jdk-sandbox: comparison test/jdk/java/net/httpclient/reactivestreams-tck/org/reactivestreams/example/unicast/AsyncIterablePublisher.java

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+/*
+* Copyright (c) 2019, Oracle and/or its affiliates. All rights reserved.
+* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+*
+* This code is free software; you can redistribute it and/or modify it
+* under the terms of the GNU General Public License version 2 only, as
+* published by the Free Software Foundation.
+*
+* This code is distributed in the hope that it will be useful, but WITHOUT
+* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+* FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+* version 2 for more details (a copy is included in the LICENSE file that
+* accompanied this code).
+*
+* You should have received a copy of the GNU General Public License version
+* 2 along with this work; if not, write to the Free Software Foundation,
+* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+*
+* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+* or visit www.oracle.com if you need additional information or have any
+* questions.
+*/
+package org.reactivestreams.example.unicast;
+import org.reactivestreams.Publisher;
+import org.reactivestreams.Subscriber;
+import org.reactivestreams.Subscription;
+import java.util.Iterator;
+import java.util.Collections;
+import java.util.concurrent.Executor;
+import java.util.concurrent.atomic.AtomicBoolean;
+import java.util.concurrent.ConcurrentLinkedQueue;
+/**
+* AsyncIterablePublisher is an implementation of Reactive Streams `Publisher`
+* which executes asynchronously, using a provided `Executor` and produces elements
+* from a given `Iterable` in a "unicast" configuration to its `Subscribers`.
+*
+* NOTE: The code below uses a lot of try-catches to show the reader where exceptions can be expected, and where they are forbidden.
+*/
+public class AsyncIterablePublisher<T> implements Publisher<T> {
+private final static int DEFAULT_BATCHSIZE = 1024;
+private final Iterable<T> elements; // This is our data source / generator
+private final Executor executor; // This is our thread pool, which will make sure that our Publisher runs asynchronously to its Subscribers
+private final int batchSize; // In general, if one uses an `Executor`, one should be nice nad not hog a thread for too long, this is the cap for that, in elements
+public AsyncIterablePublisher(final Iterable<T> elements, final Executor executor) {
+this(elements, DEFAULT_BATCHSIZE, executor);
+}
+public AsyncIterablePublisher(final Iterable<T> elements, final int batchSize, final Executor executor) {
+if (elements == null) throw null;
+if (executor == null) throw null;
+if (batchSize < 1) throw new IllegalArgumentException("batchSize must be greater than zero!");
+this.elements = elements;
+this.executor = executor;
+this.batchSize = batchSize;
+}
+@Override
+public void subscribe(final Subscriber<? super T> s) {
+// As per rule 1.11, we have decided to support multiple subscribers in a unicast configuration
+// for this `Publisher` implementation.
+// As per 2.13, this method must return normally (i.e. not throw)
+new SubscriptionImpl(s).init();
+}
+// These represent the protocol of the `AsyncIterablePublishers` SubscriptionImpls
+static interface Signal {};
+enum Cancel implements Signal { Instance; };
+enum Subscribe implements Signal { Instance; };
+enum Send implements Signal { Instance; };
+static final class Request implements Signal {
+final long n;
+Request(final long n) {
+this.n = n;
+}
+};
+// This is our implementation of the Reactive Streams `Subscription`,
+// which represents the association between a `Publisher` and a `Subscriber`.
+final class SubscriptionImpl implements Subscription, Runnable {
+final Subscriber<? super T> subscriber; // We need a reference to the `Subscriber` so we can talk to it
+private boolean cancelled = false; // This flag will track whether this `Subscription` is to be considered cancelled or not
+private long demand = 0; // Here we track the current demand, i.e. what has been requested but not yet delivered
+private Iterator<T> iterator; // This is our cursor into the data stream, which we will send to the `Subscriber`
+SubscriptionImpl(final Subscriber<? super T> subscriber) {
+// As per rule 1.09, we need to throw a `java.lang.NullPointerException` if the `Subscriber` is `null`
+if (subscriber == null) throw null;
+this.subscriber = subscriber;
+}
+// This `ConcurrentLinkedQueue` will track signals that are sent to this `Subscription`, like `request` and `cancel`
+private final ConcurrentLinkedQueue<Signal> inboundSignals = new ConcurrentLinkedQueue<Signal>();
+// We are using this `AtomicBoolean` to make sure that this `Subscription` doesn't run concurrently with itself,
+// which would violate rule 1.3 among others (no concurrent notifications).
