8173909: Miscellaneous changes imported from jsr166 CVS 2017-03
Reviewed-by: martin, psandoz
/*
* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
*
* This code is free software; you can redistribute it and/or modify it
* under the terms of the GNU General Public License version 2 only, as
* published by the Free Software Foundation. Oracle designates this
* particular file as subject to the "Classpath" exception as provided
* by Oracle in the LICENSE file that accompanied this code.
*
* This code is distributed in the hope that it will be useful, but WITHOUT
* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
* version 2 for more details (a copy is included in the LICENSE file that
* accompanied this code).
*
* You should have received a copy of the GNU General Public License version
* 2 along with this work; if not, write to the Free Software Foundation,
* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
*
* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
* or visit www.oracle.com if you need additional information or have any
* questions.
*/
/*
* This file is available under and governed by the GNU General Public
* License version 2 only, as published by the Free Software Foundation.
* However, the following notice accompanied the original version of this
* file:
*
* Written by Doug Lea with assistance from members of JCP JSR-166
* Expert Group and released to the public domain, as explained at
* http://creativecommons.org/publicdomain/zero/1.0/
*/
package java.util.concurrent;
import java.lang.invoke.MethodHandles;
import java.lang.invoke.VarHandle;
import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.locks.LockSupport;
import java.util.function.BiConsumer;
import java.util.function.BiPredicate;
import java.util.function.Consumer;
/**
* A {@link Flow.Publisher} that asynchronously issues submitted
* (non-null) items to current subscribers until it is closed. Each
* current subscriber receives newly submitted items in the same order
* unless drops or exceptions are encountered. Using a
* SubmissionPublisher allows item generators to act as compliant <a
* href="http://www.reactive-streams.org/"> reactive-streams</a>
* Publishers relying on drop handling and/or blocking for flow
* control.
*
* <p>A SubmissionPublisher uses the {@link Executor} supplied in its
* constructor for delivery to subscribers. The best choice of
* Executor depends on expected usage. If the generator(s) of
* submitted items run in separate threads, and the number of
* subscribers can be estimated, consider using a {@link
* Executors#newFixedThreadPool}. Otherwise consider using the
* default, normally the {@link ForkJoinPool#commonPool}.
*
* <p>Buffering allows producers and consumers to transiently operate
* at different rates. Each subscriber uses an independent buffer.
* Buffers are created upon first use and expanded as needed up to the
* given maximum. (The enforced capacity may be rounded up to the
* nearest power of two and/or bounded by the largest value supported
* by this implementation.) Invocations of {@link
* Flow.Subscription#request(long) request} do not directly result in
* buffer expansion, but risk saturation if unfilled requests exceed
* the maximum capacity. The default value of {@link
* Flow#defaultBufferSize()} may provide a useful starting point for
* choosing a capacity based on expected rates, resources, and usages.
*
* <p>Publication methods support different policies about what to do
* when buffers are saturated. Method {@link #submit(Object) submit}
* blocks until resources are available. This is simplest, but least
* responsive. The {@code offer} methods may drop items (either
* immediately or with bounded timeout), but provide an opportunity to
* interpose a handler and then retry.
*
* <p>If any Subscriber method throws an exception, its subscription
* is cancelled. If a handler is supplied as a constructor argument,
* it is invoked before cancellation upon an exception in method
* {@link Flow.Subscriber#onNext onNext}, but exceptions in methods
* {@link Flow.Subscriber#onSubscribe onSubscribe},
* {@link Flow.Subscriber#onError(Throwable) onError} and
* {@link Flow.Subscriber#onComplete() onComplete} are not recorded or
* handled before cancellation. If the supplied Executor throws
* {@link RejectedExecutionException} (or any other RuntimeException
* or Error) when attempting to execute a task, or a drop handler
* throws an exception when processing a dropped item, then the
* exception is rethrown. In these cases, not all subscribers will
* have been issued the published item. It is usually good practice to
* {@link #closeExceptionally closeExceptionally} in these cases.
*
* <p>Method {@link #consume(Consumer)} simplifies support for a
* common case in which the only action of a subscriber is to request
* and process all items using a supplied function.
*
* <p>This class may also serve as a convenient base for subclasses
* that generate items, and use the methods in this class to publish
* them. For example here is a class that periodically publishes the
* items generated from a supplier. (In practice you might add methods
* to independently start and stop generation, to share Executors
* among publishers, and so on, or use a SubmissionPublisher as a
* component rather than a superclass.)
*
* <pre> {@code
* class PeriodicPublisher<T> extends SubmissionPublisher<T> {
* final ScheduledFuture<?> periodicTask;
* final ScheduledExecutorService scheduler;
* PeriodicPublisher(Executor executor, int maxBufferCapacity,
* Supplier<? extends T> supplier,
* long period, TimeUnit unit) {
* super(executor, maxBufferCapacity);
* scheduler = new ScheduledThreadPoolExecutor(1);
* periodicTask = scheduler.scheduleAtFixedRate(
* () -> submit(supplier.get()), 0, period, unit);
* }
* public void close() {
* periodicTask.cancel(false);
* scheduler.shutdown();
* super.close();
* }
* }}</pre>
*
* <p>Here is an example of a {@link Flow.Processor} implementation.
* It uses single-step requests to its publisher for simplicity of
* illustration. A more adaptive version could monitor flow using the
* lag estimate returned from {@code submit}, along with other utility
* methods.
