本文详细探讨了Reactor框架中FluxSink的工作原理及其实现细节，包括其内部类如BufferAsyncSink的作用机制，以及如何通过不同策略处理背压问题。
序
本文主要研究一下FluxSink的机制
FluxSink
reactor-core-3.1.3.RELEASE-sources.jar!/reactor/core/publisher/FluxSink.java
/**
* Wrapper API around a downstream Subscriber for emitting any number of
* next signals followed by zero or one onError/onComplete.
* <p>
* @param <T> the value type
*/
public interface FluxSink<T> {
/**
* @see Subscriber#onComplete()
*/
void complete();
/**
* Return the current subscriber {@link Context}.
* <p>
* {@link Context} can be enriched via {@link Flux#subscriberContext(Function)}
* operator or directly by a child subscriber overriding
* {@link CoreSubscriber#currentContext()}
*
* @return the current subscriber {@link Context}.
*/
Context currentContext();
/**
* @see Subscriber#onError(Throwable)
* @param e the exception to signal, not null
*/
void error(Throwable e);
/**
* Try emitting, might throw an unchecked exception.
* @see Subscriber#onNext(Object)
* @param t the value to emit, not null
*/
FluxSink<T> next(T t);
/**
* The current outstanding request amount.
* @return the current outstanding request amount
*/
long requestedFromDownstream();
/**
* Returns true if the downstream cancelled the sequence.
* @return true if the downstream cancelled the sequence
*/
boolean isCancelled();
/**
* Attaches a {@link LongConsumer} to this {@link FluxSink} that will be notified of
* any request to this sink.
* <p>
* For push/pull sinks created using {@link Flux#create(java.util.function.Consumer)}
* or {@link Flux#create(java.util.function.Consumer, FluxSink.OverflowStrategy)},
* the consumer
* is invoked for every request to enable a hybrid backpressure-enabled push/pull model.
* When bridging with asynchronous listener-based APIs, the {@code onRequest} callback
* may be used to request more data from source if required and to manage backpressure
* by delivering data to sink only when requests are pending.
* <p>
* For push-only sinks created using {@link Flux#push(java.util.function.Consumer)}
* or {@link Flux#push(java.util.function.Consumer, FluxSink.OverflowStrategy)},
* the consumer is invoked with an initial request of {@code Long.MAX_VALUE} when this method
* is invoked.
*
* @param consumer the consumer to invoke on each request
* @return {@link FluxSink} with a consumer that is notified of requests
*/
FluxSink<T> onRequest(LongConsumer consumer);
/**
* Associates a disposable resource with this FluxSink
* that will be disposed in case the downstream cancels the sequence
* via {@link org.reactivestreams.Subscription#cancel()}.
* @param d the disposable callback to use
* @return the {@link FluxSink} with resource to be disposed on cancel signal
*/
FluxSink<T> onCancel(Disposable d);
/**
* Associates a disposable resource with this FluxSink
* that will be disposed on the first terminate signal which may be
* a cancel, complete or error signal.
* @param d the disposable callback to use
* @return the {@link FluxSink} with resource to be disposed on first terminate signal
*/
FluxSink<T> onDispose(Disposable d);
/**
* Enumeration for backpressure handling.
*/
enum OverflowStrategy {
/**
* Completely ignore downstream backpressure requests.
* <p>
* This may yield {@link IllegalStateException} when queues get full downstream.
*/
IGNORE,
/**
* Signal an {@link IllegalStateException} when the downstream can't keep up
*/
ERROR,
/**
* Drop the incoming signal if the downstream is not ready to receive it.
*/
DROP,
/**
* Downstream will get only the latest signals from upstream.
*/
LATEST,
/**
* Buffer all signals if the downstream can't keep up.
* <p>
* Warning! This does unbounded buffering and may lead to {@link OutOfMemoryError}.
*/
BUFFER
}
}
复制代码注意OverflowStrategy.BUFFER使用的是一个无界队列，需要额外注意OOM问题
实例
public static void main(String[] args) throws InterruptedException {
final Flux<Integer> flux = Flux.<Integer> create(fluxSink -> {
//NOTE sink:class reactor.core.publisher.FluxCreate$SerializedSink
LOGGER.info("sink:{}",fluxSink.getClass());
while (true) {
LOGGER.info("sink next");
fluxSink.next(ThreadLocalRandom.current().nextInt());
}
}, FluxSink.OverflowStrategy.BUFFER);
//NOTE flux:class reactor.core.publisher.FluxCreate,prefetch:-1
LOGGER.info("flux:{},prefetch:{}",flux.getClass(),flux.getPrefetch());
flux.subscribe(e -> {
LOGGER.info("subscribe:{}",e);
try {
TimeUnit.SECONDS.sleep(10);
} catch (InterruptedException e1) {
e1.printStackTrace();
}
});
TimeUnit.MINUTES.sleep(20);
}
复制代码这里create创建的是reactor.core.publisher.FluxCreate，而其sink是reactor.core.publisher.FluxCreate$SerializedSink
Flux.subscribe
reactor-core-3.1.3.RELEASE-sources.jar!/reactor/core/publisher/Flux.java
/**
* Subscribe {@link Consumer} to this {@link Flux} that will respectively consume all the
* elements in the sequence, handle errors, react to completion, and request upon subscription.
