Class SubmissionPublisher<T>

Type Parameters:
T - the published item type
All Implemented Interfaces:
AutoCloseable, Flow.Publisher<T> 
public class SubmissionPublisher<T>
extends Object
implements Flow.Publisher<T>, AutoCloseable

A 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 reactive-streams Publishers relying on drop handling and/or blocking for flow control.

A SubmissionPublisher uses the 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 Executors.newFixedThreadPool(int). Otherwise consider using the default, normally the ForkJoinPool.commonPool().

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 request do not directly result in buffer expansion, but risk saturation if unfilled requests exceed the maximum capacity. The default value of Flow.defaultBufferSize() may provide a useful starting point for choosing a capacity based on expected rates, resources, and usages.

A single SubmissionPublisher may be shared among multiple sources. Actions in a source thread prior to publishing an item or issuing a signal happen-before actions subsequent to the corresponding access by each subscriber. But reported estimates of lag and demand are designed for use in monitoring, not for synchronization control, and may reflect stale or inaccurate views of progress.

Publication methods support different policies about what to do when buffers are saturated. Method submit blocks until resources are available. This is simplest, but least responsive. The offer methods may drop items (either immediately or with bounded timeout), but provide an opportunity to interpose a handler and then retry.

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 onNext, but exceptions in methods onSubscribe, onError and onComplete are not recorded or handled before cancellation. If the supplied Executor throws 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 closeExceptionally in these cases.

Method 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.

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.) 

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();
   }
 }

Here is an example of a 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 submit, along with other utility methods. 

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(); }
 }
Since:
9

Constructors

Constructor Description
SubmissionPublisher()

Creates a new SubmissionPublisher using the 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 Flow.defaultBufferSize(), and no handler for Subscriber exceptions in method onNext.
SubmissionPublisher​(Executor executor, 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 no handler for Subscriber exceptions in method onNext.
SubmissionPublisher​(Executor executor, int maxBufferCapacity, BiConsumer<? super Flow.Subscriber<? super T>,​? super Throwable> handler)

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 onNext.

Methods

Modifier and Type Method Description
void close()

Unless already closed, issues onComplete signals to current subscribers, and disallows subsequent attempts to publish.
void closeExceptionally​(Throwable error)

Unless already closed, issues onError signals to current subscribers with the given error, and disallows subsequent attempts to publish.
CompletableFuture<Void> consume​(Consumer<? super T> consumer)

Processes all published items using the given Consumer function.
int estimateMaximumLag()

Returns an estimate of the maximum number of items produced but not yet consumed among all current subscribers.
long estimateMinimumDemand()

Returns an estimate of the minimum number of items requested (via request) but not yet produced, among all current subscribers.
Throwable getClosedException()

Returns the exception associated with closeExceptionally, or null if not closed or if closed normally.
Executor getExecutor()

Returns the Executor used for asynchronous delivery.
int getMaxBufferCapacity()

Returns the maximum per-subscriber buffer capacity.
int getNumberOfSubscribers()

Returns the number of current subscribers.
List<Flow.Subscriber<? super T>> getSubscribers()

Returns a list of current subscribers for monitoring and tracking purposes, not for invoking Flow.Subscriber methods on the subscribers.
boolean hasSubscribers()

Returns true if this publisher has any subscribers.
boolean isClosed()

Returns true if this publisher is not accepting submissions.
boolean isSubscribed​(Flow.Subscriber<? super T> subscriber)

Returns true if the given Subscriber is currently subscribed.
int offer​(T item, long timeout, TimeUnit unit, BiPredicate<Flow.Subscriber<? super T>,​? super T> onDrop)

Publishes the given item, if possible, to each current subscriber by asynchronously invoking its 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.
int offer​(T item, BiPredicate<Flow.Subscriber<? super T>,​? super T> onDrop)

Publishes the given item, if possible, to each current subscriber by asynchronously invoking its onNext method.
int submit​(T item)

Publishes the given item to each current subscriber by asynchronously invoking its onNext method, blocking uninterruptibly while resources for any subscriber are unavailable.
void subscribe​(Flow.Subscriber<? super T> subscriber)

Adds the given Subscriber unless already subscribed.

Methods declared in class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructors

SubmissionPublisher

public SubmissionPublisher(Executor executor,
                           int maxBufferCapacity,
                           BiConsumer<? super Flow.Subscriber<? super T>,​? super Throwable> handler)

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 onNext.

Parameters:
executor - the executor to use for async delivery, supporting creation of at least one independent thread
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 getMaxBufferCapacity() returns the actual value)
handler - if non-null, procedure to invoke upon exception thrown in method onNext
Throws:
NullPointerException - if executor is null
IllegalArgumentException - if maxBufferCapacity not positive

SubmissionPublisher

public SubmissionPublisher(Executor executor,
                           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 no handler for Subscriber exceptions in method onNext.

Parameters:
executor - the executor to use for async delivery, supporting creation of at least one independent thread
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 getMaxBufferCapacity() returns the actual value)
Throws:
NullPointerException - if executor is null
IllegalArgumentException - if maxBufferCapacity not positive

SubmissionPublisher

public SubmissionPublisher()

Creates a new SubmissionPublisher using the 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 Flow.defaultBufferSize(), and no handler for Subscriber exceptions in method onNext.