*
* <pre> {@code
* class TransformProcessor<S,T> extends SubmissionPublisher<T>
* implements Flow.Processor<S,T> {
* final Function<? super S, ? extends T> function;
* Flow.Subscription subscription;
* TransformProcessor(Executor executor, int maxBufferCapacity,
* Function<? super S, ? extends T> function) {
* super(executor, maxBufferCapacity);
* this.function = function;
* }
* public void onSubscribe(Flow.Subscription subscription) {
* (this.subscription = subscription).request(1);
* }
* public void onNext(S item) {
* subscription.request(1);
* submit(function.apply(item));
* }
* public void onError(Throwable ex) { closeExceptionally(ex); }
* public void onComplete() { close(); }
* }}</pre>
*
* @param <T> the published item type
* @author Doug Lea
* @since 9
*/
public class SubmissionPublisher<T> implements Flow.Publisher<T>,
AutoCloseable {
/*
* Most mechanics are handled by BufferedSubscription. This class
* mainly tracks subscribers and ensures sequentiality, by using
* built-in synchronization locks across public methods. (Using
* built-in locks works well in the most typical case in which
* only one thread submits items).
*/
/** The largest possible power of two array size. */
static final int BUFFER_CAPACITY_LIMIT = 1 << 30;
/** Round capacity to power of 2, at most limit. */
static final int roundCapacity(int cap) {
int n = cap - 1;
n |= n >>> 1;
n |= n >>> 2;
n |= n >>> 4;
n |= n >>> 8;
n |= n >>> 16;
return (n <= 0) ? 1 : // at least 1
(n >= BUFFER_CAPACITY_LIMIT) ? BUFFER_CAPACITY_LIMIT : n + 1;
}
// default Executor setup; nearly the same as CompletableFuture
/**
* Default executor -- ForkJoinPool.commonPool() unless it cannot
* support parallelism.
*/
private static final Executor ASYNC_POOL =
(ForkJoinPool.getCommonPoolParallelism() > 1) ?
ForkJoinPool.commonPool() : new ThreadPerTaskExecutor();
/** Fallback if ForkJoinPool.commonPool() cannot support parallelism */
private static final class ThreadPerTaskExecutor implements Executor {
ThreadPerTaskExecutor() {} // prevent access constructor creation
public void execute(Runnable r) { new Thread(r).start(); }
}
/**
* Clients (BufferedSubscriptions) are maintained in a linked list
* (via their "next" fields). This works well for publish loops.
* It requires O(n) traversal to check for duplicate subscribers,
* but we expect that subscribing is much less common than
* publishing. Unsubscribing occurs only during traversal loops,
* when BufferedSubscription methods return negative values
* signifying that they have been disabled. To reduce
* head-of-line blocking, submit and offer methods first call
* BufferedSubscription.offer on each subscriber, and place
* saturated ones in retries list (using nextRetry field), and
* retry, possibly blocking or dropping.
*/
BufferedSubscription<T> clients;
/** Run status, updated only within locks */
volatile boolean closed;
/** If non-null, the exception in closeExceptionally */
volatile Throwable closedException;
// Parameters for constructing BufferedSubscriptions
final Executor executor;
final BiConsumer<? super Flow.Subscriber<? super T>, ? super Throwable> onNextHandler;
final int maxBufferCapacity;
/**
* Creates a new SubmissionPublisher using the given Executor for
* async delivery to subscribers, with the given maximum buffer size
* for each subscriber, and, if non-null, the given handler invoked
* when any Subscriber throws an exception in method {@link
* Flow.Subscriber#onNext(Object) onNext}.
*
* @param executor the executor to use for async delivery,
* supporting creation of at least one independent thread
* @param maxBufferCapacity the maximum capacity for each
* subscriber's buffer (the enforced capacity may be rounded up to
* the nearest power of two and/or bounded by the largest value
* supported by this implementation; method {@link #getMaxBufferCapacity}
* returns the actual value)
* @param handler if non-null, procedure to invoke upon exception
* thrown in method {@code onNext}
* @throws NullPointerException if executor is null
* @throws IllegalArgumentException if maxBufferCapacity not
* positive
*/
public SubmissionPublisher(Executor executor, int maxBufferCapacity,
BiConsumer<? super Flow.Subscriber<? super T>, ? super Throwable> handler) {
if (executor == null)
throw new NullPointerException();
if (maxBufferCapacity <= 0)
throw new IllegalArgumentException("capacity must be positive");
this.executor = executor;
this.onNextHandler = handler;
this.maxBufferCapacity = roundCapacity(maxBufferCapacity);
}
/**
* Creates a new SubmissionPublisher using the given Executor for
* async delivery to subscribers, with the given maximum buffer size
* for each subscriber, and no handler for Subscriber exceptions in
* method {@link Flow.Subscriber#onNext(Object) onNext}.
*
* @param executor the executor to use for async delivery,
* supporting creation of at least one independent thread
* @param maxBufferCapacity the maximum capacity for each
* subscriber's buffer (the enforced capacity may be rounded up to
* the nearest power of two and/or bounded by the largest value
* supported by this implementation; method {@link #getMaxBufferCapacity}
* returns the actual value)
* @throws NullPointerException if executor is null
* @throws IllegalArgumentException if maxBufferCapacity not
* positive
*/
public SubmissionPublisher(Executor executor, int maxBufferCapacity) {
this(executor, maxBufferCapacity, null);
}
/**
* Creates a new SubmissionPublisher using the {@link
* ForkJoinPool#commonPool()} for async delivery to subscribers
* (unless it does not support a parallelism level of at least two,
* in which case, a new Thread is created to run each task), with
* maximum buffer capacity of {@link Flow#defaultBufferSize}, and no
* handler for Subscriber exceptions in method {@link
* Flow.Subscriber#onNext(Object) onNext}.
*/
public SubmissionPublisher() {
this(ASYNC_POOL, Flow.defaultBufferSize(), null);
}
/**
* Adds the given Subscriber unless already subscribed. If already
* subscribed, the Subscriber's {@link
* Flow.Subscriber#onError(Throwable) onError} method is invoked on
* the existing subscription with an {@link IllegalStateException}.
