UnicastSubject.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.subjects;
import java.util.Objects;
import java.util.concurrent.atomic.*;
import io.reactivex.rxjava3.annotations.*;
import io.reactivex.rxjava3.core.Observer;
import io.reactivex.rxjava3.disposables.Disposable;
import io.reactivex.rxjava3.internal.disposables.EmptyDisposable;
import io.reactivex.rxjava3.internal.functions.*;
import io.reactivex.rxjava3.internal.observers.BasicIntQueueDisposable;
import io.reactivex.rxjava3.internal.util.ExceptionHelper;
import io.reactivex.rxjava3.operators.SimpleQueue;
import io.reactivex.rxjava3.operators.SpscLinkedArrayQueue;
import io.reactivex.rxjava3.plugins.RxJavaPlugins;
/**
* A Subject that queues up events until a single {@link Observer} subscribes to it, replays
* those events to it until the {@code Observer} catches up and then switches to relaying events live to
* this single {@code Observer} until this {@code UnicastSubject} terminates or the {@code Observer} disposes.
* <p>
* <img width="640" height="370" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/UnicastSubject.png" alt="">
* <p>
* Note that {@code UnicastSubject} holds an unbounded internal buffer.
* <p>
* This subject does not have a public constructor by design; a new empty instance of this
* {@code UnicastSubject} can be created via the following {@code create} methods that
* allow specifying the retention policy for items:
* <ul>
* <li>{@link #create()} - creates an empty, unbounded {@code UnicastSubject} that
* caches all items and the terminal event it receives.</li>
* <li>{@link #create(int)} - creates an empty, unbounded {@code UnicastSubject}
* with a hint about how many <b>total</b> items one expects to retain.</li>
* <li>{@link #create(boolean)} - creates an empty, unbounded {@code UnicastSubject} that
* optionally delays an error it receives and replays it after the regular items have been emitted.</li>
* <li>{@link #create(int, Runnable)} - creates an empty, unbounded {@code UnicastSubject}
* with a hint about how many <b>total</b> items one expects to retain and a callback that will be
* called exactly once when the {@code UnicastSubject} gets terminated or the single {@code Observer} disposes.</li>
* <li>{@link #create(int, Runnable, boolean)} - creates an empty, unbounded {@code UnicastSubject}
* with a hint about how many <b>total</b> items one expects to retain and a callback that will be
* called exactly once when the {@code UnicastSubject} gets terminated or the single {@code Observer} disposes
* and optionally delays an error it receives and replays it after the regular items have been emitted.</li>
* </ul>
* <p>
* If more than one {@code Observer} attempts to subscribe to this {@code UnicastSubject}, they
* will receive an {@code IllegalStateException} indicating the single-use-only nature of this {@code UnicastSubject},
* even if the {@code UnicastSubject} already terminated with an error.
* <p>
* Since a {@code Subject} is conceptionally derived from the {@code Processor} type in the Reactive Streams specification,
* {@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 subject's state is not changed.
* <p>
* Since a {@code UnicastSubject} is an {@link io.reactivex.rxjava3.core.Observable}, it does not support backpressure.
* <p>
* When this {@code UnicastSubject} is terminated via {@link #onError(Throwable)} the current or late single {@code Observer}
* may receive the {@code Throwable} before any available items could be emitted. To make sure an onError event is delivered
* to the {@code Observer} after the normal items, create a {@code UnicastSubject} with the {@link #create(boolean)} or
* {@link #create(int, Runnable, boolean)} factory methods.
* <p>
* Even though {@code UnicastSubject} implements the {@code Observer} 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 subject is used as a standalone source. However, calling {@code onSubscribe}
* after the {@code UnicastSubject} reached its terminal state will result in the
* given {@code Disposable} being disposed 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 Subject}s
* provides such serialization and also protects against reentrance (i.e., when a downstream {@code Observer}
* consuming this subject also wants to call {@link #onNext(Object)} on this subject recursively).
