title: Spring 5 supports Java 9 Flow APIs in its reactive feature?
body: <p>In Spring 5.0.0.RC4 reference documentation, it said:</p>
  
  <blockquote>
    <p>Publisher or Flow.Publisher — any type implementing Reactive Streams Publisher is supported.</p>
  </blockquote>
  
  <p><a href="https://docs.spring.io/spring/docs/5.0.0.RC4/spring-framework-reference/reactive-web.html#webflux" rel="nofollow noreferrer">https://docs.spring.io/spring/docs/5.0.0.RC4/spring-framework-reference/reactive-web.html#webflux</a></p>
  
  <p>But when I created a simple project based on Spring 5.0.0.RC4, I got failure when return <code>Flow.Publisher</code> in a controller. It seems the <code>Flow.Publisher</code> can not be serialized by jackson.</p>
  
  <pre><code>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"]-&gt;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"]-&gt;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
  </code></pre>
  
  <p><strong>Update 1</strong>: The complete <a href="https://github.com/hantsy/spring-reactive-sample/tree/master/java9" rel="nofollow noreferrer">source codes for Java 9</a>.</p>
  
  <pre><code>@GetMapping
  public Flow.Publisher&lt;Post&gt; 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;
  }
  </code></pre>
  
  <p><strong>Update 2</strong>: In the reference doc of Spring 5.0.0.RELEASE, I can not find the statement there.</p>
  
  <p><strong>Update 3</strong>: 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 <a href="https://github.com/hantsy/spring-reactive-sample/tree/master/java9" rel="nofollow noreferrer">sample codes</a>.</p>