1 /*

     2  * Copyright (c) 2019, Oracle and/or its affiliates. All rights reserved.

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

     4  *

     5  * This code is free software; you can redistribute it and/or modify it

     6  * under the terms of the GNU General Public License version 2 only, as

     7  * published by the Free Software Foundation.

     8  *

     9  * This code is distributed in the hope that it will be useful, but WITHOUT

    10  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

    11  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

    12  * version 2 for more details (a copy is included in the LICENSE file that

    13  * accompanied this code).

    14  *

    15  * You should have received a copy of the GNU General Public License version

    16  * 2 along with this work; if not, write to the Free Software Foundation,

    17  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

    18  *

    19  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

    20  * or visit www.oracle.com if you need additional information or have any

    21  * questions.

    22  */

    23 

    24 package org.reactivestreams.example.unicast;

    25 

    26 import org.reactivestreams.Subscriber;

    27 import org.reactivestreams.Subscription;

    28 

    29 import java.util.concurrent.Executor;

    30 import java.util.concurrent.atomic.AtomicBoolean;

    31 import java.util.concurrent.ConcurrentLinkedQueue;

    32 

    33 /**

    34  * AsyncSubscriber is an implementation of Reactive Streams `Subscriber`,

    35  * it runs asynchronously (on an Executor), requests one element

    36  * at a time, and invokes a user-defined method to process each element.

    37  *

    38  * NOTE: The code below uses a lot of try-catches to show the reader where exceptions can be expected, and where they are forbidden.

    39  */

    40 public abstract class AsyncSubscriber<T> implements Subscriber<T>, Runnable {

    41 

    42   // Signal represents the asynchronous protocol between the Publisher and Subscriber

    43   private static interface Signal {}

    44 

    45   private enum OnComplete implements Signal { Instance; }

    46 

    47   private static class OnError implements Signal {

    48     public final Throwable error;

    49     public OnError(final Throwable error) { this.error = error; }

    50   }

    51 

    52   private static class OnNext<T> implements Signal {

    53     public final T next;

    54     public OnNext(final T next) { this.next = next; }

    55   }

    56 

    57   private static class OnSubscribe implements Signal {

    58     public final Subscription subscription;

    59     public OnSubscribe(final Subscription subscription) { this.subscription = subscription; }

    60   }

    61 

    62   private Subscription subscription; // Obeying rule 3.1, we make this private!

    63   private boolean done; // It's useful to keep track of whether this Subscriber is done or not

    64   private final Executor executor; // This is the Executor we'll use to be asynchronous, obeying rule 2.2

    65 

    66   // Only one constructor, and it's only accessible for the subclasses

    67   protected AsyncSubscriber(Executor executor) {

    68     if (executor == null) throw null;

    69     this.executor = executor;

    70   }

    71 

    72   // Showcases a convenience method to idempotently marking the Subscriber as "done", so we don't want to process more elements

    73   // herefor we also need to cancel our `Subscription`.

    74   private final void done() {

    75     //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.

    76     done = true; // If `whenNext` throws an exception, let's consider ourselves done (not accepting more elements)

    77     if (subscription != null) { // If we are bailing out before we got a `Subscription` there's little need for cancelling it.

    78       try {

    79         subscription.cancel(); // Cancel the subscription

    80       } catch(final Throwable t) {

    81         //Subscription.cancel is not allowed to throw an exception, according to rule 3.15

    82         (new IllegalStateException(subscription + " violated the Reactive Streams rule 3.15 by throwing an exception from cancel.", t)).printStackTrace(System.err);

    83       }

    84     }

    85   }

    86 

    87   // This method is invoked when the OnNext signals arrive

    88   // Returns whether more elements are desired or not, and if no more elements are desired,

    89   // for convenience.

    90   protected abstract boolean whenNext(final T element);

    91 

    92   // This method is invoked when the OnComplete signal arrives

    93   // override this method to implement your own custom onComplete logic.

    94   protected void whenComplete() { }

    95 

    96   // This method is invoked if the OnError signal arrives

    97   // override this method to implement your own custom onError logic.