* <p>
* This {@code UnicastSubject} supports the standard state-peeking methods {@link #hasComplete()}, {@link #hasThrowable()},
* {@link #getThrowable()} and {@link #hasObservers()}.
* <dl>
* <dt><b>Scheduler:</b></dt>
* <dd>{@code UnicastSubject} does not operate by default on a particular {@link io.reactivex.rxjava3.core.Scheduler} and
* the single {@code Observer} gets 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 UnicastSubject} enters into a terminal state
* and emits the same {@code Throwable} instance to the current single {@code Observer}. During this emission,
* if the single {@code Observer}s disposes its respective {@code Disposable}, the
* {@code Throwable} is delivered to the global error handler via
* {@link io.reactivex.rxjava3.plugins.RxJavaPlugins#onError(Throwable)}.
* If there were no {@code Observer}s subscribed to this {@code UnicastSubject} when the {@code onError()}
* was called, the global error handler is not invoked.
* </dd>
* </dl>
* <p>
* Example usage:
* <pre><code>
* UnicastSubject<Integer> subject = UnicastSubject.create();
*
* TestObserver<Integer> to1 = subject.test();
*
* // fresh UnicastSubjects are empty
* to1.assertEmpty();
*
* TestObserver<Integer> to2 = subject.test();
*
* // A UnicastSubject only allows one Observer during its lifetime
* to2.assertFailure(IllegalStateException.class);
*
* subject.onNext(1);
* to1.assertValue(1);
*
* subject.onNext(2);
* to1.assertValues(1, 2);
*
* subject.onComplete();
* to1.assertResult(1, 2);
*
* // ----------------------------------------------------
*
* UnicastSubject<Integer> subject2 = UnicastSubject.create();
*
* // a UnicastSubject caches events until its single Observer subscribes
* subject2.onNext(1);
* subject2.onNext(2);
* subject2.onComplete();
*
* TestObserver<Integer> to3 = subject2.test();
*
* // the cached events are emitted in order
* to3.assertResult(1, 2);
* </code></pre>
* @param <T> the value type received and emitted by this Subject subclass
* @since 2.0
*/
public final class UnicastSubject<T> extends Subject<T> {
/** The queue that buffers the source events. */
final SpscLinkedArrayQueue<T> queue;
/** The single Observer. */
final AtomicReference<Observer<? super T>> downstream;
/** The optional callback when the Subject gets cancelled or terminates. */
final AtomicReference<Runnable> onTerminate;
/** deliver onNext events before error event. */
final boolean delayError;
/** Indicates the single observer has cancelled. */
volatile boolean disposed;
/** Indicates the source has terminated. */
volatile boolean done;
/**
* The terminal error if not null.
* Must be set before writing to done and read after done == true.
*/
Throwable error;
/** Set to 1 atomically for the first and only Subscriber. */
final AtomicBoolean once;
/** The wip counter and QueueDisposable surface. */
final BasicIntQueueDisposable<T> wip;
boolean enableOperatorFusion;
/**
* Creates an UnicastSubject with an internal buffer capacity hint 16.
* @param <T> the value type
* @return an UnicastSubject instance
*/
@CheckReturnValue
@NonNull
public static <T> UnicastSubject<T> create() {
return new UnicastSubject<>(bufferSize(), null, true);
}
/**
* Creates an UnicastSubject with the given internal buffer capacity hint.
* @param <T> the value type
* @param capacityHint the hint to size the internal unbounded buffer
* @return an UnicastSubject instance
* @throws IllegalArgumentException if {@code capacityHint} is non-positive
*/
@CheckReturnValue
@NonNull
public static <T> UnicastSubject<T> create(int capacityHint) {
ObjectHelper.verifyPositive(capacityHint, "capacityHint");
return new UnicastSubject<>(capacityHint, null, true);
}
/**
* Creates an UnicastSubject with the given internal buffer capacity hint and a callback for
* the case when the single Subscriber cancels its subscription
* or the subject is terminated.
