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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);
}
};
}