Mocking org.springframework.web.reactive.function.client.WebClient.ResponseSpec#onStatus input parameters

Considering the following code:

    public Mono<Void> doStuff() {

        return this.requestStuff()
            .onStatus(HttpStatus::is5xxServerError,
                    clientResponse -> {
                    aMethodIWouldLikeToTest(clientResponse);
                    return Mono.error(new MyCustomException("First error I would like to test"));
                    })
            .onStatus(HttpStatus::is4xxClientError,
                    clientResponse -> {
                    aMethodIWouldLikeToTest(clientResponse);
                    return Mono.error(new MyCustomException("Second error I would like to test"));
                    })
            .bodyToMono(String.class)
            .flatMap(x -> anotherMethodIManagedToTest(x)))

    }

My first goal was to test anotherMethodIManagedToTest(x) that was achieved using:

    import org.springframework.web.reactive.function.client.WebClient;

    ...

    @Mock
    private WebClient.ResponseSpec responseSpec;

    private String desiredInputParam = "There is another black spot on the sun today!";

    ...

    @Test
    public void allGood_anotherMethodIManagedToTest_success {

        ...

        ClassUnderTest classUnderTest = new classUnderTest()
        ClassUnderTest classUnderTestSpy = spy(classUnderTestSpy);
        doReturn(responseSpec).when(classUnderTestSpy).requestStuff();

        when(responseSpec.onStatus(any(), any())).thenReturn(responseSpec);
        when(responseSpec.bodyToMono(String.class)).thenReturn(Mono.just(desiredInputParam));

        Mono<Void> result = classUnderTestSpy.doStuff();

        // Bunch of assertions for anotherMethodIManagedToTest(String desiredInputParam) performed with success ...

    }

Now I would like to create additional tests to test the event of a 5xxServerError and the event of a 4xxClientError but I am having a hard time figuring how to:

Mock the response of HttpStatus::is5xxServerError
Mock the response of HttpStatus::is4xxServerError
Mock the clientResponse in order to test aMethodIWouldLikeToTest(org.springframework.web.reactive.function.client.ClientResponse clientResponse)

Any suggestions on how to perform these actions?

Please note that I cannot really use any PowerMock alternatives (still interested if this is the only way to acheive my goal) all answers with standard Mockito are preferred.

Solution

I am sorry about my late response. It is possible to use mockito for your test case:

you need to get the HttpStatus to check it in the onStatus method
so first create an abstract class (to avoid implementing all the methods) that implements WebClient.ResponseSpec
you can create a mock from this class instead of ResponseSpec
I added 2 methods in this class:

abstract class CustomMinimalForTestResponseSpec implements WebClient.ResponseSpec {

        public abstract HttpStatus getStatus();

        public WebClient.ResponseSpec onStatus(Predicate<HttpStatus> statusPredicate, Function<ClientResponse, Mono<? extends Throwable>> exceptionFunction) {
            if (statusPredicate.test(this.getStatus())) exceptionFunction.apply(ClientResponse.create(HttpStatus.OK).build()).block();
            return this;
        }
      }

the getStatus will be used to set the HttpStatus:

when(responseSpecMock.getStatus()).thenReturn(HttpStatus.INTERNAL_SERVER_ERROR);

and for the onStatus, you can call the real method:

when(responseSpecMock.onStatus(any(Predicate.class),any(Function.class)))
   .thenCallRealMethod();

This is the test class:

package com.example.test;

import org.junit.jupiter.api.Test;
import org.junit.jupiter.api.extension.ExtendWith;
import org.mockito.InjectMocks;
import org.mockito.Mock;
import org.mockito.junit.jupiter.MockitoExtension;
import org.springframework.http.HttpStatus;
import org.springframework.web.reactive.function.client.ClientResponse;
import org.springframework.web.reactive.function.client.WebClient;
import reactor.core.publisher.Mono;

import java.util.function.Function;
import java.util.function.Predicate;

import static org.mockito.ArgumentMatchers.any;
import static org.mockito.Mockito.when;


@ExtendWith(MockitoExtension.class)
public class ExperimentalWebClientTest {

    @Mock
    private WebClient webClientMock;

    @InjectMocks
    private ExperimentalWebClient experimentalWebClient;

    @Mock
    private WebClient.RequestHeadersUriSpec requestHeadersUriSpecMock;

    @Mock
    private WebClient.RequestHeadersSpec requestHeadersSpecMock;

    @Mock
    private CustomMinimalForTestResponseSpec responseSpecMock;

    @Test
    void shouldFailsWhenHttpStatus5xx() {
        //given
        when(webClientMock.get()).thenReturn(requestHeadersUriSpecMock);
        when(requestHeadersUriSpecMock.uri(any(Function.class)))
            .thenReturn(requestHeadersSpecMock);
        when(requestHeadersSpecMock.retrieve()).thenReturn(responseSpecMock);

        when(responseSpecMock.getStatus()).thenReturn(HttpStatus.INTERNAL_SERVER_ERROR);

        when(responseSpecMock.onStatus(any(Predicate.class), any(Function.class))).thenCallRealMethod();

        //when + Then
        assertThrows(MyCustomException.class,
            () -> experimentalWebClient.doStuff(),
            "call fails with Internal Server Error") ;
    }


    abstract class CustomMinimalForTestResponseSpec implements WebClient.ResponseSpec {

        public abstract HttpStatus getStatus();

        public WebClient.ResponseSpec onStatus(Predicate<HttpStatus> statusPredicate, Function<ClientResponse, Mono<? extends Throwable>> exceptionFunction) {
            if (statusPredicate.test(this.getStatus())) exceptionFunction.apply(ClientResponse.create(HttpStatus.OK).build()).block();
            return this;
        }

    }
}