class OneShotPublisher implements Publisher<Boolean> { |
| 38 | + * private final ExecutorService executor = ForkJoinPool.commonPool(); // daemon-based |
| 39 | + * private boolean subscribed; // true after first subscribe |
| 40 | + * public synchronized void subscribe(Subscriber<? super Boolean> subscriber) { |
| 41 | + * if (subscribed) |
| 42 | + * subscriber.onError(new IllegalStateException()); // only one allowed |
| 43 | + * else { |
| 44 | + * subscribed = true; |
| 45 | + * subscriber.onSubscribe(new OneShotSubscription(subscriber, executor)); |
| 46 | + * } |
| 47 | + * } |
| 48 | + * static class OneShotSubscription implements Subscription { |
| 49 | + * private final Subscriber<? super Boolean> subscriber; |
| 50 | + * private final ExecutorService executor; |
| 51 | + * private Future<?> future; // to allow cancellation |
| 52 | + * private boolean completed; |
| 53 | + * OneShotSubscription(Subscriber<? super Boolean> subscriber, |
| 54 | + * ExecutorService executor) { |
| 55 | + * this.subscriber = subscriber; |
| 56 | + * this.executor = executor; |
| 57 | + * } |
| 58 | + * public synchronized void request(long n) { |
| 59 | + * if (n != 0 && !completed) { |
| 60 | + * completed = true; |
| 61 | + * if (n < 0) { |
| 62 | + * IllegalArgumentException ex = new IllegalArgumentException(); |
| 63 | + * executor.execute(() -> subscriber.onError(ex)); |
| 64 | + * } else { |
| 65 | + * future = executor.submit(() -> { |
| 66 | + * subscriber.onNext(Boolean.TRUE); |
| 67 | + * subscriber.onComplete(); |
| 68 | + * }); |
| 69 | + * } |
| 70 | + * } |
| 71 | + * } |
| 72 | + * public synchronized void cancel() { |
| 73 | + * completed = true; |
| 74 | + * if (future != null) future.cancel(false); |
| 75 | + * } |
| 76 | + * } |
| 77 | + * }}</pre> |
| 78 | + * |
| 79 | + * <p>A {@link Subscriber} arranges that items be requested and |
| 80 | + * processed. Items (invocations of {@link Subscriber#onNext}) are |
| 81 | + * not issued unless requested, but multiple items may be requested. |
| 82 | + * Many Subscriber implementations can arrange this in the style of |
| 83 | + * the following example, where a buffer size of 1 single-steps, and |
| 84 | + * larger sizes usually allow for more efficient overlapped processing |
| 85 | + * with less communication; for example with a value of 64, this keeps |
| 86 | + * total outstanding requests between 32 and 64. |
| 87 | + * Because Subscriber method invocations for a given {@link |
| 88 | + * Subscription} are strictly ordered, there is no need for these |
| 89 | + * methods to use locks or volatiles unless a Subscriber maintains |
| 90 | + * multiple Subscriptions (in which case it is better to instead |
| 91 | + * define multiple Subscribers, each with its own Subscription). |
| 92 | + * |
| 93 | + * <pre> {@code |
| 94 | + * class SampleSubscriber<T> implements Subscriber<T> { |
| 95 | + * final Consumer<? super T> consumer; |
| 96 | + * Subscription subscription; |
| 97 | + * final long bufferSize; |
| 98 | + * long count; |
| 99 | + * SampleSubscriber(long bufferSize, Consumer<? super T> consumer) { |
| 100 | + * this.bufferSize = bufferSize; |
| 101 | + * this.consumer = consumer; |
| 102 | + * } |
| 103 | + * public void onSubscribe(Subscription subscription) { |
| 104 | + * long initialRequestSize = bufferSize; |
| 105 | + * count = bufferSize - bufferSize / 2; // re-request when half consumed |
| 106 | + * (this.subscription = subscription).request(initialRequestSize); |
| 107 | + * } |
| 108 | + * public void onNext(T item) { |
| 109 | + * if (--count <= 0) |
| 110 | + * subscription.request(count = bufferSize - bufferSize / 2); |
| 111 | + * consumer.accept(item); |
| 112 | + * } |
| 113 | + * public void onError(Throwable ex) { ex.printStackTrace(); } |
| 114 | + * public void onComplete() {} |
| 115 | + * }}</pre> |
| 116 | + * |
| 117 | + * <p>The default value of {@link #defaultBufferSize} may provide a |