* It will let the provided {@link Subscription subscriptionConsumer}
* request the adequate amount of data, or request unbounded demand
* {@code Long.MAX_VALUE} if no such consumer is provided.
* <p>
* For a passive version that observe and forward incoming data see {@link #doOnNext(java.util.function.Consumer)},
* {@link #doOnError(java.util.function.Consumer)}, {@link #doOnComplete(Runnable)}
* and {@link #doOnSubscribe(Consumer)}.
* <p>For a version that gives you more control over backpressure and the request, see
* {@link #subscribe(Subscriber)} with a {@link BaseSubscriber}.
* <p>
* Keep in mind that since the sequence can be asynchronous, this will immediately
* return control to the calling thread. This can give the impression the consumer is
* not invoked when executing in a main thread or a unit test for instance.
*
* <p>
* <img class="marble" src="https://raw.githubusercontent.com/reactor/reactor-core/v3.1.3.RELEASE/src/docs/marble/subscribecomplete.png" alt="">
*
* @param consumer the consumer to invoke on each value
* @param errorConsumer the consumer to invoke on error signal
* @param completeConsumer the consumer to invoke on complete signal
* @param subscriptionConsumer the consumer to invoke on subscribe signal, to be used
* for the initial {@link Subscription#request(long) request}, or null for max request
*
* @return a new {@link Disposable} that can be used to cancel the underlying {@link Subscription}
*/
public final Disposable subscribe(
@Nullable Consumer<? super T> consumer,
@Nullable Consumer<? super Throwable> errorConsumer,
@Nullable Runnable completeConsumer,
@Nullable Consumer<? super Subscription> subscriptionConsumer) {
return subscribeWith(new LambdaSubscriber<>(consumer, errorConsumer,
completeConsumer,
subscriptionConsumer));
}
@Override
public final void subscribe(Subscriber<? super T> actual) {
onLastAssembly(this).subscribe(Operators.toCoreSubscriber(actual));
}
复制代码创建的是LambdaSubscriber，最后调用FluxCreate.subscribe
FluxCreate.subscribe
reactor-core-3.1.3.RELEASE-sources.jar!/reactor/core/publisher/FluxCreate.java
public void subscribe(CoreSubscriber<? super T> actual) {
BaseSink<T> sink = createSink(actual, backpressure);
actual.onSubscribe(sink);
try {
source.accept(
createMode == CreateMode.PUSH_PULL ? new SerializedSink<>(sink) :
sink);
}
catch (Throwable ex) {
Exceptions.throwIfFatal(ex);
sink.error(Operators.onOperatorError(ex, actual.currentContext()));
}
}
static <T> BaseSink<T> createSink(CoreSubscriber<? super T> t,
OverflowStrategy backpressure) {
switch (backpressure) {
case IGNORE: {
return new IgnoreSink<>(t);
}
case ERROR: {
return new ErrorAsyncSink<>(t);
}
case DROP: {
return new DropAsyncSink<>(t);
}
case LATEST: {
return new LatestAsyncSink<>(t);
}
default: {
return new BufferAsyncSink<>(t, Queues.SMALL_BUFFER_SIZE);
}
}
}
复制代码先创建sink，这里创建的是BufferAsyncSink，然后调用LambdaSubscriber.onSubscribe 然后再调用source.accept，也就是调用fluxSink的lambda方法产生数据，开启stream模式
LambdaSubscriber.onSubscribe
reactor-core-3.1.3.RELEASE-sources.jar!/reactor/core/publisher/LambdaSubscriber.java
public final void onSubscribe(Subscription s) {
if (Operators.validate(subscription, s)) {
this.subscription = s;
if (subscriptionConsumer != null) {
try {
subscriptionConsumer.accept(s);
}
catch (Throwable t) {
Exceptions.throwIfFatal(t);
s.cancel();
onError(t);
}
}
else {
s.request(Long.MAX_VALUE);
}
}
}
复制代码这里又调用了BufferAsyncSink的request(Long.MAX_VALUE)，实际是调用BaseSink的request
public final void request(long n) {
if (Operators.validate(n)) {
Operators.addCap(REQUESTED, this, n);
