Spring 5 supports Java 9 Flow APIs in its reactive feature? - spring

Question

In Spring 5.0.0.RC4 reference documentation, it said:
Publisher or Flow.Publisher — any type implementing Reactive Streams Publisher is supported.
https://docs.spring.io/spring/docs/5.0.0.RC4/spring-framework-reference/reactive-web.html#webflux
But when I created a simple project based on Spring 5.0.0.RC4, I got failure when return Flow.Publisher in a controller. It seems the Flow.Publisher can not be serialized by jackson.
org.springframework.core.codec.CodecException: Type definition error: [simple type, class java.util.concurrent.ForkJoinPool$DefaultForkJoinWorkerThreadFactory]; nested exception is com.fasterxml.jackson.databind.exc.InvalidDefinitionException: No serializer found for class java.util.concurrent.ForkJoinPool$DefaultForkJoinWorkerThreadFactory and no properties discovered to create BeanSerializer (to avoid exception, disable SerializationFeature.FAIL_ON_EMPTY_BEANS) (through reference chain: java.util.concurrent.SubmissionPublisher["executor"]->java.util.concurrent.ForkJoinPool["factory"])
at org.springframework.http.codec.json.AbstractJackson2Encoder.encodeValue(AbstractJackson2Encoder.java:132)
at org.springframework.http.codec.json.AbstractJackson2Encoder.lambda$encode$0(AbstractJackson2Encoder.java:96)
at reactor.core.publisher.FluxMapFuseable$MapFuseableSubscriber.onNext(FluxMapFuseable.java:107)
at reactor.core.publisher.FluxJust$WeakScalarSubscription.request(FluxJust.java:91)
at reactor.core.publisher.FluxMapFuseable$MapFuseableSubscriber.request(FluxMapFuseable.java:156)
at org.springframework.http.server.reactive.ChannelSendOperator$WriteBarrier.onSubscribe(ChannelSendOperator.java:143)
at reactor.core.publisher.FluxMapFuseable$MapFuseableSubscriber.onSubscribe(FluxMapFuseable.java:90)
at reactor.core.publisher.FluxJust.subscribe(FluxJust.java:68)
at reactor.core.publisher.FluxMapFuseable.subscribe(FluxMapFuseable.java:63)
at org.springframework.http.server.reactive.ChannelSendOperator.subscribe(ChannelSendOperator.java:76)
at reactor.core.publisher.MonoOnErrorResume.subscribe(MonoOnErrorResume.java:44)
at reactor.core.publisher.MonoFlatMap$FlatMapMain.onNext(MonoFlatMap.java:150)
at reactor.core.publisher.Operators$MonoSubscriber.complete(Operators.java:1068)
at reactor.core.publisher.MonoFlatMap$FlatMapInner.onNext(MonoFlatMap.java:241)
at reactor.core.publisher.FluxOnErrorResume$ResumeSubscriber.onNext(FluxOnErrorResume.java:72)
at reactor.core.publisher.FluxPeekFuseable$PeekFuseableSubscriber.onNext(FluxPeekFuseable.java:198)
at reactor.core.publisher.FluxPeekFuseable$PeekFuseableSubscriber.onNext(FluxPeekFuseable.java:198)
at reactor.core.publisher.Operators$MonoSubscriber.complete(Operators.java:1068)
at reactor.core.publisher.MonoIgnoreThen$ThenAcceptInner.onNext(MonoIgnoreThen.java:290)
at reactor.core.publisher.Operators$ScalarSubscription.request(Operators.java:1625)
at reactor.core.publisher.MonoIgnoreThen$ThenAcceptInner.onSubscribe(MonoIgnoreThen.java:279)
at reactor.core.publisher.FluxFlatMap.trySubscribeScalarMap(FluxFlatMap.java:161)
at reactor.core.publisher.MonoFlatMap.subscribe(MonoFlatMap.java:53)
at reactor.core.publisher.MonoDefer.subscribe(MonoDefer.java:52)
at reactor.core.publisher.MonoIgnoreThen$ThenIgnoreMain.drain(MonoIgnoreThen.java:148)