    98   protected void whenError(Throwable error) { }

    99 

   100   private final void handleOnSubscribe(final Subscription s) {

   101     if (s == null) {

   102       // Getting a null `Subscription` here is not valid so lets just ignore it.

   103     } else if (subscription != null) { // If someone has made a mistake and added this Subscriber multiple times, let's handle it gracefully

   104       try {

   105         s.cancel(); // Cancel the additional subscription to follow rule 2.5

   106       } catch(final Throwable t) {

   107         //Subscription.cancel is not allowed to throw an exception, according to rule 3.15

   108         (new IllegalStateException(s + " violated the Reactive Streams rule 3.15 by throwing an exception from cancel.", t)).printStackTrace(System.err);

   109       }

   110     } else {

   111       // We have to assign it locally before we use it, if we want to be a synchronous `Subscriber`

   112       // Because according to rule 3.10, the Subscription is allowed to call `onNext` synchronously from within `request`

   113       subscription = s;

   114       try {

   115         // If we want elements, according to rule 2.1 we need to call `request`

   116         // And, according to rule 3.2 we are allowed to call this synchronously from within the `onSubscribe` method

   117         s.request(1); // Our Subscriber is unbuffered and modest, it requests one element at a time

   118       } catch(final Throwable t) {

   119         // Subscription.request is not allowed to throw according to rule 3.16

   120         (new IllegalStateException(s + " violated the Reactive Streams rule 3.16 by throwing an exception from request.", t)).printStackTrace(System.err);

   121       }

   122     }

   123   }

   124 

   125   private final void handleOnNext(final T element) {

   126     if (!done) { // If we aren't already done

   127       if(subscription == null) { // Technically this check is not needed, since we are expecting Publishers to conform to the spec

   128         // Check for spec violation of 2.1 and 1.09

   129         (new IllegalStateException("Someone violated the Reactive Streams rule 1.09 and 2.1 by signalling OnNext before `Subscription.request`. (no Subscription)")).printStackTrace(System.err);

   130       } else {

   131         try {

   132           if (whenNext(element)) {

   133             try {

   134               subscription.request(1); // Our Subscriber is unbuffered and modest, it requests one element at a time

   135             } catch(final Throwable t) {

   136               // Subscription.request is not allowed to throw according to rule 3.16

   137               (new IllegalStateException(subscription + " violated the Reactive Streams rule 3.16 by throwing an exception from request.", t)).printStackTrace(System.err);

   138             }

   139           } else {

   140             done(); // This is legal according to rule 2.6

   141           }

   142         } catch(final Throwable t) {

   143           done();

   144           try {

   145             onError(t);

   146           } catch(final Throwable t2) {

   147             //Subscriber.onError is not allowed to throw an exception, according to rule 2.13

   148             (new IllegalStateException(this + " violated the Reactive Streams rule 2.13 by throwing an exception from onError.", t2)).printStackTrace(System.err);

   149           }

   150         }

   151       }

   152     }

   153   }

   154 

   155   // Here it is important that we do not violate 2.2 and 2.3 by calling methods on the `Subscription` or `Publisher`

   156   private void handleOnComplete() {

   157     if (subscription == null) { // Technically this check is not needed, since we are expecting Publishers to conform to the spec

   158       // Publisher is not allowed to signal onComplete before onSubscribe according to rule 1.09

   159       (new IllegalStateException("Publisher violated the Reactive Streams rule 1.09 signalling onComplete prior to onSubscribe.")).printStackTrace(System.err);

   160     } else {

   161       done = true; // Obey rule 2.4

   162       whenComplete();

   163     }

   164   }

   165 

   166   // Here it is important that we do not violate 2.2 and 2.3 by calling methods on the `Subscription` or `Publisher`

   167   private void handleOnError(final Throwable error) {

   168     if (subscription == null) { // Technically this check is not needed, since we are expecting Publishers to conform to the spec

   169       // Publisher is not allowed to signal onError before onSubscribe according to rule 1.09

   170       (new IllegalStateException("Publisher violated the Reactive Streams rule 1.09 signalling onError prior to onSubscribe.")).printStackTrace(System.err);