* Otherwise, upon success, the Subscriber's {@link
* Flow.Subscriber#onSubscribe onSubscribe} method is invoked
* asynchronously with a new {@link Flow.Subscription}. If {@link
* Flow.Subscriber#onSubscribe onSubscribe} throws an exception, the
* subscription is cancelled. Otherwise, if this SubmissionPublisher
* was closed exceptionally, then the subscriber's {@link
* Flow.Subscriber#onError onError} method is invoked with the
* corresponding exception, or if closed without exception, the
* subscriber's {@link Flow.Subscriber#onComplete() onComplete}
* method is invoked. Subscribers may enable receiving items by
* invoking the {@link Flow.Subscription#request(long) request}
* method of the new Subscription, and may unsubscribe by invoking
* its {@link Flow.Subscription#cancel() cancel} method.
*
* @param subscriber the subscriber
* @throws NullPointerException if subscriber is null
*/
public void subscribe(Flow.Subscriber<? super T> subscriber) {
if (subscriber == null) throw new NullPointerException();
BufferedSubscription<T> subscription =
new BufferedSubscription<T>(subscriber, executor,
onNextHandler, maxBufferCapacity);
synchronized (this) {
for (BufferedSubscription<T> b = clients, pred = null;;) {
if (b == null) {
Throwable ex;
subscription.onSubscribe();
if ((ex = closedException) != null)
subscription.onError(ex);
else if (closed)
subscription.onComplete();
else if (pred == null)
clients = subscription;
else
pred.next = subscription;
break;
}
BufferedSubscription<T> next = b.next;
if (b.isDisabled()) { // remove
b.next = null; // detach
if (pred == null)
clients = next;
else
pred.next = next;
}
else if (subscriber.equals(b.subscriber)) {
b.onError(new IllegalStateException("Duplicate subscribe"));
break;
}
else
pred = b;
b = next;
}
}
}
/**
* Publishes the given item to each current subscriber by
* asynchronously invoking its {@link Flow.Subscriber#onNext(Object)
* onNext} method, blocking uninterruptibly while resources for any
* subscriber are unavailable. This method returns an estimate of
* the maximum lag (number of items submitted but not yet consumed)
* among all current subscribers. This value is at least one
* (accounting for this submitted item) if there are any
* subscribers, else zero.
*
* <p>If the Executor for this publisher throws a
* RejectedExecutionException (or any other RuntimeException or
* Error) when attempting to asynchronously notify subscribers,
* then this exception is rethrown, in which case not all
* subscribers will have been issued this item.
*
* @param item the (non-null) item to publish
* @return the estimated maximum lag among subscribers
* @throws IllegalStateException if closed
* @throws NullPointerException if item is null
* @throws RejectedExecutionException if thrown by Executor
*/
public int submit(T item) {
if (item == null) throw new NullPointerException();
int lag = 0;
boolean complete;
synchronized (this) {
complete = closed;
BufferedSubscription<T> b = clients;
if (!complete) {
BufferedSubscription<T> pred = null, r = null, rtail = null;
while (b != null) {
BufferedSubscription<T> next = b.next;
int stat = b.offer(item);
if (stat < 0) { // disabled
b.next = null;
if (pred == null)
clients = next;
else
pred.next = next;
}
else {
if (stat > lag)
lag = stat;
else if (stat == 0) { // place on retry list
b.nextRetry = null;
if (rtail == null)
r = b;
else
rtail.nextRetry = b;
rtail = b;
}
pred = b;
}
b = next;
}
while (r != null) {
BufferedSubscription<T> nextRetry = r.nextRetry;
r.nextRetry = null;
int stat = r.submit(item);
if (stat > lag)
lag = stat;
else if (stat < 0 && clients == r)
clients = r.next; // postpone internal unsubscribes
r = nextRetry;
}
}
}
if (complete)
throw new IllegalStateException("Closed");
else
return lag;
}
/**
* Publishes the given item, if possible, to each current subscriber
* by asynchronously invoking its {@link
* Flow.Subscriber#onNext(Object) onNext} method. The item may be
* dropped by one or more subscribers if resource limits are
* exceeded, in which case the given handler (if non-null) is
* invoked, and if it returns true, retried once. Other calls to
* methods in this class by other threads are blocked while the
* handler is invoked. Unless recovery is assured, options are
* usually limited to logging the error and/or issuing an {@link
* Flow.Subscriber#onError(Throwable) onError} signal to the
* subscriber.
*
* <p>This method returns a status indicator: If negative, it
* represents the (negative) number of drops (failed attempts to
* issue the item to a subscriber). Otherwise it is an estimate of
* the maximum lag (number of items submitted but not yet
* consumed) among all current subscribers. This value is at least
* one (accounting for this submitted item) if there are any
* subscribers, else zero.
*
* <p>If the Executor for this publisher throws a
* RejectedExecutionException (or any other RuntimeException or
* Error) when attempting to asynchronously notify subscribers, or
* the drop handler throws an exception when processing a dropped
* item, then this exception is rethrown.
*
* @param item the (non-null) item to publish
* @param onDrop if non-null, the handler invoked upon a drop to a
* subscriber, with arguments of the subscriber and item; if it
* returns true, an offer is re-attempted (once)
* @return if negative, the (negative) number of drops; otherwise
* an estimate of maximum lag
* @throws IllegalStateException if closed
* @throws NullPointerException if item is null
* @throws RejectedExecutionException if thrown by Executor
*/
public int offer(T item,
BiPredicate<Flow.Subscriber<? super T>, ? super T> onDrop) {
return doOffer(0L, item, onDrop);
}
/**
* Publishes the given item, if possible, to each current subscriber
* by asynchronously invoking its {@link
* Flow.Subscriber#onNext(Object) onNext} method, blocking while
* resources for any subscription are unavailable, up to the
* specified timeout or until the caller thread is interrupted, at
* which point the given handler (if non-null) is invoked, and if it
* returns true, retried once. (The drop handler may distinguish
* timeouts from interrupts by checking whether the current thread
* is interrupted.) Other calls to methods in this class by other
* threads are blocked while the handler is invoked. Unless
* recovery is assured, options are usually limited to logging the
* error and/or issuing an {@link Flow.Subscriber#onError(Throwable)
* onError} signal to the subscriber.
