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package org.reactivestreams.example.unicast;
import org.reactivestreams.Subscriber;
import org.reactivestreams.Subscription;
import java.util.concurrent.Executor;
import java.util.concurrent.atomic.AtomicBoolean;
import java.util.concurrent.ConcurrentLinkedQueue;
/**
* AsyncSubscriber is an implementation of Reactive Streams `Subscriber`,
* it runs asynchronously (on an Executor), requests one element
* at a time, and invokes a user-defined method to process each element.
*
* 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 abstract class AsyncSubscriber<T> implements Subscriber<T>, Runnable {
// Signal represents the asynchronous protocol between the Publisher and Subscriber
private static interface Signal {}
private enum OnComplete implements Signal { Instance; }
private static class OnError implements Signal {
public final Throwable error;
public OnError(final Throwable error) { this.error = error; }
}
private static class OnNext<T> implements Signal {
public final T next;
public OnNext(final T next) { this.next = next; }
}
private static class OnSubscribe implements Signal {
public final Subscription subscription;
public OnSubscribe(final Subscription subscription) { this.subscription = subscription; }
}
private Subscription subscription; // Obeying rule 3.1, we make this private!
private boolean done; // It's useful to keep track of whether this Subscriber is done or not
private final Executor executor; // This is the Executor we'll use to be asynchronous, obeying rule 2.2
// Only one constructor, and it's only accessible for the subclasses
protected AsyncSubscriber(Executor executor) {
if (executor == null) throw null;
this.executor = executor;
}
// Showcases a convenience method to idempotently marking the Subscriber as "done", so we don't want to process more elements
// herefor we also need to cancel our `Subscription`.
private final void done() {
//On this line we could add a guard against `!done`, but since rule 3.7 says that `Subscription.cancel()` is idempotent, we don't need to.
done = true; // If `whenNext` throws an exception, let's consider ourselves done (not accepting more elements)
if (subscription != null) { // If we are bailing out before we got a `Subscription` there's little need for cancelling it.
try {
subscription.cancel(); // Cancel the subscription
} catch(final Throwable t) {
//Subscription.cancel is not allowed to throw an exception, according to rule 3.15
(new IllegalStateException(subscription + " violated the Reactive Streams rule 3.15 by throwing an exception from cancel.", t)).printStackTrace(System.err);
}
}
}
// This method is invoked when the OnNext signals arrive
// Returns whether more elements are desired or not, and if no more elements are desired,
// for convenience.
protected abstract boolean whenNext(final T element);
// This method is invoked when the OnComplete signal arrives
// override this method to implement your own custom onComplete logic.
protected void whenComplete() { }
// This method is invoked if the OnError signal arrives
// override this method to implement your own custom onError logic.
protected void whenError(Throwable error) { }
private final void handleOnSubscribe(final Subscription s) {
if (s == null) {
// Getting a null `Subscription` here is not valid so lets just ignore it.
} else if (subscription != null) { // If someone has made a mistake and added this Subscriber multiple times, let's handle it gracefully
try {
s.cancel(); // Cancel the additional subscription to follow rule 2.5
} catch(final Throwable t) {
//Subscription.cancel is not allowed to throw an exception, according to rule 3.15
(new IllegalStateException(s + " violated the Reactive Streams rule 3.15 by throwing an exception from cancel.", t)).printStackTrace(System.err);
}
} else {
// We have to assign it locally before we use it, if we want to be a synchronous `Subscriber`
// Because according to rule 3.10, the Subscription is allowed to call `onNext` synchronously from within `request`
subscription = s;
try {
// If we want elements, according to rule 2.1 we need to call `request`
// And, according to rule 3.2 we are allowed to call this synchronously from within the `onSubscribe` method
s.request(1); // Our Subscriber is unbuffered and modest, it requests one element at a time
} catch(final Throwable t) {
// Subscription.request is not allowed to throw according to rule 3.16
(new IllegalStateException(s + " violated the Reactive Streams rule 3.16 by throwing an exception from request.", t)).printStackTrace(System.err);
}
}
}
private final void handleOnNext(final T element) {
if (!done) { // If we aren't already done
if(subscription == null) { // Technically this check is not needed, since we are expecting Publishers to conform to the spec