+private final AtomicBoolean on = new AtomicBoolean(false);
+// This method will register inbound demand from our `Subscriber` and validate it against rule 3.9 and rule 3.17
+private void doRequest(final long n) {
+if (n < 1)
+terminateDueTo(new IllegalArgumentException(subscriber + " violated the Reactive Streams rule 3.9 by requesting a non-positive number of elements."));
+else if (demand + n < 1) {
+// As governed by rule 3.17, when demand overflows `Long.MAX_VALUE` we treat the signalled demand as "effectively unbounded"
+demand = Long.MAX_VALUE;  // Here we protect from the overflow and treat it as "effectively unbounded"
+doSend(); // Then we proceed with sending data downstream
+} else {
+demand += n; // Here we record the downstream demand
+doSend(); // Then we can proceed with sending data downstream
+}
+}
+// This handles cancellation requests, and is idempotent, thread-safe and not synchronously performing heavy computations as specified in rule 3.5
+private void doCancel() {
+cancelled = true;
+}
+// Instead of executing `subscriber.onSubscribe` synchronously from within `Publisher.subscribe`
+// we execute it asynchronously, this is to avoid executing the user code (`Iterable.iterator`) on the calling thread.
+// It also makes it easier to follow rule 1.9
+private void doSubscribe() {
+try {
+iterator = elements.iterator();
+if (iterator == null)
+iterator = Collections.<T>emptyList().iterator(); // So we can assume that `iterator` is never null
+} catch(final Throwable t) {
+subscriber.onSubscribe(new Subscription() { // We need to make sure we signal onSubscribe before onError, obeying rule 1.9
+@Override public void cancel() {}
+@Override public void request(long n) {}
+});
+terminateDueTo(t); // Here we send onError, obeying rule 1.09
+}
+if (!cancelled) {
+// Deal with setting up the subscription with the subscriber
+try {
+subscriber.onSubscribe(this);
+} catch(final Throwable t) { // Due diligence to obey 2.13
+terminateDueTo(new IllegalStateException(subscriber + " violated the Reactive Streams rule 2.13 by throwing an exception from onSubscribe.", t));
+}
+// Deal with already complete iterators promptly
+boolean hasElements = false;
+try {
+hasElements = iterator.hasNext();
+} catch(final Throwable t) {
+terminateDueTo(t); // If hasNext throws, there's something wrong and we need to signal onError as per 1.2, 1.4,
+}
+// If we don't have anything to deliver, we're already done, so lets do the right thing and
+// not wait for demand to deliver `onComplete` as per rule 1.2 and 1.3
+if (!hasElements) {
+try {
+doCancel(); // Rule 1.6 says we need to consider the `Subscription` cancelled when `onComplete` is signalled
+subscriber.onComplete();
+} catch(final Throwable t) { // As per rule 2.13, `onComplete` is not allowed to throw exceptions, so we do what we can, and log this.
+(new IllegalStateException(subscriber + " violated the Reactive Streams rule 2.13 by throwing an exception from onComplete.", t)).printStackTrace(System.err);
+}
+}
+}
+}
+// This is our behavior for producing elements downstream
+private void doSend() {
+try {
+// In order to play nice with the `Executor` we will only send at-most `batchSize` before
+// rescheduing ourselves and relinquishing the current thread.
+int leftInBatch = batchSize;
+do {
+T next;
+boolean hasNext;
+try {
+next = iterator.next(); // We have already checked `hasNext` when subscribing, so we can fall back to testing -after- `next` is called.