at reactor.core.publisher.MonoIgnoreThen.subscribe(MonoIgnoreThen.java:56)
at reactor.core.publisher.MonoPeekFuseable.subscribe(MonoPeekFuseable.java:74)
at reactor.core.publisher.MonoPeekFuseable.subscribe(MonoPeekFuseable.java:74)
at reactor.core.publisher.MonoOnErrorResume.subscribe(MonoOnErrorResume.java:44)
at reactor.core.publisher.MonoFlatMap$FlatMapMain.onNext(MonoFlatMap.java:150)
at reactor.core.publisher.FluxSwitchIfEmpty$SwitchIfEmptySubscriber.onNext(FluxSwitchIfEmpty.java:67)
at reactor.core.publisher.MonoNext$NextSubscriber.onNext(MonoNext.java:76)
at reactor.core.publisher.FluxConcatMap$ConcatMapImmediate.innerNext(FluxConcatMap.java:271)
at reactor.core.publisher.FluxConcatMap$ConcatMapInner.onNext(FluxConcatMap.java:798)
at reactor.core.publisher.FluxMapFuseable$MapFuseableSubscriber.onNext(FluxMapFuseable.java:115)
at reactor.core.publisher.Operators$ScalarSubscription.request(Operators.java:1625)
at reactor.core.publisher.FluxMapFuseable$MapFuseableSubscriber.request(FluxMapFuseable.java:156)
at reactor.core.publisher.Operators$MultiSubscriptionSubscriber.set(Operators.java:1439)
at reactor.core.publisher.Operators$MultiSubscriptionSubscriber.onSubscribe(Operators.java:1313)
at reactor.core.publisher.FluxMapFuseable$MapFuseableSubscriber.onSubscribe(FluxMapFuseable.java:90)
at reactor.core.publisher.MonoJust.subscribe(MonoJust.java:54)
at reactor.core.publisher.MonoMapFuseable.subscribe(MonoMapFuseable.java:59)
at reactor.core.publisher.Mono.subscribe(Mono.java:2757)
at reactor.core.publisher.FluxConcatMap$ConcatMapImmediate.drain(FluxConcatMap.java:418)
at reactor.core.publisher.FluxConcatMap$ConcatMapImmediate.onSubscribe(FluxConcatMap.java:210)
at reactor.core.publisher.FluxIterable.subscribe(FluxIterable.java:91)
at reactor.core.publisher.FluxIterable.subscribe(FluxIterable.java:55)
at reactor.core.publisher.FluxConcatMap.subscribe(FluxConcatMap.java:121)
at reactor.core.publisher.MonoNext.subscribe(MonoNext.java:40)
at reactor.core.publisher.MonoSwitchIfEmpty.subscribe(MonoSwitchIfEmpty.java:44)
at reactor.core.publisher.MonoFlatMap.subscribe(MonoFlatMap.java:60)
at reactor.core.publisher.MonoFlatMap.subscribe(MonoFlatMap.java:60)
at reactor.core.publisher.MonoOnErrorResume.subscribe(MonoOnErrorResume.java:44)
at reactor.core.publisher.MonoOnErrorResume.subscribe(MonoOnErrorResume.java:44)
at reactor.core.publisher.Mono.subscribe(Mono.java:2757)
at reactor.core.publisher.MonoIgnoreThen$ThenIgnoreMain.drain(MonoIgnoreThen.java:167)
at reactor.core.publisher.MonoIgnoreThen.subscribe(MonoIgnoreThen.java:56)
at reactor.core.publisher.MonoOnErrorResume.subscribe(MonoOnErrorResume.java:44)
at reactor.core.publisher.MonoPeekTerminal.subscribe(MonoPeekTerminal.java:61)
at reactor.ipc.netty.channel.ChannelOperations.applyHandler(ChannelOperations.java:380)
at reactor.ipc.netty.http.server.HttpServerOperations.onHandlerStart(HttpServerOperations.java:354)
at io.netty.util.concurrent.AbstractEventExecutor.safeExecute(AbstractEventExecutor.java:163)
at io.netty.util.concurrent.SingleThreadEventExecutor.runAllTasks(SingleThreadEventExecutor.java:403)
at io.netty.channel.nio.NioEventLoop.run(NioEventLoop.java:463)
at io.netty.util.concurrent.SingleThreadEventExecutor$5.run(SingleThreadEventExecutor.java:858)
at java.base/java.lang.Thread.run(Thread.java:844)