*
* <p>This method returns a status indicator: If negative, it
* represents the (negative) number of drops (failed attempts to
* issue the item to a subscriber). Otherwise it is an estimate of
* the maximum lag (number of items submitted but not yet
* consumed) among all current subscribers. This value is at least
* one (accounting for this submitted item) if there are any
* subscribers, else zero.
*
* <p>If the Executor for this publisher throws a
* RejectedExecutionException (or any other RuntimeException or
* Error) when attempting to asynchronously notify subscribers, or
* the drop handler throws an exception when processing a dropped
* item, then this exception is rethrown.
*
* @param item the (non-null) item to publish
* @param timeout how long to wait for resources for any subscriber
* before giving up, in units of {@code unit}
* @param unit a {@code TimeUnit} determining how to interpret the
* {@code timeout} parameter
* @param onDrop if non-null, the handler invoked upon a drop to a
* subscriber, with arguments of the subscriber and item; if it
* returns true, an offer is re-attempted (once)
* @return if negative, the (negative) number of drops; otherwise
* an estimate of maximum lag
* @throws IllegalStateException if closed
* @throws NullPointerException if item is null
* @throws RejectedExecutionException if thrown by Executor
*/
public int offer(T item, long timeout, TimeUnit unit,
BiPredicate<Flow.Subscriber<? super T>, ? super T> onDrop) {
return doOffer(unit.toNanos(timeout), item, onDrop);
}
/** Common implementation for both forms of offer */
final int doOffer(long nanos, T item,
BiPredicate<Flow.Subscriber<? super T>, ? super T> onDrop) {
if (item == null) throw new NullPointerException();
int lag = 0, drops = 0;
boolean complete;
synchronized (this) {
complete = closed;
BufferedSubscription<T> b = clients;
if (!complete) {
BufferedSubscription<T> pred = null, r = null, rtail = null;
while (b != null) {
BufferedSubscription<T> next = b.next;
int stat = b.offer(item);
if (stat < 0) {
b.next = null;
if (pred == null)
clients = next;
else
pred.next = next;
}
else {
if (stat > lag)
lag = stat;
else if (stat == 0) {
b.nextRetry = null;
if (rtail == null)
r = b;
else
rtail.nextRetry = b;
rtail = b;
}
else if (stat > lag)
lag = stat;
pred = b;
}
b = next;
}
while (r != null) {
BufferedSubscription<T> nextRetry = r.nextRetry;
r.nextRetry = null;
int stat = (nanos > 0L)
? r.timedOffer(item, nanos)
: r.offer(item);
if (stat == 0 && onDrop != null &&
onDrop.test(r.subscriber, item))
stat = r.offer(item);
if (stat == 0)
++drops;
else if (stat > lag)
lag = stat;
else if (stat < 0 && clients == r)
clients = r.next;
r = nextRetry;
}
}
}
if (complete)
throw new IllegalStateException("Closed");
else
return (drops > 0) ? -drops : lag;
}
/**
* Unless already closed, issues {@link
* Flow.Subscriber#onComplete() onComplete} signals to current
* subscribers, and disallows subsequent attempts to publish.
* Upon return, this method does <em>NOT</em> guarantee that all
* subscribers have yet completed.
*/
public void close() {
if (!closed) {
BufferedSubscription<T> b;
synchronized (this) {
b = clients;
clients = null;
closed = true;
}
while (b != null) {
BufferedSubscription<T> next = b.next;
b.next = null;
b.onComplete();
b = next;
}
}
}
/**
* Unless already closed, issues {@link
* Flow.Subscriber#onError(Throwable) onError} signals to current
* subscribers with the given error, and disallows subsequent
* attempts to publish. Future subscribers also receive the given
* error. Upon return, this method does <em>NOT</em> guarantee
* that all subscribers have yet completed.
*
* @param error the {@code onError} argument sent to subscribers
* @throws NullPointerException if error is null
*/
public void closeExceptionally(Throwable error) {
if (error == null)
throw new NullPointerException();
if (!closed) {
BufferedSubscription<T> b;
synchronized (this) {
b = clients;
clients = null;
closed = true;
closedException = error;
}
while (b != null) {
BufferedSubscription<T> next = b.next;
b.next = null;
b.onError(error);
b = next;
}
}
}
/**
* Returns true if this publisher is not accepting submissions.
*
* @return true if closed
*/
public boolean isClosed() {
return closed;
}
/**
* Returns the exception associated with {@link
* #closeExceptionally(Throwable) closeExceptionally}, or null if
* not closed or if closed normally.
*
* @return the exception, or null if none
*/
public Throwable getClosedException() {
return closedException;
}
/**
* Returns true if this publisher has any subscribers.
*
* @return true if this publisher has any subscribers
*/
public boolean hasSubscribers() {
boolean nonEmpty = false;
if (!closed) {
synchronized (this) {
for (BufferedSubscription<T> b = clients; b != null;) {
BufferedSubscription<T> next = b.next;
if (b.isDisabled()) {
b.next = null;
b = clients = next;
}
else {
nonEmpty = true;
break;
}
}
}
}
return nonEmpty;
}
/**
* Returns the number of current subscribers.
*
* @return the number of current subscribers
*/
public int getNumberOfSubscribers() {
int count = 0;
if (!closed) {
synchronized (this) {
BufferedSubscription<T> pred = null, next;
for (BufferedSubscription<T> b = clients; b != null; b = next) {
next = b.next;
if (b.isDisabled()) {
b.next = null;
if (pred == null)
clients = next;
else
pred.next = next;
}
else {
pred = b;
++count;
}
}
}
}
return count;
}
/**
* Returns the Executor used for asynchronous delivery.