| 118 | + * useful starting point for choosing request sizes and capacities in |
| 119 | + * Flow components based on expected rates, resources, and usages. |
| 120 | + * Or, when flow control is never needed, a subscriber may initially |
| 121 | + * request an effectively unbounded number of items, as in: |
| 122 | + * |
| 123 | + * <pre> {@code |
| 124 | + * class UnboundedSubscriber<T> implements Subscriber<T> { |
| 125 | + * public void onSubscribe(Subscription subscription) { |
| 126 | + * subscription.request(Long.MAX_VALUE); // effectively unbounded |
| 127 | + * } |
| 128 | + * public void onNext(T item) { use(item); } |
| 129 | + * public void onError(Throwable ex) { ex.printStackTrace(); } |
| 130 | + * public void onComplete() {} |
| 131 | + * void use(T item) { ... } |
| 132 | + * }}</pre> |
| 133 | + * |
| 134 | + * @author Doug Lea |
| 135 | + * @since 9 |
| 136 | + */ |
| 137 | +public final class Flow { |
| 138 | + |
| 139 | + private Flow() {} // uninstantiable |
| 140 | + |
| 141 | + /** |
| 142 | + * A producer of items (and related control messages) received by |
| 143 | + * Subscribers. Each current {@link Subscriber} receives the same |
| 144 | + * items (via method {@code onNext}) in the same order, unless |
| 145 | + * drops or errors are encountered. If a Publisher encounters an |
| 146 | + * error that does not allow items to be issued to a Subscriber, |
| 147 | + * that Subscriber receives {@code onError}, and then receives no |
| 148 | + * further messages. Otherwise, when it is known that no further |
| 149 | + * messages will be issued to it, a subscriber receives {@code |
| 150 | + * onComplete}. Publishers ensure that Subscriber method |
| 151 | + * invocations for each subscription are strictly ordered in <a |
| 152 | + * href="package-summary.html#MemoryVisibility"><i>happens-before</i></a> |
| 153 | + * order. |
| 154 | + * |
| 155 | + * <p>Publishers may vary in policy about whether drops (failures |
| 156 | + * to issue an item because of resource limitations) are treated |
| 157 | + * as unrecoverable errors. Publishers may also vary about |
| 158 | + * whether Subscribers receive items that were produced or |
| 159 | + * available before they subscribed. |
| 160 | + * |
| 161 | + * @param <T> the published item type |
| 162 | + */ |
| 163 | + @FunctionalInterface |
| 164 | + public static interface Publisher<T> { |
| 165 | + /** |
| 166 | + * Adds the given Subscriber if possible. If already |
| 167 | + * subscribed, or the attempt to subscribe fails due to policy |
| 168 | + * violations or errors, the Subscriber's {@code onError} |
| 169 | + * method is invoked with an {@link IllegalStateException}. |
| 170 | + * Otherwise, the Subscriber's {@code onSubscribe} method is |
| 171 | + * invoked with a new {@link Subscription}. Subscribers may |
| 172 | + * enable receiving items by invoking the {@code request} |
| 173 | + * method of this Subscription, and may unsubscribe by |
| 174 | + * invoking its {@code cancel} method. |
| 175 | + * |
| 176 | + * @param subscriber the subscriber |
| 177 | + * @throws NullPointerException if subscriber is null |
| 178 | + */ |
| 179 | + public void subscribe(Subscriber<? super T> subscriber); |
| 180 | + } |
| 181 | + |
| 182 | + /** |
| 183 | + * A receiver of messages. The methods in this interface are |
| 184 | + * invoked in strict sequential order for each {@link |
| 185 | + * Subscription}. |
| 186 | + * |
| 187 | + * @param <T> the subscribed item type |
| 188 | + */ |
| 189 | + public static interface Subscriber<T> { |
| 190 | + /** |
| 191 | + * Method invoked prior to invoking any other Subscriber |
| 192 | + * methods for the given Subscription. If this method throws |
| 193 | + * an exception, resulting behavior is not guaranteed, but may |
| 194 | + * cause the Subscription not to be established or to be cancelled. |
| 195 | + * |
| 196 | + * <p>Typically, implementations of this method invoke {@code |