+hasNext = iterator.hasNext(); // Need to keep track of End-of-Stream
+} catch (final Throwable t) {
+terminateDueTo(t); // If `next` or `hasNext` throws (they can, since it is user-provided), we need to treat the stream as errored as per rule 1.4
+return;
+}
+subscriber.onNext(next); // Then we signal the next element downstream to the `Subscriber`
+if (!hasNext) { // If we are at End-of-Stream
+doCancel(); // We need to consider this `Subscription` as cancelled as per rule 1.6
+subscriber.onComplete(); // Then we signal `onComplete` as per rule 1.2 and 1.5
+}
+} while (!cancelled           // This makes sure that rule 1.8 is upheld, i.e. we need to stop signalling "eventually"
+&& --leftInBatch > 0 // This makes sure that we only send `batchSize` number of elements in one go (so we can yield to other Runnables)
+&& --demand > 0);    // This makes sure that rule 1.1 is upheld (sending more than was demanded)
+if (!cancelled && demand > 0) // If the `Subscription` is still alive and well, and we have demand to satisfy, we signal ourselves to send more data
+signal(Send.Instance);
+} catch(final Throwable t) {
+// We can only get here if `onNext` or `onComplete` threw, and they are not allowed to according to 2.13, so we can only cancel and log here.
+doCancel(); // Make sure that we are cancelled, since we cannot do anything else since the `Subscriber` is faulty.
+(new IllegalStateException(subscriber + " violated the Reactive Streams rule 2.13 by throwing an exception from onNext or onComplete.", t)).printStackTrace(System.err);
+}
+}
+// This is a helper method to ensure that we always `cancel` when we signal `onError` as per rule 1.6
+private void terminateDueTo(final Throwable t) {
+cancelled = true; // When we signal onError, the subscription must be considered as cancelled, as per rule 1.6
+try {
+subscriber.onError(t); // Then we signal the error downstream, to the `Subscriber`
+} catch(final Throwable t2) { // If `onError` throws an exception, this is a spec violation according to rule 1.9, and all we can do is to log it.
+(new IllegalStateException(subscriber + " violated the Reactive Streams rule 2.13 by throwing an exception from onError.", t2)).printStackTrace(System.err);
+}
+}
+// What `signal` does is that it sends signals to the `Subscription` asynchronously
+private void signal(final Signal signal) {
+if (inboundSignals.offer(signal)) // No need to null-check here as ConcurrentLinkedQueue does this for us
+tryScheduleToExecute(); // Then we try to schedule it for execution, if it isn't already
+}
+// This is the main "event loop" if you so will
+@Override public final void run() {
+if(on.get()) { // establishes a happens-before relationship with the end of the previous run
+try {
+final Signal s = inboundSignals.poll(); // We take a signal off the queue
+if (!cancelled) { // to make sure that we follow rule 1.8, 3.6 and 3.7
+// Below we simply unpack the `Signal`s and invoke the corresponding methods
+if (s instanceof Request)
+doRequest(((Request)s).n);
+else if (s == Send.Instance)
+doSend();
+else if (s == Cancel.Instance)
+doCancel();
+else if (s == Subscribe.Instance)
+doSubscribe();
+}
+} finally {
+on.set(false); // establishes a happens-before relationship with the beginning of the next run
+if(!inboundSignals.isEmpty()) // If we still have signals to process
+tryScheduleToExecute(); // Then we try to schedule ourselves to execute again
+}
+}
+}
+// This method makes sure that this `Subscription` is only running on one Thread at a time,
+// this is important to make sure that we follow rule 1.3
+private final void tryScheduleToExecute() {
+if(on.compareAndSet(false, true)) {
+try {
+executor.execute(this);
+} catch(Throwable t) { // If we can't run on the `Executor`, we need to fail gracefully
+if (!cancelled) {
+doCancel(); // First of all, this failure is not recoverable, so we need to follow rule 1.4 and 1.6
+try {
+terminateDueTo(new IllegalStateException("Publisher terminated due to unavailable Executor.", t));
+} finally {
+inboundSignals.clear(); // We're not going to need these anymore
+// This subscription is cancelled by now, but letting it become schedulable again means
+// that we can drain the inboundSignals queue if anything arrives after clearing
+on.set(false);
+}
+}
+}
+}
+}
+// Our implementation of `Subscription.request` sends a signal to the Subscription that more elements are in demand
+@Override public void request(final long n) {
+signal(new Request(n));
+}
+// Our implementation of `Subscription.cancel` sends a signal to the Subscription that the `Subscriber` is not interested in any more elements
+@Override public void cancel() {
+signal(Cancel.Instance);
+}
+// The reason for the `init` method is that we want to ensure the `SubscriptionImpl`
+// is completely constructed before it is exposed to the thread pool, therefor this
+// method is only intended to be invoked once, and immediately after the constructor has
+// finished.
+void init() {
+signal(Subscribe.Instance);
+}
+};
+}