Caused by: com.fasterxml.jackson.databind.exc.InvalidDefinitionException: No serializer found for class java.util.concurrent.ForkJoinPool$DefaultForkJoinWorkerThreadFactory and no properties discovered to create BeanSerializer (to avoid exception, disable SerializationFeature.FAIL_ON_EMPTY_BEANS) (through reference chain: java.util.concurrent.SubmissionPublisher["executor"]->java.util.concurrent.ForkJoinPool["factory"])
at com.fasterxml.jackson.databind.exc.InvalidDefinitionException.from(InvalidDefinitionException.java:77)
at com.fasterxml.jackson.databind.SerializerProvider.reportBadDefinition(SerializerProvider.java:1191)
at com.fasterxml.jackson.databind.DatabindContext.reportBadDefinition(DatabindContext.java:312)
at com.fasterxml.jackson.databind.ser.impl.UnknownSerializer.failForEmpty(UnknownSerializer.java:71)
at com.fasterxml.jackson.databind.ser.impl.UnknownSerializer.serialize(UnknownSerializer.java:33)
at com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:727)
at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:719)
at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:155)
at com.fasterxml.jackson.databind.ser.BeanPropertyWriter.serializeAsField(BeanPropertyWriter.java:727)
at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:719)
at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:155)
at com.fasterxml.jackson.databind.ser.DefaultSerializerProvider._serialize(DefaultSerializerProvider.java:480)
at com.fasterxml.jackson.databind.ser.DefaultSerializerProvider.serializeValue(DefaultSerializerProvider.java:319)
at com.fasterxml.jackson.databind.ObjectWriter$Prefetch.serialize(ObjectWriter.java:1396)
at com.fasterxml.jackson.databind.ObjectWriter._configAndWriteValue(ObjectWriter.java:1120)
at com.fasterxml.jackson.databind.ObjectWriter.writeValue(ObjectWriter.java:950)
at org.springframework.http.codec.json.AbstractJackson2Encoder.encodeValue(AbstractJackson2Encoder.java:129)
... 65 common frames omitted
Update 1: The complete source codes for Java 9.
#GetMapping
public Flow.Publisher<Post> all() {
SubmissionPublisher publisher = new SubmissionPublisher();
publisher.submit(new Post(1L, "post one", "content of post one"));
publisher.submit(new Post(2L, "post two", "content of post two"));
return publisher;
}
Update 2: In the reference doc of Spring 5.0.0.RELEASE, I can not find the statement there.
Update 3: Updated the Spring 5.0.2(managed by Spring Boot 2.0.0.M7), the codes is running well without any exceptions, but it will block the request when access it. Check my updated sample codes.

Answer 1

Indeed, this is not supported right now.
Please subscribe to the issue SPR-16052 to know when it's available.

Answer 2

What about this
#RestController
public class Controller {
#GetMapping("/one")
Publisher<String> one(){
return JdkFlowAdapter.publisherToFlowPublisher(Flux.just("one", "two"));
}
}

Answer 3

If you read carefully the first line of the stacktrace, you see that the encoder cannot serialize the object of type java.util.concurrent.ForkJoinPool referenced
from the SubmissionPublisher. Indeed, threads (and so thread pools) cannot be serialized.
You can avoid direct reference to the ForkJoinPool with a proxy executor:
Executor proxyExecutor = (Runnable command)-> ForkJoinPool.commonPool().execute(command);
SubmissionPublisher publisher = new SubmissionPublisher(proxyExecutor);