*
* @return the Executor used for asynchronous delivery
*/
public Executor getExecutor() {
return executor;
}
/**
* Returns the maximum per-subscriber buffer capacity.
*
* @return the maximum per-subscriber buffer capacity
*/
public int getMaxBufferCapacity() {
return maxBufferCapacity;
}
/**
* Returns a list of current subscribers for monitoring and
* tracking purposes, not for invoking {@link Flow.Subscriber}
* methods on the subscribers.
*
* @return list of current subscribers
*/
public List<Flow.Subscriber<? super T>> getSubscribers() {
ArrayList<Flow.Subscriber<? super T>> subs = new ArrayList<>();
synchronized (this) {
BufferedSubscription<T> pred = null, next;
for (BufferedSubscription<T> b = clients; b != null; b = next) {
next = b.next;
if (b.isDisabled()) {
b.next = null;
if (pred == null)
clients = next;
else
pred.next = next;
}
else
subs.add(b.subscriber);
}
}
return subs;
}
/**
* Returns true if the given Subscriber is currently subscribed.
*
* @param subscriber the subscriber
* @return true if currently subscribed
* @throws NullPointerException if subscriber is null
*/
public boolean isSubscribed(Flow.Subscriber<? super T> subscriber) {
if (subscriber == null) throw new NullPointerException();
if (!closed) {
synchronized (this) {
BufferedSubscription<T> pred = null, next;
for (BufferedSubscription<T> b = clients; b != null; b = next) {
next = b.next;
if (b.isDisabled()) {
b.next = null;
if (pred == null)
clients = next;
else
pred.next = next;
}
else if (subscriber.equals(b.subscriber))
return true;
else
pred = b;
}
}
}
return false;
}
/**
* Returns an estimate of the minimum number of items requested
* (via {@link Flow.Subscription#request(long) request}) but not
* yet produced, among all current subscribers.
*
* @return the estimate, or zero if no subscribers
*/
public long estimateMinimumDemand() {
long min = Long.MAX_VALUE;
boolean nonEmpty = false;
synchronized (this) {
BufferedSubscription<T> pred = null, next;
for (BufferedSubscription<T> b = clients; b != null; b = next) {
int n; long d;
next = b.next;
if ((n = b.estimateLag()) < 0) {
b.next = null;
if (pred == null)
clients = next;
else
pred.next = next;
}
else {
if ((d = b.demand - n) < min)
min = d;
nonEmpty = true;
pred = b;
}
}
}
return nonEmpty ? min : 0;
}
/**
* Returns an estimate of the maximum number of items produced but
* not yet consumed among all current subscribers.
*
* @return the estimate
*/
public int estimateMaximumLag() {
int max = 0;
synchronized (this) {
BufferedSubscription<T> pred = null, next;
for (BufferedSubscription<T> b = clients; b != null; b = next) {
int n;
next = b.next;
if ((n = b.estimateLag()) < 0) {
b.next = null;
if (pred == null)
clients = next;
else
pred.next = next;
}
else {
if (n > max)
max = n;
pred = b;
}
}
}
return max;
}
/**
* Processes all published items using the given Consumer function.
* Returns a CompletableFuture that is completed normally when this
* publisher signals {@link Flow.Subscriber#onComplete()
* onComplete}, or completed exceptionally upon any error, or an
* exception is thrown by the Consumer, or the returned
* CompletableFuture is cancelled, in which case no further items
* are processed.
*
* @param consumer the function applied to each onNext item
* @return a CompletableFuture that is completed normally
* when the publisher signals onComplete, and exceptionally
* upon any error or cancellation
* @throws NullPointerException if consumer is null
*/
public CompletableFuture<Void> consume(Consumer<? super T> consumer) {
if (consumer == null)
throw new NullPointerException();
CompletableFuture<Void> status = new CompletableFuture<>();
subscribe(new ConsumerSubscriber<T>(status, consumer));
return status;
}
/** Subscriber for method consume */
private static final class ConsumerSubscriber<T>
implements Flow.Subscriber<T> {
final CompletableFuture<Void> status;
final Consumer<? super T> consumer;
Flow.Subscription subscription;
ConsumerSubscriber(CompletableFuture<Void> status,
Consumer<? super T> consumer) {
this.status = status; this.consumer = consumer;
}
public final void onSubscribe(Flow.Subscription subscription) {
this.subscription = subscription;
status.whenComplete((v, e) -> subscription.cancel());
if (!status.isDone())
subscription.request(Long.MAX_VALUE);
}
public final void onError(Throwable ex) {
status.completeExceptionally(ex);
}
public final void onComplete() {
status.complete(null);
}
public final void onNext(T item) {
try {
consumer.accept(item);
} catch (Throwable ex) {
subscription.cancel();
status.completeExceptionally(ex);
}
}
}
/**
* A task for consuming buffer items and signals, created and
* executed whenever they become available. A task consumes as
* many items/signals as possible before terminating, at which
* point another task is created when needed. The dual Runnable
* and ForkJoinTask declaration saves overhead when executed by
* ForkJoinPools, without impacting other kinds of Executors.
*/
@SuppressWarnings("serial")
static final class ConsumerTask<T> extends ForkJoinTask<Void>
implements Runnable, CompletableFuture.AsynchronousCompletionTask {
final BufferedSubscription<T> consumer;
ConsumerTask(BufferedSubscription<T> consumer) {
this.consumer = consumer;
}
public final Void getRawResult() { return null; }
public final void setRawResult(Void v) {}
public final boolean exec() { consumer.consume(); return false; }
public final void run() { consumer.consume(); }
}
/**
* A bounded (ring) buffer with integrated control to start a
* consumer task whenever items are available. The buffer
* algorithm is similar to one used inside ForkJoinPool (see its
* internal documentation for details) specialized for the case of
* at most one concurrent producer and consumer, and power of two
* buffer sizes. This allows methods to operate without locks even
* while supporting resizing, blocking, task-triggering, and
* garbage-free buffers (nulling out elements when consumed),
* although supporting these does impose a bit of overhead
* compared to plain fixed-size ring buffers.
