PublishProcessor.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.*;
import org.reactivestreams.*;
import io.reactivex.rxjava3.annotations.*;
import io.reactivex.rxjava3.exceptions.MissingBackpressureException;
import io.reactivex.rxjava3.internal.subscriptions.SubscriptionHelper;
import io.reactivex.rxjava3.internal.util.*;
import io.reactivex.rxjava3.plugins.RxJavaPlugins;
/**
* Processor that multicasts all subsequently observed items to its current {@link Subscriber}s.
*
* <p>
* <img width="640" height="278" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/PublishProcessor.png" alt="">
* <p>
* This processor does not have a public constructor by design; a new empty instance of this
* {@code PublishProcessor} can be created via the {@link #create()} method.
* <p>
* Since a {@code PublishProcessor} 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 PublishProcessor} is a {@link io.reactivex.rxjava3.core.Flowable} as well as a {@link FlowableProcessor},
* however, it does not coordinate backpressure between different subscribers and between an
* upstream source and a subscriber. If an upstream item is received via {@link #onNext(Object)}, if
* a subscriber is not ready to receive an item, that subscriber is terminated via a {@link MissingBackpressureException}.
* To avoid this case, use {@link #offer(Object)} and retry sometime later if it returned false.
* The {@code PublishProcessor}'s {@link Subscriber}-side consumes items in an unbounded manner.
* <p>
* For a multicasting processor type that also coordinates between the downstream {@code Subscriber}s and the upstream
* source as well, consider using {@link MulticastProcessor}.
* <p>
* When this {@code PublishProcessor} is terminated via {@link #onError(Throwable)} or {@link #onComplete()},
* late {@link Subscriber}s only receive the respective terminal event.
* <p>
* Unlike a {@link BehaviorProcessor}, a {@code PublishProcessor} doesn't retain/cache items, therefore, a new
* {@code Subscriber} won't receive any past items.
* <p>
* Even though {@code PublishProcessor} 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 PublishProcessor} reached its terminal state will result in the
* given {@link Subscription} being canceled immediately.
* <p>
* Calling {@link #onNext(Object)}, {@link #offer(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 {@link 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).
* Note that serializing over {@link #offer(Object)} is not supported through {@code toSerialized()} because it is a method
* available on the {@code PublishProcessor} and {@code BehaviorProcessor} classes only.
* <p>
* This {@code PublishProcessor} supports the standard state-peeking methods {@link #hasComplete()}, {@link #hasThrowable()},
* {@link #getThrowable()} and {@link #hasSubscribers()}.
* <dl>
* <dt><b>Backpressure:</b></dt>
* <dd>The processor does not coordinate backpressure for its subscribers and implements a weaker {@code onSubscribe} which
* calls requests {@link Long#MAX_VALUE} from the incoming Subscriptions. This makes it possible to subscribe the {@code PublishProcessor}
* to multiple sources (note on serialization though) unlike the standard {@code Subscriber} contract. Child subscribers, however, are not overflown but receive an
* {@link IllegalStateException} in case their requested amount is zero.</dd>
* <dt><b>Scheduler:</b></dt>
* <dd>{@code PublishProcessor} does not operate by default on a particular {@link io.reactivex.rxjava3.core.Scheduler} and
* the {@code Subscriber}s get notified on the thread the respective {@code onXXX} methods were invoked.</dd>
* <dt><b>Error handling:</b></dt>
* <dd>When the {@link #onError(Throwable)} is called, the {@code PublishProcessor} 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 cancel 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 PublishProcessor} when the {@code onError()}
* was called, the global error handler is not invoked.
* </dd>
* </dl>
*
* Example usage:
* <pre> {@code
PublishProcessor<Object> processor = PublishProcessor.create();
// subscriber1 will receive all onNext and onComplete events
processor.subscribe(subscriber1);
processor.onNext("one");
processor.onNext("two");
// subscriber2 will only receive "three" and onComplete
processor.subscribe(subscriber2);
processor.onNext("three");
processor.onComplete();
} </pre>
* @param <T> the value type multicasted to Subscribers.
* @see MulticastProcessor
*/
public final class PublishProcessor<@NonNull T> extends FlowableProcessor<T> {
/** The terminated indicator for the subscribers array. */
@SuppressWarnings("rawtypes")
static final PublishSubscription[] TERMINATED = new PublishSubscription[0];
/** An empty subscribers array to avoid allocating it all the time. */
@SuppressWarnings("rawtypes")
static final PublishSubscription[] EMPTY = new PublishSubscription[0];
/** The array of currently subscribed subscribers. */
final AtomicReference<PublishSubscription<T>[]> subscribers;
/** The error, write before terminating and read after checking subscribers. */
Throwable error;
/**
* Constructs a PublishProcessor.
* @param <T> the value type
* @return the new PublishProcessor
*/
@CheckReturnValue
@NonNull
public static <T> PublishProcessor<T> create() {
return new PublishProcessor<>();
}
/**
* Constructs a PublishProcessor.
