AsyncProcessor.java
/*
* Copyright (c) 2016-present, RxJava Contributors.
*
* Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in
* compliance with the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software distributed under the License is
* distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See
* the License for the specific language governing permissions and limitations under the License.
*/
package io.reactivex.rxjava3.processors;
import java.util.concurrent.atomic.AtomicReference;
import org.reactivestreams.*;
import io.reactivex.rxjava3.annotations.*;
import io.reactivex.rxjava3.internal.subscriptions.DeferredScalarSubscription;
import io.reactivex.rxjava3.internal.util.ExceptionHelper;
import io.reactivex.rxjava3.plugins.RxJavaPlugins;
/**
* Processor that emits the very last value followed by a completion event or the received error
* to {@link Subscriber}s.
* <p>
* <img width="640" height="239" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/AsyncProcessor.png" alt="">
* <p>
* This processor does not have a public constructor by design; a new empty instance of this
* {@code AsyncProcessor} can be created via the {@link #create()} method.
* <p>
* Since an {@code AsyncProcessor} is a Reactive Streams {@code Processor} type,
* {@code null}s are not allowed (<a href="https://github.com/reactive-streams/reactive-streams-jvm#2.13">Rule 2.13</a>)
* as parameters to {@link #onNext(Object)} and {@link #onError(Throwable)}. Such calls will result in a
* {@link NullPointerException} being thrown and the processor's state is not changed.
* <p>
* {@code AsyncProcessor} is a {@link io.reactivex.rxjava3.core.Flowable} as well as a {@link FlowableProcessor} and supports backpressure from the downstream but
* its {@link Subscriber}-side consumes items in an unbounded manner.
* <p>
* When this {@code AsyncProcessor} is terminated via {@link #onError(Throwable)}, the
* last observed item (if any) is cleared and late {@link Subscriber}s only receive
* the {@code onError} event.
* <p>
* The {@code AsyncProcessor} caches the latest item internally and it emits this item only when {@code onComplete} is called.
* Therefore, it is not recommended to use this {@code Processor} with infinite or never-completing sources.
* <p>
* Even though {@code AsyncProcessor} implements the {@link Subscriber} interface, calling
* {@code onSubscribe} is not required (<a href="https://github.com/reactive-streams/reactive-streams-jvm#2.12">Rule 2.12</a>)
* if the processor is used as a standalone source. However, calling {@code onSubscribe}
* after the {@code AsyncProcessor} reached its terminal state will result in the
* given {@link Subscription} being canceled immediately.
* <p>
* Calling {@link #onNext(Object)}, {@link #onError(Throwable)} and {@link #onComplete()}
* is required to be serialized (called from the same thread or called non-overlappingly from different threads
* through external means of serialization). The {@link #toSerialized()} method available to all {@code FlowableProcessor}s
* provides such serialization and also protects against reentrance (i.e., when a downstream {@code Subscriber}
* consuming this processor also wants to call {@link #onNext(Object)} on this processor recursively).
* The implementation of {@code onXXX} methods are technically thread-safe but non-serialized calls
* to them may lead to undefined state in the currently subscribed {@code Subscriber}s.
* <p>
* This {@code AsyncProcessor} supports the standard state-peeking methods {@link #hasComplete()}, {@link #hasThrowable()},
* {@link #getThrowable()} and {@link #hasSubscribers()} as well as means to read the very last observed value -
* after this {@code AsyncProcessor} has been completed - in a non-blocking and thread-safe
* manner via {@link #hasValue()} or {@link #getValue()}.
* <dl>
* <dt><b>Backpressure:</b></dt>
* <dd>The {@code AsyncProcessor} honors the backpressure of the downstream {@code Subscriber}s and won't emit
* its single value to a particular {@code Subscriber} until that {@code Subscriber} has requested an item.
* When the {@code AsyncProcessor} is subscribed to a {@link io.reactivex.rxjava3.core.Flowable}, the processor consumes this
* {@code Flowable} in an unbounded manner (requesting {@link Long#MAX_VALUE}) as only the very last upstream item is
* retained by it.