*
* <p>The callback, if not null, is called exactly once and
* non-overlapped with any active replay.
*
* @param <T> the value type
* @param capacityHint the hint to size the internal unbounded buffer
* @param onTerminate the callback to run when the Subject is terminated or cancelled, null not allowed
* @return an UnicastSubject instance
* @throws NullPointerException if {@code onTerminate} is {@code null}
* @throws IllegalArgumentException if {@code capacityHint} is non-positive
*/
@CheckReturnValue
@NonNull
public static <T> UnicastSubject<T> create(int capacityHint, @NonNull Runnable onTerminate) {
ObjectHelper.verifyPositive(capacityHint, "capacityHint");
Objects.requireNonNull(onTerminate, "onTerminate");
return new UnicastSubject<>(capacityHint, onTerminate, true);
}
/**
* Creates an UnicastSubject with the given internal buffer capacity hint, delay error flag and
* a callback for the case when the single Observer disposes its {@link Disposable}
* or the subject is terminated.
*
* <p>The callback, if not null, is called exactly once and
* non-overlapped with any active replay.
* <p>History: 2.0.8 - experimental
* @param <T> the value type
* @param capacityHint the hint to size the internal unbounded buffer
* @param onTerminate the callback to run when the Subject is terminated or cancelled, null not allowed
* @param delayError deliver pending onNext events before onError
* @return an UnicastSubject instance
* @throws NullPointerException if {@code onTerminate} is {@code null}
* @throws IllegalArgumentException if {@code capacityHint} is non-positive
* @since 2.2
*/
@CheckReturnValue
@NonNull
public static <T> UnicastSubject<T> create(int capacityHint, @NonNull Runnable onTerminate, boolean delayError) {
ObjectHelper.verifyPositive(capacityHint, "capacityHint");
Objects.requireNonNull(onTerminate, "onTerminate");
return new UnicastSubject<>(capacityHint, onTerminate, delayError);
}
/**
* Creates an UnicastSubject with an internal buffer capacity hint 16 and given delay error flag.
*
* <p>The callback, if not null, is called exactly once and
* non-overlapped with any active replay.
* <p>History: 2.0.8 - experimental
* @param <T> the value type
* @param delayError deliver pending onNext events before onError
* @return an UnicastSubject instance
* @since 2.2
*/
@CheckReturnValue
@NonNull
public static <T> UnicastSubject<T> create(boolean delayError) {
return new UnicastSubject<>(bufferSize(), null, delayError);
}
/**
* Creates an UnicastSubject with the given capacity hint, delay error flag and callback
* for when the Subject is terminated normally or its single Subscriber cancels.
* <p>History: 2.0.8 - experimental
* @param capacityHint the capacity hint for the internal, unbounded queue
* @param onTerminate the callback to run when the Subject is terminated or cancelled, null not allowed
* @param delayError deliver pending onNext events before onError
* @since 2.2
*/
UnicastSubject(int capacityHint, Runnable onTerminate, boolean delayError) {
this.queue = new SpscLinkedArrayQueue<>(capacityHint);
this.onTerminate = new AtomicReference<>(onTerminate);
this.delayError = delayError;
this.downstream = new AtomicReference<>();
this.once = new AtomicBoolean();
this.wip = new UnicastQueueDisposable();
}
@Override
protected void subscribeActual(Observer<? super T> observer) {
if (!once.get() && once.compareAndSet(false, true)) {
observer.onSubscribe(wip);
downstream.lazySet(observer); // full barrier in drain
if (disposed) {
downstream.lazySet(null);
return;
}
drain();
} else {
