Reactor - 在flatMap的输出上调用块会导致ClassCastException(Reactor - Calling block on output of flatMap causes ClassCastException)

 我正在使用Spring Boot 2.0.0.M7和Project Reactor。 我的问题涉及编写单元测试时遇到的一些奇怪的行为。 在尝试将flatMap的输出提供给存储库时，我也遇到了这个问题。 
 
Mono<Foo> create(Mono<FooResource> resourceMono) {
    resourceMono.flatMap({
        // convert resource into Foo domain Entity
        return new Foo()
    })
}
 
 由于flatMap的返回值，这个闭包应该发出一个Mono<Foo> 。 但是，在调用block()来订阅并获取生成的Foo ， FluxFlatMap.trySubscribeScalarMap存在ClassCastException 
 
 测试代码： 
 
   def createdFoo = Foo.create(Mono.just(fooResource)).block() 
 
 堆栈跟踪： 
 
java.lang.ClassCastException: com.example.Foo cannot be cast to org.reactivestreams.Publisher

at reactor.core.publisher.FluxFlatMap.trySubscribeScalarMap(FluxFlatMap.java:141)
at reactor.core.publisher.MonoFlatMap.subscribe(MonoFlatMap.java:53)
at reactor.core.publisher.Mono.block(Mono.java:1161)
 
 这似乎是因为MonoJust实现了Callable而发生的，所以trySubscribeScalarMap尝试解包失败。 
 
 在非测试案例场景中，会发生类似的错误。 
 
Mono<ServerResponse> createFoo(ServerRequest request) {
    def body = request.body(BodyExtractors.toMono(FooResource))
    ok().body(fromPublisher(Foo.create(body)
        .flatMap({fooRepository.save(it)}), Foo))
}
 
 堆栈跟踪： 
 
java.lang.ClassCastException: com.example.Foo cannot be cast to reactor.core.publisher.Mono
at reactor.core.publisher.MonoFlatMap$FlatMapMain.onNext(MonoFlatMap.java:118) [reactor-core-3.1.2.RELEASE.jar:3.1.2.RELEASE]
at reactor.core.publisher.FluxOnAssembly$OnAssemblySubscriber.onNext(FluxOnAssembly.java:450) ~[reactor-core-3.1.2.RELEASE.jar:3.1.2.RELEASE]
at reactor.core.publisher.Operators$MonoSubscriber.complete(Operators.java:1092) ~[reactor-core-3.1.2.RELEASE.jar:3.1.2.RELEASE]
at reactor.core.publisher.MonoSingle$SingleSubscriber.onComplete(MonoSingle.java:171) ~[reactor-core-3.1.2.RELEASE.jar:3.1.2.RELEASE]
<...>
Assembly trace from producer [reactor.core.publisher.MonoFlatMap] :
reactor.core.publisher.Mono.flatMap(Mono.java:2059)
org.codehaus.groovy.runtime.callsite.PojoMetaMethodSite$PojoCachedMethodSiteNoUnwrap.invoke(PojoMetaMethodSite.java:213)
org.codehaus.groovy.runtime.callsite.PojoMetaMethodSite.call(PojoMetaMethodSite.java:56)
org.codehaus.groovy.runtime.callsite.CallSiteArray.defaultCall(CallSiteArray.java:48)
org.codehaus.groovy.runtime.callsite.AbstractCallSite.call(AbstractCallSite.java:113)
org.codehaus.groovy.runtime.callsite.AbstractCallSite.call(AbstractCallSite.java:125)
com.example.Foo.create(Foo.groovy:28)
org.codehaus.groovy.runtime.callsite.CallSiteArray.defaultCall(CallSiteArray.java:48)
org.codehaus.groovy.runtime.callsite.AbstractCallSite.call(AbstractCallSite.java:113)
org.codehaus.groovy.runtime.callsite.AbstractCallSite.call(AbstractCallSite.java:125)
com.example.FooHandlerFunctions.createFoo(FooHandlerFunctions.groovy:48)
 
 在另一个Mono.just(foo)封装flatMap闭包的输出解决了这两个问题。 但是，这似乎不应该需要。 我做错了什么或者误解了flatMap在这里的工作原理？ 

I'm using Spring Boot 2.0.0.M7 and Project Reactor. My issue is relating to some strange behavior encountered while writing a Unit test. I also ran into this while trying to feed the output of flatMap into the repository.
 