* </dd>
* <dt><b>Scheduler:</b></dt>
* <dd>{@code AsyncProcessor} does not operate by default on a particular {@link io.reactivex.rxjava3.core.Scheduler} and
* the {@code Subscriber}s get notified on the thread where the terminating {@code onError} or {@code onComplete}
* methods were invoked.</dd>
* <dt><b>Error handling:</b></dt>
* <dd>When the {@link #onError(Throwable)} is called, the {@code AsyncProcessor} enters into a terminal state
* and emits the same {@code Throwable} instance to the last set of {@code Subscriber}s. During this emission,
* if one or more {@code Subscriber}s dispose their respective {@code Subscription}s, the
* {@code Throwable} is delivered to the global error handler via
* {@link io.reactivex.rxjava3.plugins.RxJavaPlugins#onError(Throwable)} (multiple times if multiple {@code Subscriber}s
* cancel at once).
* If there were no {@code Subscriber}s subscribed to this {@code AsyncProcessor} when the {@code onError()}
* was called, the global error handler is not invoked.
* </dd>
* </dl>
* <p>
* Example usage:
* <pre><code>
* AsyncProcessor<Object> processor = AsyncProcessor.create();
*
* TestSubscriber<Object> ts1 = processor.test();
*
* ts1.assertEmpty();
*
* processor.onNext(1);
*
* // AsyncProcessor only emits when onComplete was called.
* ts1.assertEmpty();
*
* processor.onNext(2);
* processor.onComplete();
*
* // onComplete triggers the emission of the last cached item and the onComplete event.
* ts1.assertResult(2);
*
* TestSubscriber<Object> ts2 = processor.test();
*
* // late Subscribers receive the last cached item too
* ts2.assertResult(2);
* </code></pre>
* @param <T> the value type
*/
public final class AsyncProcessor<@NonNull T> extends FlowableProcessor<T> {
@SuppressWarnings("rawtypes")
static final AsyncSubscription[] EMPTY = new AsyncSubscription[0];
@SuppressWarnings("rawtypes")
static final AsyncSubscription[] TERMINATED = new AsyncSubscription[0];
final AtomicReference<AsyncSubscription<T>[]> subscribers;
/** Write before updating subscribers, read after reading subscribers as TERMINATED. */
Throwable error;
/** Write before updating subscribers, read after reading subscribers as TERMINATED. */
T value;
/**
* Creates a new AsyncProcessor.
* @param <T> the value type to be received and emitted
* @return the new AsyncProcessor instance
*/
@CheckReturnValue
@NonNull
public static <T> AsyncProcessor<T> create() {
return new AsyncProcessor<>();
}
/**
* Constructs an AsyncProcessor.
* @since 2.0
*/
@SuppressWarnings("unchecked")
AsyncProcessor() {
this.subscribers = new AtomicReference<>(EMPTY);
}
@Override
public void onSubscribe(@NonNull Subscription s) {
if (subscribers.get() == TERMINATED) {
s.cancel();
return;
}
// AsyncProcessor doesn't bother with request coordination.