* @since 2.0
*/
@SuppressWarnings("unchecked")
PublishProcessor() {
subscribers = new AtomicReference<>(EMPTY);
}
@Override
protected void subscribeActual(@NonNull Subscriber<? super T> t) {
PublishSubscription<T> ps = new PublishSubscription<>(t, this);
t.onSubscribe(ps);
if (add(ps)) {
// if cancellation happened while a successful add, the remove() didn't work
// so we need to do it again
if (ps.isCancelled()) {
remove(ps);
}
} else {
Throwable ex = error;
if (ex != null) {
t.onError(ex);
} else {
t.onComplete();
}
}
}
/**
* Tries to add the given subscriber to the subscribers array atomically
* or returns false if this processor has terminated.
* @param ps the subscriber to add
* @return true if successful, false if this processor has terminated
*/
boolean add(PublishSubscription<T> ps) {
for (;;) {
PublishSubscription<T>[] a = subscribers.get();
if (a == TERMINATED) {
return false;
}
int n = a.length;
@SuppressWarnings("unchecked")
PublishSubscription<T>[] b = new PublishSubscription[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 subscription wrapping a subscriber to remove
*/
@SuppressWarnings("unchecked")
void remove(PublishSubscription<T> ps) {
for (;;) {
PublishSubscription<T>[] a = subscribers.get();
if (a == TERMINATED || a == EMPTY) {
return;
}
int n = a.length;
int j = -1;
for (int i = 0; i < n; i++) {
if (a[i] == ps) {
j = i;
break;
}
}
if (j < 0) {
return;
}
PublishSubscription<T>[] b;
if (n == 1) {
b = EMPTY;
} else {
b = new PublishSubscription[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;
}
}
}
@Override
public void onSubscribe(@NonNull Subscription s) {
if (subscribers.get() == TERMINATED) {
s.cancel();
return;
}
// PublishProcessor 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.");
for (PublishSubscription<T> s : subscribers.get()) {
s.onNext(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;
}
error = t;
for (PublishSubscription<T> s : subscribers.getAndSet(TERMINATED)) {
s.onError(t);
}
}
@SuppressWarnings("unchecked")
@Override
public void onComplete() {
if (subscribers.get() == TERMINATED) {
return;
}
for (PublishSubscription<T> s : subscribers.getAndSet(TERMINATED)) {
s.onComplete();
}
}
/**
* Tries to emit the item to all currently subscribed {@link Subscriber}s if all of them
* has requested some value, returns {@code false} otherwise.
* <p>
* This method should be called in a sequential manner just like the {@code onXXX} methods
* of this {@code PublishProcessor}.
* <p>History: 2.0.8 - experimental
* @param t the item to emit, not {@code null}
* @return {@code true} if the item was emitted to all {@code Subscriber}s
* @throws NullPointerException if {@code t} is {@code null}
* @since 2.2
*/
@CheckReturnValue
public boolean offer(@NonNull T t) {
ExceptionHelper.nullCheck(t, "offer called with a null value.");
PublishSubscription<T>[] array = subscribers.get();
for (PublishSubscription<T> s : array) {
if (s.isFull()) {
return false;
}
}
for (PublishSubscription<T> s : array) {
s.onNext(t);
}
return true;
}
@Override
@CheckReturnValue
public boolean hasSubscribers() {
return subscribers.get().length != 0;
}
@Override
@Nullable
@CheckReturnValue
public Throwable getThrowable() {
if (subscribers.get() == TERMINATED) {
return error;
}
return null;
}
@Override
@CheckReturnValue
public boolean hasThrowable() {
return subscribers.get() == TERMINATED && error != null;
}
@Override
@CheckReturnValue
public boolean hasComplete() {
return subscribers.get() == TERMINATED && error == null;
}
/**
* Wraps the actual subscriber, tracks its requests and makes cancellation
* to remove itself from the current subscribers array.
*
* @param <T> the value type
*/
static final class PublishSubscription<@NonNull T> extends AtomicLong implements Subscription {
private static final long serialVersionUID = 3562861878281475070L;
/** The actual subscriber. */
final Subscriber<? super T> downstream;
/** The parent processor servicing this subscriber. */
final PublishProcessor<T> parent;
/**
* Constructs a PublishSubscriber, wraps the actual subscriber and the state.
* @param actual the actual subscriber
* @param parent the parent PublishProcessor
*/
PublishSubscription(Subscriber<? super T> actual, PublishProcessor<T> parent) {
this.downstream = actual;
this.parent = parent;
}
public void onNext(T t) {
long r = get();
if (r == Long.MIN_VALUE) {
return;
}
if (r != 0L) {
downstream.onNext(t);
BackpressureHelper.producedCancel(this, 1);
} else {
cancel();
downstream.onError(MissingBackpressureException.createDefault());
}
}
public void onError(Throwable t) {
if (get() != Long.MIN_VALUE) {
downstream.onError(t);
} else {
RxJavaPlugins.onError(t);
}
}
public void onComplete() {
if (get() != Long.MIN_VALUE) {
downstream.onComplete();
}
}
@Override
public void request(long n) {
if (SubscriptionHelper.validate(n)) {
BackpressureHelper.addCancel(this, n);
}
}
@Override
public void cancel() {
if (getAndSet(Long.MIN_VALUE) != Long.MIN_VALUE) {
parent.remove(this);
}
}
public boolean isCancelled() {
return get() == Long.MIN_VALUE;
}
boolean isFull() {
return get() == 0L;
}
}
}