Methods

subscribe

public void subscribe(Flow.Subscriber<? super T> subscriber)

Adds the given Subscriber unless already subscribed. If already subscribed, the Subscriber's onError method is invoked on the existing subscription with an IllegalStateException. Otherwise, upon success, the Subscriber's onSubscribe method is invoked asynchronously with a new Flow.Subscription. If onSubscribe throws an exception, the subscription is cancelled. Otherwise, if this SubmissionPublisher was closed exceptionally, then the subscriber's onError method is invoked with the corresponding exception, or if closed without exception, the subscriber's onComplete method is invoked. Subscribers may enable receiving items by invoking the request method of the new Subscription, and may unsubscribe by invoking its cancel method.

Specified by:
subscribe in interface Flow.Publisher<T>
Parameters:
subscriber - the subscriber
Throws:
NullPointerException - if subscriber is null

submit

public int submit(T item)

Publishes the given item to each current subscriber by asynchronously invoking its 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.

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.

Parameters:
item - the (non-null) item to publish
Returns:
the estimated maximum lag among subscribers
Throws:
IllegalStateException - if closed
NullPointerException - if item is null
RejectedExecutionException - if thrown by Executor

offer

public int offer(T item,
                 BiPredicate<Flow.Subscriber<? super T>,​? super T> onDrop)

Publishes the given item, if possible, to each current subscriber by asynchronously invoking its 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 onError signal to the subscriber.

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.

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.

Parameters:
item - the (non-null) item to publish
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)
Returns:
if negative, the (negative) number of drops; otherwise an estimate of maximum lag
Throws:
IllegalStateException - if closed
NullPointerException - if item is null
RejectedExecutionException - if thrown by Executor

offer

public int offer(T item,
                 long timeout,
                 TimeUnit unit,
                 BiPredicate<Flow.Subscriber<? super T>,​? super T> onDrop)

Publishes the given item, if possible, to each current subscriber by asynchronously invoking its 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 onError signal to the subscriber.

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.

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.

Parameters:
item - the (non-null) item to publish
timeout - how long to wait for resources for any subscriber before giving up, in units of unit
unit - a TimeUnit determining how to interpret the timeout parameter
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)
Returns:
if negative, the (negative) number of drops; otherwise an estimate of maximum lag
Throws:
IllegalStateException - if closed
NullPointerException - if item is null
RejectedExecutionException - if thrown by Executor

close

public void close()

Unless already closed, issues onComplete signals to current subscribers, and disallows subsequent attempts to publish. Upon return, this method does NOT guarantee that all subscribers have yet completed.

Specified by:
close in interface AutoCloseable

closeExceptionally

public void closeExceptionally(Throwable error)

Unless already closed, issues 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 NOT guarantee that all subscribers have yet completed.

Parameters:
error - the onError argument sent to subscribers
Throws:
NullPointerException - if error is null

isClosed

public boolean isClosed()

Returns true if this publisher is not accepting submissions.

Returns:
true if closed

getClosedException

public Throwable getClosedException()

Returns the exception associated with closeExceptionally, or null if not closed or if closed normally.

Returns:
the exception, or null if none

hasSubscribers

public boolean hasSubscribers()

Returns true if this publisher has any subscribers.

Returns:
true if this publisher has any subscribers

getNumberOfSubscribers

public int getNumberOfSubscribers()

Returns the number of current subscribers.

Returns:
the number of current subscribers

getExecutor

public Executor getExecutor()

Returns the Executor used for asynchronous delivery.

Returns:
the Executor used for asynchronous delivery

getMaxBufferCapacity

public int getMaxBufferCapacity()

Returns the maximum per-subscriber buffer capacity.

Returns:
the maximum per-subscriber buffer capacity

getSubscribers

public List<Flow.Subscriber<? super T>> getSubscribers()

Returns a list of current subscribers for monitoring and tracking purposes, not for invoking Flow.Subscriber methods on the subscribers.

Returns:
list of current subscribers

isSubscribed

public boolean isSubscribed(Flow.Subscriber<? super T> subscriber)

Returns true if the given Subscriber is currently subscribed.

Parameters:
subscriber - the subscriber
Returns:
true if currently subscribed
Throws:
NullPointerException - if subscriber is null

estimateMinimumDemand

public long estimateMinimumDemand()

Returns an estimate of the minimum number of items requested (via request) but not yet produced, among all current subscribers.

Returns:
the estimate, or zero if no subscribers

estimateMaximumLag

public int estimateMaximumLag()

Returns an estimate of the maximum number of items produced but not yet consumed among all current subscribers.

Returns:
the estimate

consume

public CompletableFuture<Void> consume(Consumer<? super T> consumer)

Processes all published items using the given Consumer function. Returns a CompletableFuture that is completed normally when this publisher signals 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.

Parameters:
consumer - the function applied to each onNext item
Returns:
a CompletableFuture that is completed normally when the publisher signals onComplete, and exceptionally upon any error or cancellation
Throws:
NullPointerException - if consumer is null

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