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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

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 *

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 * 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,

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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);

          }

        }

      }

    }

  }

}