   171     } else {

   172       done = true; // Obey rule 2.4

   173       whenError(error);

   174     }

   175   }

   176 

   177   // We implement the OnX methods on `Subscriber` to send Signals that we will process asycnhronously, but only one at a time

   178 

   179   @Override public final void onSubscribe(final Subscription s) {

   180     // As per rule 2.13, we need to throw a `java.lang.NullPointerException` if the `Subscription` is `null`

   181     if (s == null) throw null;

   182 

   183     signal(new OnSubscribe(s));

   184   }

   185 

   186   @Override public final void onNext(final T element) {

   187     // As per rule 2.13, we need to throw a `java.lang.NullPointerException` if the `element` is `null`

   188     if (element == null) throw null;

   189 

   190     signal(new OnNext<T>(element));

   191   }

   192 

   193   @Override public final void onError(final Throwable t) {

   194     // As per rule 2.13, we need to throw a `java.lang.NullPointerException` if the `Throwable` is `null`

   195     if (t == null) throw null;

   196 

   197     signal(new OnError(t));

   198   }

   199 

   200   @Override public final void onComplete() {

   201      signal(OnComplete.Instance);

   202   }

   203 

   204   // This `ConcurrentLinkedQueue` will track signals that are sent to this `Subscriber`, like `OnComplete` and `OnNext` ,

   205   // and obeying rule 2.11

   206   private final ConcurrentLinkedQueue<Signal> inboundSignals = new ConcurrentLinkedQueue<Signal>();

   207 

   208   // We are using this `AtomicBoolean` to make sure that this `Subscriber` doesn't run concurrently with itself,

   209   // obeying rule 2.7 and 2.11

   210   private final AtomicBoolean on = new AtomicBoolean(false);

   211 

   212    @SuppressWarnings("unchecked")

   213    @Override public final void run() {

   214     if(on.get()) { // establishes a happens-before relationship with the end of the previous run

   215       try {

   216         final Signal s = inboundSignals.poll(); // We take a signal off the queue

   217         if (!done) { // If we're done, we shouldn't process any more signals, obeying rule 2.8

   218           // Below we simply unpack the `Signal`s and invoke the corresponding methods

   219           if (s instanceof OnNext<?>)

   220             handleOnNext(((OnNext<T>)s).next);

   221           else if (s instanceof OnSubscribe)

   222             handleOnSubscribe(((OnSubscribe)s).subscription);

   223           else if (s instanceof OnError) // We are always able to handle OnError, obeying rule 2.10

   224             handleOnError(((OnError)s).error);

   225           else if (s == OnComplete.Instance) // We are always able to handle OnComplete, obeying rule 2.9

   226             handleOnComplete();

   227         }

   228       } finally {

   229         on.set(false); // establishes a happens-before relationship with the beginning of the next run

   230         if(!inboundSignals.isEmpty()) // If we still have signals to process

   231           tryScheduleToExecute(); // Then we try to schedule ourselves to execute again

   232       }

   233     }

   234   }

   235 

   236   // What `signal` does is that it sends signals to the `Subscription` asynchronously

   237   private void signal(final Signal signal) {

   238     if (inboundSignals.offer(signal)) // No need to null-check here as ConcurrentLinkedQueue does this for us

   239       tryScheduleToExecute(); // Then we try to schedule it for execution, if it isn't already

   240   }

   241 

   242   // This method makes sure that this `Subscriber` is only executing on one Thread at a time

   243   private final void tryScheduleToExecute() {

   244     if(on.compareAndSet(false, true)) {

   245       try {

   246         executor.execute(this);

   247       } catch(Throwable t) { // If we can't run on the `Executor`, we need to fail gracefully and not violate rule 2.13

   248         if (!done) {

   249           try {

   250             done(); // First of all, this failure is not recoverable, so we need to cancel our subscription

   251           } finally {

   252             inboundSignals.clear(); // We're not going to need these anymore

   253             // This subscription is cancelled by now, but letting the Subscriber become schedulable again means

   254             // that we can drain the inboundSignals queue if anything arrives after clearing

   255             on.set(false);

   256           }

   257         }

   258       }

   259     }

   260   }

   261 }