Mono<Foo> create(Mono<FooResource> resourceMono) {
    resourceMono.flatMap({
        // convert resource into Foo domain Entity
        return new Foo()
    })
}
 
This closure should emit a Mono<Foo> due to the return value of flatMap. However, when calling block() to subscribe and get the resulting Foo, there is a ClassCastException in FluxFlatMap.trySubscribeScalarMap
 
Test code:
 
   def createdFoo = Foo.create(Mono.just(fooResource)).block() 
 
Stack Trace:
 
java.lang.ClassCastException: com.example.Foo cannot be cast to org.reactivestreams.Publisher

at reactor.core.publisher.FluxFlatMap.trySubscribeScalarMap(FluxFlatMap.java:141)
at reactor.core.publisher.MonoFlatMap.subscribe(MonoFlatMap.java:53)
at reactor.core.publisher.Mono.block(Mono.java:1161)
 
This appears to occur because MonoJust implements Callable, so trySubscribeScalarMap tries to unwrap it unsuccessfully.
 
In a non-test case scenario, a similar error occurs.
 
Mono<ServerResponse> createFoo(ServerRequest request) {
    def body = request.body(BodyExtractors.toMono(FooResource))
    ok().body(fromPublisher(Foo.create(body)
        .flatMap({fooRepository.save(it)}), Foo))
}
 
Stack trace:
 
java.lang.ClassCastException: com.example.Foo cannot be cast to reactor.core.publisher.Mono
at reactor.core.publisher.MonoFlatMap$FlatMapMain.onNext(MonoFlatMap.java:118) [reactor-core-3.1.2.RELEASE.jar:3.1.2.RELEASE]
at reactor.core.publisher.FluxOnAssembly$OnAssemblySubscriber.onNext(FluxOnAssembly.java:450) ~[reactor-core-3.1.2.RELEASE.jar:3.1.2.RELEASE]
at reactor.core.publisher.Operators$MonoSubscriber.complete(Operators.java:1092) ~[reactor-core-3.1.2.RELEASE.jar:3.1.2.RELEASE]
at reactor.core.publisher.MonoSingle$SingleSubscriber.onComplete(MonoSingle.java:171) ~[reactor-core-3.1.2.RELEASE.jar:3.1.2.RELEASE]
<...>
Assembly trace from producer [reactor.core.publisher.MonoFlatMap] :
reactor.core.publisher.Mono.flatMap(Mono.java:2059)
org.codehaus.groovy.runtime.callsite.PojoMetaMethodSite$PojoCachedMethodSiteNoUnwrap.invoke(PojoMetaMethodSite.java:213)
org.codehaus.groovy.runtime.callsite.PojoMetaMethodSite.call(PojoMetaMethodSite.java:56)
org.codehaus.groovy.runtime.callsite.CallSiteArray.defaultCall(CallSiteArray.java:48)
org.codehaus.groovy.runtime.callsite.AbstractCallSite.call(AbstractCallSite.java:113)
org.codehaus.groovy.runtime.callsite.AbstractCallSite.call(AbstractCallSite.java:125)
com.example.Foo.create(Foo.groovy:28)
org.codehaus.groovy.runtime.callsite.CallSiteArray.defaultCall(CallSiteArray.java:48)
org.codehaus.groovy.runtime.callsite.AbstractCallSite.call(AbstractCallSite.java:113)
org.codehaus.groovy.runtime.callsite.AbstractCallSite.call(AbstractCallSite.java:125)
com.example.FooHandlerFunctions.createFoo(FooHandlerFunctions.groovy:48)
 
Wrapping the output of the flatMap closure in another Mono.just(foo) solves both issues. However, it seems like that shouldn't be needed. Am I doing something wrong or just misunderstanding how flatMap works here?

原文：https://stackoverflow.com/questions/47994303