*
* The publisher guarantees a single producer via its lock. We
* ensure in this class that there is at most one consumer. The
* request and cancel methods must be fully thread-safe but are
* coded to exploit the most common case in which they are only
* called by consumers (usually within onNext).
*
* Execution control is managed using the ACTIVE ctl bit. We
* ensure that a task is active when consumable items (and
* usually, SUBSCRIBE, ERROR or COMPLETE signals) are present and
* there is demand (unfilled requests). This is complicated on
* the creation side by the possibility of exceptions when trying
* to execute tasks. These eventually force DISABLED state, but
* sometimes not directly. On the task side, termination (clearing
* ACTIVE) that would otherwise race with producers or request()
* calls uses the CONSUME keep-alive bit to force a recheck.
*
* The ctl field also manages run state. When DISABLED, no further
* updates are possible. Disabling may be preceded by setting
* ERROR or COMPLETE (or both -- ERROR has precedence), in which
* case the associated Subscriber methods are invoked, possibly
* synchronously if there is no active consumer task (including
* cases where execute() failed). The cancel() method is supported
* by treating as ERROR but suppressing onError signal.
*
* Support for blocking also exploits the fact that there is only
* one possible waiter. ManagedBlocker-compatible control fields
* are placed in this class itself rather than in wait-nodes.
* Blocking control relies on the "waiter" field. Producers set
* the field before trying to block, but must then recheck (via
* offer) before parking. Signalling then just unparks and clears
* waiter field. If the producer and/or consumer are using a
* ForkJoinPool, the producer attempts to help run consumer tasks
* via ForkJoinPool.helpAsyncBlocker before blocking.
*
* This class uses @Contended and heuristic field declaration
* ordering to reduce false-sharing-based memory contention among
* instances of BufferedSubscription, but it does not currently
* attempt to avoid memory contention among buffers. This field
* and element packing can hurt performance especially when each
* publisher has only one client operating at a high rate.
* Addressing this may require allocating substantially more space
* than users expect.
*/
@SuppressWarnings("serial")
@jdk.internal.vm.annotation.Contended
private static final class BufferedSubscription<T>
implements Flow.Subscription, ForkJoinPool.ManagedBlocker {
// Order-sensitive field declarations
long timeout; // > 0 if timed wait
volatile long demand; // # unfilled requests
int maxCapacity; // reduced on OOME
int putStat; // offer result for ManagedBlocker
volatile int ctl; // atomic run state flags
volatile int head; // next position to take
int tail; // next position to put
Object[] array; // buffer: null if disabled
Flow.Subscriber<? super T> subscriber; // null if disabled
Executor executor; // null if disabled
BiConsumer<? super Flow.Subscriber<? super T>, ? super Throwable> onNextHandler;
volatile Throwable pendingError; // holds until onError issued
volatile Thread waiter; // blocked producer thread
T putItem; // for offer within ManagedBlocker
BufferedSubscription<T> next; // used only by publisher
BufferedSubscription<T> nextRetry; // used only by publisher
// ctl values
static final int ACTIVE = 0x01; // consumer task active
static final int CONSUME = 0x02; // keep-alive for consumer task
static final int DISABLED = 0x04; // final state
static final int ERROR = 0x08; // signal onError then disable
static final int SUBSCRIBE = 0x10; // signal onSubscribe
static final int COMPLETE = 0x20; // signal onComplete when done
static final long INTERRUPTED = -1L; // timeout vs interrupt sentinel
/**
* Initial buffer capacity used when maxBufferCapacity is
* greater. Must be a power of two.
*/
static final int DEFAULT_INITIAL_CAP = 32;
BufferedSubscription(Flow.Subscriber<? super T> subscriber,
Executor executor,
BiConsumer<? super Flow.Subscriber<? super T>,
? super Throwable> onNextHandler,
int maxBufferCapacity) {
this.subscriber = subscriber;
this.executor = executor;
this.onNextHandler = onNextHandler;
this.maxCapacity = maxBufferCapacity;
this.array = new Object[maxBufferCapacity < DEFAULT_INITIAL_CAP ?
(maxBufferCapacity < 2 ? // at least 2 slots
2 : maxBufferCapacity) :
DEFAULT_INITIAL_CAP];
}
final boolean isDisabled() {
return ctl == DISABLED;
}
/**
* Returns estimated number of buffered items, or -1 if
* disabled.
*/
final int estimateLag() {
int n;
return (ctl == DISABLED) ? -1 : ((n = tail - head) > 0) ? n : 0;
}
/**
* Tries to add item and start consumer task if necessary.
* @return -1 if disabled, 0 if dropped, else estimated lag
*/
final int offer(T item) {
int h = head, t = tail, cap, size, stat;
Object[] a = array;
if (a != null && (cap = a.length) > 0 && cap >= (size = t + 1 - h)) {
a[(cap - 1) & t] = item; // relaxed writes OK
tail = t + 1;
stat = size;
}
else
stat = growAndAdd(a, item);
return (stat > 0 &&
(ctl & (ACTIVE | CONSUME)) != (ACTIVE | CONSUME)) ?
startOnOffer(stat) : stat;
}
/**
* Tries to create or expand buffer, then adds item if possible.