EmptyDisposable.error(new IllegalStateException("Only a single observer allowed."), observer);
}
}
void doTerminate() {
Runnable r = onTerminate.get();
if (r != null && onTerminate.compareAndSet(r, null)) {
r.run();
}
}
@Override
public void onSubscribe(Disposable d) {
if (done || disposed) {
d.dispose();
}
}
@Override
public void onNext(T t) {
ExceptionHelper.nullCheck(t, "onNext called with a null value.");
if (done || disposed) {
return;
}
queue.offer(t);
drain();
}
@Override
public void onError(Throwable t) {
ExceptionHelper.nullCheck(t, "onError called with a null Throwable.");
if (done || disposed) {
RxJavaPlugins.onError(t);
return;
}
error = t;
done = true;
doTerminate();
drain();
}
@Override
public void onComplete() {
if (done || disposed) {
return;
}
done = true;
doTerminate();
drain();
}
void drainNormal(Observer<? super T> a) {
int missed = 1;
SimpleQueue<T> q = queue;
boolean failFast = !this.delayError;
boolean canBeError = true;
for (;;) {
for (;;) {
if (disposed) {
downstream.lazySet(null);
q.clear();
return;
}
boolean d = this.done;
T v = queue.poll();
boolean empty = v == null;
if (d) {
if (failFast && canBeError) {
if (failedFast(q, a)) {
return;
} else {
canBeError = false;
}
}
if (empty) {
errorOrComplete(a);
return;
}
}
if (empty) {
break;
}
a.onNext(v);
}
missed = wip.addAndGet(-missed);
if (missed == 0) {
break;
}
}
}
void drainFused(Observer<? super T> a) {
int missed = 1;
final SpscLinkedArrayQueue<T> q = queue;
final boolean failFast = !delayError;
for (;;) {
if (disposed) {
downstream.lazySet(null);
return;
}
boolean d = done;
if (failFast && d) {
if (failedFast(q, a)) {
return;
}
}
a.onNext(null);
if (d) {
errorOrComplete(a);
return;
}
missed = wip.addAndGet(-missed);
if (missed == 0) {
break;
}
}
}
void errorOrComplete(Observer<? super T> a) {
downstream.lazySet(null);
Throwable ex = error;
if (ex != null) {
a.onError(ex);
} else {
a.onComplete();
}
}
boolean failedFast(final SimpleQueue<T> q, Observer<? super T> a) {
Throwable ex = error;
if (ex != null) {
downstream.lazySet(null);
q.clear();
a.onError(ex);
return true;
} else {
return false;
}
}
void drain() {
if (wip.getAndIncrement() != 0) {
return;
}
Observer<? super T> a = downstream.get();
int missed = 1;
for (;;) {
if (a != null) {
if (enableOperatorFusion) {
drainFused(a);
} else {
drainNormal(a);
}
return;
}
missed = wip.addAndGet(-missed);
if (missed == 0) {
break;
}
a = downstream.get();
}
}
@Override
@CheckReturnValue
public boolean hasObservers() {
return downstream.get() != null;
}
@Override
@Nullable
@CheckReturnValue
public Throwable getThrowable() {
if (done) {
return error;
}
return null;
}
@Override
@CheckReturnValue
public boolean hasThrowable() {
return done && error != null;
}
@Override
@CheckReturnValue
public boolean hasComplete() {
return done && error == null;
}
final class UnicastQueueDisposable extends BasicIntQueueDisposable<T> {
private static final long serialVersionUID = 7926949470189395511L;
@Override
public int requestFusion(int mode) {
if ((mode & ASYNC) != 0) {
enableOperatorFusion = true;
return ASYNC;
}
return NONE;
}
@Nullable
@Override
public T poll() {
return queue.poll();
}
@Override
public boolean isEmpty() {
return queue.isEmpty();
}
@Override
public void clear() {
queue.clear();
}
@Override
public void dispose() {
if (!disposed) {
disposed = true;
doTerminate();
downstream.lazySet(null);
if (wip.getAndIncrement() == 0) {
downstream.lazySet(null);
if (!enableOperatorFusion) {
queue.clear();
}
}
}
}
@Override
public boolean isDisposed() {
return disposed;
}
}
}