PublishSubject.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.concurrent.atomic.*;
import io.reactivex.rxjava3.annotations.*;
import io.reactivex.rxjava3.core.Observer;
import io.reactivex.rxjava3.disposables.Disposable;
import io.reactivex.rxjava3.internal.util.ExceptionHelper;
import io.reactivex.rxjava3.plugins.RxJavaPlugins;
/**
* A Subject that emits (multicasts) items to currently subscribed {@link Observer}s and terminal events to current
* or late {@code Observer}s.
* <p>
* <img width="640" height="281" src="https://raw.github.com/wiki/ReactiveX/RxJava/images/rx-operators/PublishSubject.png" alt="">
* <p>
* This subject does not have a public constructor by design; a new empty instance of this
* {@code PublishSubject} can be created via the {@link #create()} method.
* <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 PublishSubject} is an {@link io.reactivex.rxjava3.core.Observable}, it does not support backpressure.
* <p>
* When this {@code PublishSubject} is terminated via {@link #onError(Throwable)} or {@link #onComplete()},
* late {@link io.reactivex.rxjava3.core.Observer}s only receive the respective terminal event.
* <p>
* Unlike a {@link BehaviorSubject}, a {@code PublishSubject} doesn't retain/cache items, therefore, a new
* {@code Observer} won't receive any past items.
* <p>
* Even though {@code PublishSubject} 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 PublishSubject} 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 PublishSubject} supports the standard state-peeking methods {@link #hasComplete()}, {@link #hasThrowable()},
* {@link #getThrowable()} and {@link #hasObservers()}.
* <dl>
* <dt><b>Scheduler:</b></dt>
* <dd>{@code PublishSubject} does not operate by default on a particular {@link io.reactivex.rxjava3.core.Scheduler} and
* the {@code Observer}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 PublishSubject} enters into a terminal state
* and emits the same {@code Throwable} instance to the last set of {@code Observer}s. During this emission,
* if one or more {@code Observer}s dispose their respective {@code Disposable}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 Observer}s
* cancel at once).
* If there were no {@code Observer}s subscribed to this {@code PublishSubject} when the {@code onError()}
* was called, the global error handler is not invoked.
* </dd>
* </dl>
* <p>
* Example usage:
* <pre> {@code
PublishSubject<Object> subject = PublishSubject.create();
// observer1 will receive all onNext and onComplete events
subject.subscribe(observer1);
subject.onNext("one");
subject.onNext("two");
// observer2 will only receive "three" and onComplete
subject.subscribe(observer2);
subject.onNext("three");
subject.onComplete();
// late Observers only receive the terminal event
subject.test().assertEmpty();
} </pre>
*
* @param <T>
* the type of items observed and emitted by the Subject
*/
public final class PublishSubject<T> extends Subject<T> {
/** The terminated indicator for the subscribers array. */
@SuppressWarnings("rawtypes")
static final PublishDisposable[] TERMINATED = new PublishDisposable[0];
/** An empty subscribers array to avoid allocating it all the time. */
@SuppressWarnings("rawtypes")
static final PublishDisposable[] EMPTY = new PublishDisposable[0];
/** The array of currently subscribed subscribers. */
final AtomicReference<PublishDisposable<T>[]> subscribers;
/** The error, write before terminating and read after checking subscribers. */
Throwable error;
/**
* Constructs a PublishSubject.
* @param <T> the value type
* @return the new PublishSubject
*/
@CheckReturnValue
@NonNull
public static <T> PublishSubject<T> create() {
return new PublishSubject<>();
}
/**
* Constructs a PublishSubject.
* @since 2.0
*/
@SuppressWarnings("unchecked")
PublishSubject() {
subscribers = new AtomicReference<>(EMPTY);
}
@Override
protected void subscribeActual(Observer<? super T> t) {
PublishDisposable<T> ps = new PublishDisposable<>(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.isDisposed()) {
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 the subject has terminated.
* @param ps the subscriber to add
* @return true if successful, false if the subject has terminated
*/
boolean add(PublishDisposable<T> ps) {
for (;;) {
PublishDisposable<T>[] a = subscribers.get();
if (a == TERMINATED) {
return false;
}
int n = a.length;
@SuppressWarnings("unchecked")
PublishDisposable<T>[] b = new PublishDisposable[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 the subject.
* @param ps the subject to remove
*/
@SuppressWarnings("unchecked")
void remove(PublishDisposable<T> ps) {
for (;;) {
PublishDisposable<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;
}
PublishDisposable<T>[] b;
if (n == 1) {
b = EMPTY;
} else {
b = new PublishDisposable[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(Disposable d) {
if (subscribers.get() == TERMINATED) {
d.dispose();
}
}
@Override
public void onNext(T t) {
ExceptionHelper.nullCheck(t, "onNext called with a null value.");
for (PublishDisposable<T> pd : subscribers.get()) {
pd.onNext(t);
}
}
@SuppressWarnings("unchecked")
@Override
public void onError(Throwable t) {
ExceptionHelper.nullCheck(t, "onError called with a null Throwable.");
if (subscribers.get() == TERMINATED) {
RxJavaPlugins.onError(t);
return;
}
error = t;
for (PublishDisposable<T> pd : subscribers.getAndSet(TERMINATED)) {
pd.onError(t);
}
}
@SuppressWarnings("unchecked")
@Override
public void onComplete() {
if (subscribers.get() == TERMINATED) {
return;
}
for (PublishDisposable<T> pd : subscribers.getAndSet(TERMINATED)) {
pd.onComplete();
}
}
@Override
@CheckReturnValue
public boolean hasObservers() {
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 PublishDisposable<T> extends AtomicBoolean implements Disposable {
private static final long serialVersionUID = 3562861878281475070L;
/** The actual subscriber. */
final Observer<? super T> downstream;
/** The subject state. */
final PublishSubject<T> parent;
/**
* Constructs a PublishSubscriber, wraps the actual subscriber and the state.
* @param actual the actual subscriber
* @param parent the parent PublishProcessor
*/
PublishDisposable(Observer<? super T> actual, PublishSubject<T> parent) {
this.downstream = actual;
this.parent = parent;
}
public void onNext(T t) {
if (!get()) {
downstream.onNext(t);
}
}
public void onError(Throwable t) {
if (get()) {
RxJavaPlugins.onError(t);
} else {
downstream.onError(t);
}
}
public void onComplete() {
if (!get()) {
downstream.onComplete();
}
}
@Override
public void dispose() {
if (compareAndSet(false, true)) {
parent.remove(this);
}
}
@Override
public boolean isDisposed() {
return get();
}
}
}