*/
private int growAndAdd(Object[] a, T item) {
boolean alloc;
int cap, stat;
if ((ctl & (ERROR | DISABLED)) != 0) {
cap = 0;
stat = -1;
alloc = false;
}
else if (a == null || (cap = a.length) <= 0) {
cap = 0;
stat = 1;
alloc = true;
}
else {
VarHandle.fullFence(); // recheck
int h = head, t = tail, size = t + 1 - h;
if (cap >= size) {
a[(cap - 1) & t] = item;
tail = t + 1;
stat = size;
alloc = false;
}
else if (cap >= maxCapacity) {
stat = 0; // cannot grow
alloc = false;
}
else {
stat = cap + 1;
alloc = true;
}
}
if (alloc) {
int newCap = (cap > 0) ? cap << 1 : 1;
if (newCap <= cap)
stat = 0;
else {
Object[] newArray = null;
try {
newArray = new Object[newCap];
} catch (Throwable ex) { // try to cope with OOME
}
if (newArray == null) {
if (cap > 0)
maxCapacity = cap; // avoid continuous failure
stat = 0;
}
else {
array = newArray;
int t = tail;
int newMask = newCap - 1;
if (a != null && cap > 0) {
int mask = cap - 1;
for (int j = head; j != t; ++j) {
int k = j & mask;
Object x = QA.getAcquire(a, k);
if (x != null && // races with consumer
QA.compareAndSet(a, k, x, null))
newArray[j & newMask] = x;
}
}
newArray[t & newMask] = item;
tail = t + 1;
}
}
}
return stat;
}
/**
* Spins/helps/blocks while offer returns 0. Called only if
* initial offer return 0.
*/
final int submit(T item) {
int stat;
if ((stat = offer(item)) == 0) {
putItem = item;
timeout = 0L;
putStat = 0;
ForkJoinPool.helpAsyncBlocker(executor, this);
if ((stat = putStat) == 0) {
try {
ForkJoinPool.managedBlock(this);
} catch (InterruptedException ie) {
timeout = INTERRUPTED;
}
stat = putStat;
}
if (timeout < 0L)
Thread.currentThread().interrupt();
}
return stat;
}
/**
* Timeout version; similar to submit.
*/
final int timedOffer(T item, long nanos) {
int stat;
if ((stat = offer(item)) == 0 && (timeout = nanos) > 0L) {
putItem = item;
putStat = 0;
ForkJoinPool.helpAsyncBlocker(executor, this);
if ((stat = putStat) == 0) {
try {
ForkJoinPool.managedBlock(this);
} catch (InterruptedException ie) {
timeout = INTERRUPTED;
}
stat = putStat;
}
if (timeout < 0L)
Thread.currentThread().interrupt();
}
return stat;
}
/**
* Tries to start consumer task after offer.
* @return -1 if now disabled, else argument
*/
private int startOnOffer(int stat) {
for (;;) {
Executor e; int c;
if ((c = ctl) == DISABLED || (e = executor) == null) {
stat = -1;
break;
}
else if ((c & ACTIVE) != 0) { // ensure keep-alive
if ((c & CONSUME) != 0 ||
CTL.compareAndSet(this, c, c | CONSUME))
break;
}
else if (demand == 0L || tail == head)
break;
else if (CTL.compareAndSet(this, c, c | (ACTIVE | CONSUME))) {
try {
e.execute(new ConsumerTask<T>(this));
break;
} catch (RuntimeException | Error ex) { // back out
do {} while (((c = ctl) & DISABLED) == 0 &&
(c & ACTIVE) != 0 &&
!CTL.weakCompareAndSet
(this, c, c & ~ACTIVE));
throw ex;
}
}
}
return stat;
}
private void signalWaiter(Thread w) {
waiter = null;
LockSupport.unpark(w); // release producer
}
/**
* Nulls out most fields, mainly to avoid garbage retention
* until publisher unsubscribes, but also to help cleanly stop
* upon error by nulling required components.
*/
private void detach() {
Thread w = waiter;
executor = null;
subscriber = null;
pendingError = null;
signalWaiter(w);
}
/**
* Issues error signal, asynchronously if a task is running,
* else synchronously.
*/
final void onError(Throwable ex) {
for (int c;;) {
if (((c = ctl) & (ERROR | DISABLED)) != 0)
break;
else if ((c & ACTIVE) != 0) {
pendingError = ex;
if (CTL.compareAndSet(this, c, c | ERROR))
break; // cause consumer task to exit
}
else if (CTL.compareAndSet(this, c, DISABLED)) {
Flow.Subscriber<? super T> s = subscriber;
if (s != null && ex != null) {
try {
s.onError(ex);
} catch (Throwable ignore) {
}
}
detach();
break;
}
}
}
/**
* Tries to start consumer task upon a signal or request;
* disables on failure.
*/
private void startOrDisable() {
Executor e;
if ((e = executor) != null) { // skip if already disabled
try {
e.execute(new ConsumerTask<T>(this));
} catch (Throwable ex) { // back out and force signal
for (int c;;) {
if ((c = ctl) == DISABLED || (c & ACTIVE) == 0)
break;
if (CTL.compareAndSet(this, c, c & ~ACTIVE)) {
onError(ex);
break;
}
}
}
}
}
final void onComplete() {
for (int c;;) {
if ((c = ctl) == DISABLED)
break;
if (CTL.compareAndSet(this, c,
c | (ACTIVE | CONSUME | COMPLETE))) {
if ((c & ACTIVE) == 0)
startOrDisable();
break;
}
}
}
final void onSubscribe() {
for (int c;;) {
if ((c = ctl) == DISABLED)
break;
if (CTL.compareAndSet(this, c,
c | (ACTIVE | CONSUME | SUBSCRIBE))) {
if ((c & ACTIVE) == 0)
startOrDisable();
break;
}
}
}
/**
* Causes consumer task to exit if active (without reporting
* onError unless there is already a pending error), and
* disables.
*/
public void cancel() {
for (int c;;) {
if ((c = ctl) == DISABLED)
break;
else if ((c & ACTIVE) != 0) {
if (CTL.compareAndSet(this, c,
c | (CONSUME | ERROR)))
break;
}
else if (CTL.compareAndSet(this, c, DISABLED)) {
detach();
break;
}
}
}
/**
* Adds to demand and possibly starts task.
