Example synchronous range Publisher

examples/src/main/java/org/reactivestreams/example/lazycast/RangePublisher.java

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+/************************************************************************
+ * Licensed under Public Domain (CC0)                                    *
+ *                                                                       *
+ * To the extent possible under law, the person who associated CC0 with  *
+ * this code has waived all copyright and related or neighboring         *
+ * rights to this code.                                                  *
+ *                                                                       *
+ * You should have received a copy of the CC0 legalcode along with this  *
+ * work. If not, see <http://creativecommons.org/publicdomain/zero/1.0/>.*
+ ************************************************************************/
+
+package org.reactivestreams.example.lazycast;
+
+import org.reactivestreams.*;
+
+import java.util.concurrent.atomic.AtomicLong;
+
+/**
+ * A synchronous implementation of the {@link Publisher} that can
+ * be subscribed to multiple times and each individual subscription
+ * will receive range of monotonically increasing integer values on demand.
+ */
+public final class RangePublisher implements Publisher<Integer> {
+
+    /** The starting value of the range. */
+    final int start;
+
+    /** The number of items to emit. */
+    final int count;
+
+    /**
+     * Constructs a RangePublisher instance with the given start and count values
+     * that yields a sequence of [start, start + count).
+     * @param start the starting value of the range
+     * @param count the number of items to emit
+     */
+    public RangePublisher(int start, int count) {
+        this.start = start;
+        this.count = count;
+    }
+
+    @Override
+    public void subscribe(Subscriber<? super Integer> subscriber) {
+        // As per rule 1.11, we have decided to support multiple subscribers
+        // in a unicast configuration for this `Publisher` implementation.
+
+        // As per rule 1.09, we need to throw a `java.lang.NullPointerException`
+        // if the `Subscriber` is `null`
+        if (subscriber == null) throw null;
+
+        // As per 2.13, this method must return normally (i.e. not throw).
+        try {
+            subscriber.onSubscribe(new RangeSubscription(subscriber, start, start + count));
+        } catch (Throwable ex) {
+            new IllegalStateException(subscriber + " violated the Reactive Streams rule 2.13 " +
+                    "by throwing an exception from onSubscribe.", ex)
+                    // When onSubscribe fails this way, we don't know what state the
+                    // subscriber is thus calling onError may cause more crashes.
+                    .printStackTrace();
+        }
+    }
+
+    /**
+     * A Subscription implementation that holds the current downstream
+     * requested amount and responds to the downstream's request() and
+     * cancel() calls.
+     */
+    static final class RangeSubscription
+            // We are using this `AtomicLong` to make sure that this `Subscription`
+            // doesn't run concurrently with itself, which would violate rule 1.3
+            // among others (no concurrent notifications).
+            // The atomic transition from 0L to N > 0L will ensure this.
+            extends AtomicLong implements Subscription {
+
+        private static final long serialVersionUID = -9000845542177067735L;
+
+        /** The Subscriber we are emitting integer values to. */
+        final Subscriber<? super Integer> downstream;
+
+        /** The end index (exclusive). */
+        final int end;
+
+        /**
+         * The current index and within the [start, start + count) range that
+         * will be emitted as downstream.onNext().
+         */
+        int index;
+
+        /**
+         * Indicates the emission should stop.
+         */
+        volatile boolean cancelled;
+
+        /**
+         * Holds onto the IllegalArgumentException (containing the offending stacktrace)
+         * indicating there was a non-positive request() call from the downstream.
+         */
+        volatile Throwable invalidRequest;
+
+        /**
+         * Constructs a stateful RangeSubscription that emits signals to the given
+         * downstream from an integer range of [start, end).
+         * @param downstream the Subscriber receiving the integer values and the completion signal.
+         * @param start the first integer value emitted, start of the range
+         * @param end the end of the range, exclusive
+         */
+        RangeSubscription(Subscriber<? super Integer> downstream, int start, int end) {
+            this.downstream = downstream;
+            this.index = start;
+            this.end = end;
+        }
+
+        // This method will register inbound demand from our `Subscriber` and
+        // validate it against rule 3.9 and rule 3.17
+        @Override
+        public void request(long n) {
+            // Non-positive requests should be honored with IllegalArgumentException
+            if (n <= 0L) {
+                invalidRequest = new IllegalArgumentException("§3.9: non-positive requests are not allowed!");
+                n = 1;
+            }
+            // Downstream requests are cumulative and may come from any thread
+            for (;;) {
+                long requested = get();
+                long update = requested + n;
+                // As governed by rule 3.17, when demand overflows `Long.MAX_VALUE`
+                // we treat the signalled demand as "effectively unbounded"
+                if (update < 0L) {
+                    update = Long.MAX_VALUE;
+                }
+                // atomically update the current requested amount
+                if (compareAndSet(requested, update)) {
+                    // if there was no prior request amount, we start the emission loop
+                    if (requested == 0L) {
+                        emit(update);
+                    }
+                    break;
+                }
+            }
+        }
+
+        // This handles cancellation requests, and is idempotent, thread-safe and not
+        // synchronously performing heavy computations as specified in rule 3.5
+        @Override
+        public void cancel() {
