ClassCastException Mono<Void> unit test Java

Question

i have the method in springboot that consume a endpoint

public Mono<Void> mesaVisadoCalculadoraNotificar(NotificarIn in) {
 setTimeoutErrorCode("MVC-01");
 return Mono.fromCallable(() -> {
 WebTarget path = target.path(MesaVisadoCalculadora.NOTIFICAR.getUri());
 
 return restHelper.performRequest(path, in, new GenericType<Void>(){}, HttpAction.POST);
 });
 }
}

and this test

@Test
public void mesavisadoCalculadoraNotificaTest(){
 when(target.path(Mockito.anyString())).thenReturn(target);
 when(restHelper.performRequest(Mockito.any(WebTarget.class), Mockito.any(), Mockito.any(GenericType.class),Mockito.any(HttpAction.class))).thenReturn(Mono.empty());
 Mono<Void> promise = agent.mesaVisadoCalculadoraNotificar(in);
 promise.block()
 Assertions.assertNotNull(promise);
 
}

but in the line promise.block() return ClassCastException, any idea what it might be,

this is a error in console:

MesavisadoCalculadoraAgentTest > mesavisadoCalculadoraNotificaTest() FAILED
 java.lang.ClassCastException at MesavisadoCalculadoraAgentTest.java:65
java.lang.ClassCastException: reactor.core.publisher.MonoIgnorePublisher cannot be cast to java.lang.Void
 at infrastructure.agent.apimesavisadocalculadora.MesaVisadoCalculadoraAgent.lambda$mesaVisadoCalculadoraNotificar0ドル(MesaVisadoCalculadoraAgent.java:45)
 at reactor.core.publisher.MonoCallable.block(MonoCallable.java:81)
 at reactor.core.publisher.MonoCallable.block(MonoCallable.java:74)
 at infrastructure.agent.apimesavisadocalculadora.MesavisadoCalculadoraAgentTest.mesavisadoCalculadoraNotificaTest(MesavisadoCalculadoraAgentTest.java:65)
 at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
 at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
 at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
 at java.lang.reflect.Method.invoke(Method.java:498)
 at org.junit.platform.commons.util.ReflectionUtils.invokeMethod(ReflectionUtils.java:686)
 at org.junit.jupiter.engine.execution.MethodInvocation.proceed(MethodInvocation.java:60)

• Robert: Please edit your question and add the full exception details.
• Roman Gajardo Robles: It doesn't provide much information.
• Andrew Henle: @RomanGajardoRobles You need to provide enough information so others can replicate your exact problem.
• Roman Gajardo Robles: @Andre Henle, Update the error details
• knittl: What's the signature of restHelper.performRequest? Does it return a Mono<?>?

Answer 1

I think the problem is that:

• you are setting up your restClient.performRequest method to return a Mono<Void>,
• if you have a callable that returns a value of type T, then wrapping this callable in Mono.fromCallable(...) returns a value of type Mono<T>.

If you want Mono.fromCallable(...) to return Mono<Void>, then the callable needs to return Void. Therefore, the inner callable should return null rather than a Mono.

Try replacing the Mono.empty() in your mock setup with null.

Disclaimer: I haven't tested this.