*/
public void request(long n) {
if (n > 0L) {
for (;;) {
long prev = demand, d;
if ((d = prev + n) < prev) // saturate
d = Long.MAX_VALUE;
if (DEMAND.compareAndSet(this, prev, d)) {
for (int c, h;;) {
if ((c = ctl) == DISABLED)
break;
else if ((c & ACTIVE) != 0) {
if ((c & CONSUME) != 0 ||
CTL.compareAndSet(this, c, c | CONSUME))
break;
}
else if ((h = head) != tail) {
if (CTL.compareAndSet(this, c,
c | (ACTIVE|CONSUME))) {
startOrDisable();
break;
}
}
else if (head == h && tail == h)
break; // else stale
if (demand == 0L)
break;
}
break;
}
}
}
else if (n < 0L)
onError(new IllegalArgumentException(
"negative subscription request"));
}
public final boolean isReleasable() { // for ManagedBlocker
T item = putItem;
if (item != null) {
if ((putStat = offer(item)) == 0)
return false;
putItem = null;
}
return true;
}
public final boolean block() { // for ManagedBlocker
T item = putItem;
if (item != null) {
putItem = null;
long nanos = timeout;
long deadline = (nanos > 0L) ? System.nanoTime() + nanos : 0L;
while ((putStat = offer(item)) == 0) {
if (Thread.interrupted()) {
timeout = INTERRUPTED;
if (nanos > 0L)
break;
}
else if (nanos > 0L &&
(nanos = deadline - System.nanoTime()) <= 0L)
break;
else if (waiter == null)
waiter = Thread.currentThread();
else {
if (nanos > 0L)
LockSupport.parkNanos(this, nanos);
else
LockSupport.park(this);
waiter = null;
}
}
}
waiter = null;
return true;
}
/**
* Consumer loop, called from ConsumerTask, or indirectly
* when helping during submit.
*/
final void consume() {
Flow.Subscriber<? super T> s;
int h = head;
if ((s = subscriber) != null) { // else disabled
for (;;) {
long d = demand;
int c; Object[] a; int n, i; Object x; Thread w;
if (((c = ctl) & (ERROR | SUBSCRIBE | DISABLED)) != 0) {
if (!checkControl(s, c))
break;
}
else if ((a = array) == null || h == tail ||
(n = a.length) == 0 ||
(x = QA.getAcquire(a, i = (n - 1) & h)) == null) {
if (!checkEmpty(s, c))
break;
}
else if (d == 0L) {
if (!checkDemand(c))
break;
}
else if (((c & CONSUME) != 0 ||
CTL.compareAndSet(this, c, c | CONSUME)) &&
QA.compareAndSet(a, i, x, null)) {
HEAD.setRelease(this, ++h);
DEMAND.getAndAdd(this, -1L);
if ((w = waiter) != null)
signalWaiter(w);
try {
@SuppressWarnings("unchecked") T y = (T) x;
s.onNext(y);
} catch (Throwable ex) {
handleOnNext(s, ex);
}
}
}
}
}
/**
* Responds to control events in consume().
*/
private boolean checkControl(Flow.Subscriber<? super T> s, int c) {
boolean stat = true;
if ((c & SUBSCRIBE) != 0) {
if (CTL.compareAndSet(this, c, c & ~SUBSCRIBE)) {
try {
if (s != null)
s.onSubscribe(this);
} catch (Throwable ex) {
onError(ex);
}
}
}
else if ((c & ERROR) != 0) {
Throwable ex = pendingError;
ctl = DISABLED; // no need for CAS
if (ex != null) { // null if errorless cancel
try {
if (s != null)
s.onError(ex);
} catch (Throwable ignore) {
}
}
}
else {
detach();
stat = false;
}
return stat;
}
/**
* Responds to apparent emptiness in consume().
*/
private boolean checkEmpty(Flow.Subscriber<? super T> s, int c) {
boolean stat = true;
if (head == tail) {
if ((c & CONSUME) != 0)
CTL.compareAndSet(this, c, c & ~CONSUME);
else if ((c & COMPLETE) != 0) {
if (CTL.compareAndSet(this, c, DISABLED)) {
try {
if (s != null)
s.onComplete();
} catch (Throwable ignore) {
}
}
}
else if (CTL.compareAndSet(this, c, c & ~ACTIVE))
stat = false;
}
return stat;
}
/**
* Responds to apparent zero demand in consume().
*/
private boolean checkDemand(int c) {
boolean stat = true;
if (demand == 0L) {
if ((c & CONSUME) != 0)
CTL.compareAndSet(this, c, c & ~CONSUME);
else if (CTL.compareAndSet(this, c, c & ~ACTIVE))
stat = false;
}
return stat;
}
/**
* Processes exception in Subscriber.onNext.
*/
private void handleOnNext(Flow.Subscriber<? super T> s, Throwable ex) {
BiConsumer<? super Flow.Subscriber<? super T>, ? super Throwable> h;
if ((h = onNextHandler) != null) {
try {
h.accept(s, ex);
} catch (Throwable ignore) {
}
}
onError(ex);
}
// VarHandle mechanics
private static final VarHandle CTL;
private static final VarHandle TAIL;
private static final VarHandle HEAD;
private static final VarHandle DEMAND;
private static final VarHandle QA;
static {
try {
MethodHandles.Lookup l = MethodHandles.lookup();
CTL = l.findVarHandle(BufferedSubscription.class, "ctl",
int.class);
TAIL = l.findVarHandle(BufferedSubscription.class, "tail",
int.class);
HEAD = l.findVarHandle(BufferedSubscription.class, "head",
int.class);
DEMAND = l.findVarHandle(BufferedSubscription.class, "demand",
long.class);
QA = MethodHandles.arrayElementVarHandle(Object[].class);
} catch (ReflectiveOperationException e) {
throw new Error(e);
}
// Reduce the risk of rare disastrous classloading in first call to
// LockSupport.park: https://bugs.openjdk.java.net/browse/JDK-8074773
Class<?> ensureLoaded = LockSupport.class;
}
}
}