CODEWITHSUNDEEP
spring-dataproject-reactor
JudgingNotJudging
JudgingNotJudging

Reputation: 679

Reactor - Calling block on output of flatMap causes ClassCastException

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?

Upvotes: 0

Views: 1928

Answers (1)

Simon Basl&#233;
Simon Basl&#233;

Reputation: 28301

the flatmap takes a Function that should return a Mono (I guess groovy let you return the wrong type, as Foo doesn't seem to implement Publisher?)

Upvotes: 1

Related Questions