+            // Indicate to the emission loop it should stop.
+            cancelled = true;
+        }
+
+        void emit(long currentRequested) {
+            // Load fields to avoid re-reading them from memory due to volatile accesses in the loop.
+            Subscriber<? super Integer> downstream = this.downstream;
+            int index = this.index;
+            int end = this.end;
+            int emitted = 0;
+
+            try {
+                for (; ; ) {
+                    // Check if there was an invalid request and then report its exception
+                    // as mandated by rule 3.9. The stacktrace in it should
+                    // help locate the faulty logic in the Subscriber.
+                    Throwable invalidRequest = this.invalidRequest;
+                    if (invalidRequest != null) {
+                        // When we signal onError, the subscription must be considered as cancelled, as per rule 1.6
+                        cancelled = true;
+
+                        downstream.onError(invalidRequest);
+                        return;
+                    }
+
+                    // Loop while the index hasn't reached the end and we haven't
+                    // emitted all that's been requested
+                    while (index != end && emitted != currentRequested) {
+                        // to make sure that we follow rule 1.8, 3.6 and 3.7
+                        // We stop if cancellation was requested.
+                        if (cancelled) {
+                            return;
+                        }
+
+                        downstream.onNext(index);
+
+                        // Increment the index for the next possible emission.
+                        index++;
+                        // Increment the emitted count to prevent overflowing the downstream.
+                        emitted++;
+                    }
+
+                    // If the index reached the end, we complete the downstream.
+                    if (index == end) {
+                        // to make sure that we follow rule 1.8, 3.6 and 3.7
+                        // Unless cancellation was requested by the last onNext.
+                        if (!cancelled) {
+                            // We need to consider this `Subscription` as cancelled as per rule 1.6
+                            // Note, however, that this state is not observable from the outside
+                            // world and since we leave the loop with requested > 0L, any
+                            // further request() will never trigger the loop.
+                            cancelled = true;
+
+                            downstream.onComplete();
+                        }
+                        return;
+                    }
+
+                    // Did the requested amount change while we were looping?
+                    long freshRequested = get();
+                    if (freshRequested == currentRequested) {
+                        // Save where the loop has left off: the next value to be emitted
+                        this.index = index;
+                        // Atomically subtract the previously requested (also emitted) amount
+                        currentRequested = addAndGet(-currentRequested);
+                        // If there was no new request in between get() and addAndGet(), we simply quit
+                        // The next 0 to N transition in request() will trigger the next emission loop.
+                        if (currentRequested == 0L) {
+                            break;
+                        }
+                        // Looks like there were more async requests, reset the emitted count and continue.
+                        emitted = 0;
+                    } else {
+                        // Yes, avoid the atomic subtraction and resume.
+                        // emitted != currentRequest in this case and index
+                        // still points to the next value to be emitted
+                        currentRequested = freshRequested;
+                    }
+                }
+            } catch (Throwable ex) {
+                // We can only get here if `onNext`, `onError` or `onComplete` threw, and they
+                // are not allowed to according to 2.13, so we can only cancel and log here.
+                // If `onError` throws an exception, this is a spec violation according to rule 1.9,
+                // and all we can do is to log it.
+
+                // Make sure that we are cancelled, since we cannot do anything else
+                // since the `Subscriber` is faulty.
+                cancelled = true;
+
+                // We can't report the failure to onError as the Subscriber is unreliable.
+                (new IllegalStateException(downstream + " violated the Reactive Streams rule 2.13 by " +
+                        "throwing an exception from onNext, onError or onComplete.", ex))
+                        .printStackTrace();
+            }
+        }
+    }
+}

examples/src/test/java/org/reactivestreams/example/lazycast/RangePublisherTest.java

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+/************************************************************************
+ * Licensed under Public Domain (CC0)                                    *
+ *                                                                       *
+ * To the extent possible under law, the person who associated CC0 with  *
+ * this code has waived all copyright and related or neighboring         *
+ * rights to this code.                                                  *
+ *                                                                       *
+ * You should have received a copy of the CC0 legalcode along with this  *
+ * work. If not, see <http://creativecommons.org/publicdomain/zero/1.0/>.*
+ ************************************************************************/
+
+package org.reactivestreams.example.lazycast;
+
+import org.reactivestreams.Publisher;
+import org.reactivestreams.tck.*;
+
+public class RangePublisherTest extends PublisherVerification<Integer> {
+    public RangePublisherTest() {
+        super(new TestEnvironment(50, 50));
+    }
+
+    @Override
+    public Publisher<Integer> createPublisher(long elements) {
+        return new RangePublisher(1, (int)elements);
+    }
+
+    @Override
+    public Publisher<Integer> createFailedPublisher() {
+        return null;
+    }
+}