// Check for spec violation of 2.1 and 1.09
(new IllegalStateException("Someone violated the Reactive Streams rule 1.09 and 2.1 by signalling OnNext before `Subscription.request`. (no Subscription)")).printStackTrace(System.err);
} else {
try {
if (whenNext(element)) {
try {
subscription.request(1); // Our Subscriber is unbuffered and modest, it requests one element at a time
} catch(final Throwable t) {
// Subscription.request is not allowed to throw according to rule 3.16
(new IllegalStateException(subscription + " violated the Reactive Streams rule 3.16 by throwing an exception from request.", t)).printStackTrace(System.err);
}
} else {
done(); // This is legal according to rule 2.6
}
} catch(final Throwable t) {
done();
try {
onError(t);
} catch(final Throwable t2) {
//Subscriber.onError is not allowed to throw an exception, according to rule 2.13
(new IllegalStateException(this + " violated the Reactive Streams rule 2.13 by throwing an exception from onError.", t2)).printStackTrace(System.err);
}
}
}
}
}
// Here it is important that we do not violate 2.2 and 2.3 by calling methods on the `Subscription` or `Publisher`
private void handleOnComplete() {
if (subscription == null) { // Technically this check is not needed, since we are expecting Publishers to conform to the spec
// Publisher is not allowed to signal onComplete before onSubscribe according to rule 1.09
(new IllegalStateException("Publisher violated the Reactive Streams rule 1.09 signalling onComplete prior to onSubscribe.")).printStackTrace(System.err);
} else {
done = true; // Obey rule 2.4
whenComplete();
}
}
// Here it is important that we do not violate 2.2 and 2.3 by calling methods on the `Subscription` or `Publisher`
private void handleOnError(final Throwable error) {
if (subscription == null) { // Technically this check is not needed, since we are expecting Publishers to conform to the spec
// Publisher is not allowed to signal onError before onSubscribe according to rule 1.09
(new IllegalStateException("Publisher violated the Reactive Streams rule 1.09 signalling onError prior to onSubscribe.")).printStackTrace(System.err);
} else {
done = true; // Obey rule 2.4
whenError(error);
}
}
// We implement the OnX methods on `Subscriber` to send Signals that we will process asycnhronously, but only one at a time
@Override public final void onSubscribe(final Subscription s) {
// As per rule 2.13, we need to throw a `java.lang.NullPointerException` if the `Subscription` is `null`
if (s == null) throw null;
signal(new OnSubscribe(s));
}
@Override public final void onNext(final T element) {
// As per rule 2.13, we need to throw a `java.lang.NullPointerException` if the `element` is `null`
if (element == null) throw null;
signal(new OnNext<T>(element));
}
@Override public final void onError(final Throwable t) {
// As per rule 2.13, we need to throw a `java.lang.NullPointerException` if the `Throwable` is `null`
if (t == null) throw null;
signal(new OnError(t));
}
@Override public final void onComplete() {
signal(OnComplete.Instance);
}
// This `ConcurrentLinkedQueue` will track signals that are sent to this `Subscriber`, like `OnComplete` and `OnNext` ,
// and obeying rule 2.11
private final ConcurrentLinkedQueue<Signal> inboundSignals = new ConcurrentLinkedQueue<Signal>();
// We are using this `AtomicBoolean` to make sure that this `Subscriber` doesn't run concurrently with itself,
// obeying rule 2.7 and 2.11
private final AtomicBoolean on = new AtomicBoolean(false);
@SuppressWarnings("unchecked")
@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 (!done) { // If we're done, we shouldn't process any more signals, obeying rule 2.8
// Below we simply unpack the `Signal`s and invoke the corresponding methods
if (s instanceof OnNext<?>)
handleOnNext(((OnNext<T>)s).next);
else if (s instanceof OnSubscribe)
handleOnSubscribe(((OnSubscribe)s).subscription);
else if (s instanceof OnError) // We are always able to handle OnError, obeying rule 2.10
handleOnError(((OnError)s).error);
else if (s == OnComplete.Instance) // We are always able to handle OnComplete, obeying rule 2.9
handleOnComplete();
}
} 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
}
}
}
// 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 method makes sure that this `Subscriber` is only executing on one Thread at a time
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 and not violate rule 2.13
if (!done) {
try {
done(); // First of all, this failure is not recoverable, so we need to cancel our subscription
} finally {
inboundSignals.clear(); // We're not going to need these anymore
// This subscription is cancelled by now, but letting the Subscriber become schedulable again means
// that we can drain the inboundSignals queue if anything arrives after clearing
on.set(false);
}
}
}
}
}
}