s.request(Long.MAX_VALUE);
}
@Override
public void onNext(@NonNull T t) {
ExceptionHelper.nullCheck(t, "onNext called with a null value.");
if (subscribers.get() == TERMINATED) {
return;
}
value = t;
}
@SuppressWarnings("unchecked")
@Override
public void onError(@NonNull Throwable t) {
ExceptionHelper.nullCheck(t, "onError called with a null Throwable.");
if (subscribers.get() == TERMINATED) {
RxJavaPlugins.onError(t);
return;
}
value = null;
error = t;
for (AsyncSubscription<T> as : subscribers.getAndSet(TERMINATED)) {
as.onError(t);
}
}
@SuppressWarnings("unchecked")
@Override
public void onComplete() {
if (subscribers.get() == TERMINATED) {
return;
}
T v = value;
AsyncSubscription<T>[] array = subscribers.getAndSet(TERMINATED);
if (v == null) {
for (AsyncSubscription<T> as : array) {
as.onComplete();
}
} else {
for (AsyncSubscription<T> as : array) {
as.complete(v);
}
}
}
@Override
@CheckReturnValue
public boolean hasSubscribers() {
return subscribers.get().length != 0;
}
@Override
@CheckReturnValue
public boolean hasThrowable() {
return subscribers.get() == TERMINATED && error != null;
}
@Override
@CheckReturnValue
public boolean hasComplete() {
return subscribers.get() == TERMINATED && error == null;
}
@Override
@Nullable
@CheckReturnValue
public Throwable getThrowable() {
return subscribers.get() == TERMINATED ? error : null;
}
@Override
protected void subscribeActual(@NonNull Subscriber<? super T> s) {
AsyncSubscription<T> as = new AsyncSubscription<>(s, this);
s.onSubscribe(as);
if (add(as)) {
if (as.isCancelled()) {
remove(as);
}
} else {
Throwable ex = error;
if (ex != null) {
s.onError(ex);
} else {
T v = value;
if (v != null) {
as.complete(v);
} else {
as.onComplete();
}
}
}
}
/**
* Tries to add the given subscriber to the subscribers array atomically
* or returns false if the processor has terminated.
* @param ps the subscriber to add
* @return true if successful, false if the processor has terminated
*/
boolean add(AsyncSubscription<T> ps) {
for (;;) {
AsyncSubscription<T>[] a = subscribers.get();
if (a == TERMINATED) {
return false;
}
int n = a.length;
@SuppressWarnings("unchecked")
AsyncSubscription<T>[] b = new AsyncSubscription[n + 1];
System.arraycopy(a, 0, b, 0, n);
b[n] = ps;
if (subscribers.compareAndSet(a, b)) {
return true;
}
}
}
/**
* Atomically removes the given subscriber if it is subscribed to this processor.
* @param ps the subscriber's subscription wrapper to remove
*/
@SuppressWarnings("unchecked")
void remove(AsyncSubscription<T> ps) {
for (;;) {
AsyncSubscription<T>[] a = subscribers.get();
int n = a.length;
if (n == 0) {
return;
}
int j = -1;
for (int i = 0; i < n; i++) {
if (a[i] == ps) {
j = i;
break;
}
}
if (j < 0) {
return;
}
AsyncSubscription<T>[] b;
if (n == 1) {
b = EMPTY;
} else {
b = new AsyncSubscription[n - 1];
System.arraycopy(a, 0, b, 0, j);
System.arraycopy(a, j + 1, b, j, n - j - 1);
}
if (subscribers.compareAndSet(a, b)) {
return;
}
}
}
/**
* Returns true if this processor has any value.
* <p>The method is thread-safe.
* @return true if this processor has any value
*/
@CheckReturnValue
public boolean hasValue() {
return subscribers.get() == TERMINATED && value != null;
}
/**
* Returns a single value this processor currently has or null if no such value exists.
* <p>The method is thread-safe.
* @return a single value this processor currently has or null if no such value exists
*/
@Nullable
@CheckReturnValue
public T getValue() {
return subscribers.get() == TERMINATED ? value : null;
}
static final class AsyncSubscription<@NonNull T> extends DeferredScalarSubscription<T> {
private static final long serialVersionUID = 5629876084736248016L;
final AsyncProcessor<T> parent;
AsyncSubscription(Subscriber<? super T> actual, AsyncProcessor<T> parent) {
super(actual);
this.parent = parent;
}
@Override
public void cancel() {
if (super.tryCancel()) {
parent.remove(this);
}
}
void onComplete() {
if (!isCancelled()) {
downstream.onComplete();
}
}
void onError(Throwable t) {
if (isCancelled()) {
RxJavaPlugins.onError(t);
} else {
downstream.onError(t);
}
}
}
}