LongConsumer consumer = requestConsumer;
if (n > 0 && consumer != null && !isCancelled()) {
consumer.accept(n);
}
onRequestedFromDownstream();
}
}
复制代码这里的onRequestedFromDownstream调用了BufferAsyncSink的onRequestedFromDownstream
@Override
void onRequestedFromDownstream() {
drain();
}
复制代码调用的是BufferAsyncSink的drain
BufferAsyncSink.drain
void drain() {
if (WIP.getAndIncrement(this) != 0) {
return;
}
int missed = 1;
final Subscriber<? super T> a = actual;
final Queue<T> q = queue;
for (; ; ) {
long r = requested;
long e = 0L;
while (e != r) {
if (isCancelled()) {
q.clear();
return;
}
boolean d = done;
T o = q.poll();
boolean empty = o == null;
if (d && empty) {
Throwable ex = error;
if (ex != null) {
super.error(ex);
}
else {
super.complete();
}
return;
}
if (empty) {
break;
}
a.onNext(o);
e++;
}
if (e == r) {
if (isCancelled()) {
q.clear();
return;
}
boolean d = done;
boolean empty = q.isEmpty();
if (d && empty) {
Throwable ex = error;
if (ex != null) {
super.error(ex);
}
else {
super.complete();
}
return;
}
}
if (e != 0) {
Operators.produced(REQUESTED, this, e);
}
missed = WIP.addAndGet(this, -missed);
if (missed == 0) {
break;
}
}
}
复制代码这里的queue是创建BufferAsyncSink指定的，默认是Queues.SMALL_BUFFER_SIZE(
Math.max(16,Integer.parseInt(System.getProperty("reactor.bufferSize.small", "256")))) 而这里的onNext则是同步调用LambdaSubscriber的consumer
FluxCreate.subscribe#source.accept
source.accept(
createMode == CreateMode.PUSH_PULL ? new SerializedSink<>(sink) :
sink);
复制代码CreateMode.PUSH_PULL这里对sink包装为SerializedSink，然后调用Flux.create自定义的lambda consumer
fluxSink -> {
//NOTE sink:class reactor.core.publisher.FluxCreate$SerializedSink
LOGGER.info("sink:{}",fluxSink.getClass());
while (true) {
LOGGER.info("sink next");
fluxSink.next(ThreadLocalRandom.current().nextInt());
}
}
复制代码之后就开启数据推送
SerializedSink.next
reactor-core-3.1.3.RELEASE-sources.jar!/reactor/core/publisher/FluxCreate.java#SerializedSink.next
public FluxSink<T> next(T t) {
Objects.requireNonNull(t, "t is null in sink.next(t)");
if (sink.isCancelled() || done) {
Operators.onNextDropped(t, sink.currentContext());
return this;
}
if (WIP.get(this) == 0 && WIP.compareAndSet(this, 0, 1)) {
try {
sink.next(t);
}
catch (Throwable ex) {
Operators.onOperatorError(sink, ex, t, sink.currentContext());
}
if (WIP.decrementAndGet(this) == 0) {
return this;
}
}
else {
Queue<T> q = queue;
synchronized (this) {
q.offer(t);
}
if (WIP.getAndIncrement(this) != 0) {
return this;
}
}
drainLoop();
return this;
}
复制代码这里调用BufferAsyncSink.next，然后drainLoop之后才返回
BufferAsyncSink.next
public FluxSink<T> next(T t) {
queue.offer(t);
drain();
return this;
}
复制代码这里将数据放入queue中，然后调用drain取数据，同步调用LambdaSubscriber的onNext
reactor-core-3.1.3.RELEASE-sources.jar!/reactor/core/publisher/LambdaSubscriber.java
@Override
public final void onNext(T x) {
try {
if (consumer != null) {
consumer.accept(x);
}
}
catch (Throwable t) {
Exceptions.throwIfFatal(t);
this.subscription.cancel();
onError(t);
}
}
复制代码即同步调用自定义的subscribe方法，实例中除了log还会sleep，这里是同步阻塞的 这里调用完之后，fluxSink这里的next方法返回，然后继续循环
fluxSink -> {
//NOTE sink:class reactor.core.publisher.FluxCreate$SerializedSink
LOGGER.info("sink:{}",fluxSink.getClass());
while (true) {
LOGGER.info("sink next");
fluxSink.next(ThreadLocalRandom.current().nextInt());
}
}
复制代码小结
fluxSink这里看是无限循环next产生数据，实则不用担心，如果subscribe与fluxSink都是同一个线程的话(本实例都是在main线程)，它们是同步阻塞调用的。
subscribe的时候调用LambdaSubscriber.onSubscribe，request(N)请求数据，然后再调用source.accept，也就是调用fluxSink的lambda方法产生数据，开启stream模式
这里的fluxSink.next里头阻塞调用了subscribe的consumer，返回之后才继续循环。
至于BUFFER模式OOM的问题，可以思考下如何产生。
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