| 197 | + * subscription.request} to enable receiving items. |
| 198 | + * |
| 199 | + * @param subscription a new subscription |
| 200 | + */ |
| 201 | + public void onSubscribe(Subscription subscription); |
| 202 | + |
| 203 | + /** |
| 204 | + * Method invoked with a Subscription's next item. If this |
| 205 | + * method throws an exception, resulting behavior is not |
| 206 | + * guaranteed, but may cause the Subscription to be cancelled. |
| 207 | + * |
| 208 | + * @param item the item |
| 209 | + */ |
| 210 | + public void onNext(T item); |
| 211 | + |
| 212 | + /** |
| 213 | + * Method invoked upon an unrecoverable error encountered by a |
| 214 | + * Publisher or Subscription, after which no other Subscriber |
| 215 | + * methods are invoked by the Subscription. If this method |
| 216 | + * itself throws an exception, resulting behavior is |
| 217 | + * undefined. |
| 218 | + * |
| 219 | + * @param throwable the exception |
| 220 | + */ |
| 221 | + public void onError(Throwable throwable); |
| 222 | + |
| 223 | + /** |
| 224 | + * Method invoked when it is known that no additional |
| 225 | + * Subscriber method invocations will occur for a Subscription |
| 226 | + * that is not already terminated by error, after which no |
| 227 | + * other Subscriber methods are invoked by the Subscription. |
| 228 | + * If this method throws an exception, resulting behavior is |
| 229 | + * undefined. |
| 230 | + */ |
| 231 | + public void onComplete(); |
| 232 | + } |
| 233 | + |
| 234 | + /** |
| 235 | + * Message control linking a {@link Publisher} and {@link |
| 236 | + * Subscriber}. Subscribers receive items only when requested, |
| 237 | + * and may cancel at any time. The methods in this interface are |
| 238 | + * intended to be invoked only by their Subscribers; usages in |
| 239 | + * other contexts have undefined effects. |
| 240 | + */ |
| 241 | + public static interface Subscription { |
| 242 | + /** |
| 243 | + * Adds the given number {@code n} of items to the current |
| 244 | + * unfulfilled demand for this subscription. If {@code n} is |
| 245 | + * negative, the Subscriber will receive an {@code onError} |
| 246 | + * signal with an {@link IllegalArgumentException} argument. |
| 247 | + * Otherwise, the Subscriber will receive up to {@code n} |
| 248 | + * additional {@code onNext} invocations (or fewer if |
| 249 | + * terminated). |
| 250 | + * |
| 251 | + * @param n the increment of demand; a value of {@code |
| 252 | + * Long.MAX_VALUE} may be considered as effectively unbounded |
| 253 | + */ |
| 254 | + public void request(long n); |
| 255 | + |
| 256 | + /** |
| 257 | + * Causes the Subscriber to (eventually) stop receiving |
| 258 | + * messages. Implementation is best-effort -- additional |
| 259 | + * messages may be received after invoking this method. |
| 260 | + * A cancelled subscription need not ever receive an |
| 261 | + * {@code onComplete} or {@code onError} signal. |
| 262 | + */ |
| 263 | + public void cancel(); |
| 264 | + } |
| 265 | + |
| 266 | + /** |
| 267 | + * A component that acts as both a Subscriber and Publisher. |
| 268 | + * |
| 269 | + * @param <T> the subscribed item type |
| 270 | + * @param <R> the published item type |
| 271 | + */ |
| 272 | + public static interface Processor<T, R> extends Subscriber<T>, Publisher<R> { |
| 273 | + } |
| 274 | + |
| 275 | + static final int DEFAULT_BUFFER_SIZE = 256; |
| 276 | + |
| 277 | + /** |
| 278 | + * Returns a default value for Publisher or Subscriber buffering, |
| 279 | + * that may be used in the absence of other constraints. |
| 280 | + * |
| 281 | + * @implNote |
| 282 | + * The current value returned is 256. |
| 283 | + * |
| 284 | + * @return the buffer size value |
| 285 | + */ |
| 286 | + public static int defaultBufferSize() { |
| 287 | + return DEFAULT_BUFFER_SIZE; |
| 288 | + } |
| 289 | + |
| 290 | +} |
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