Mono.doOnError() reactor block unit test - unit-testing

Question

I have a rest controller using spring webflux and reactor, I am writing unit test for the controller. Please find below the code snippets and help me to write the unit test method to test the .doOnError() block.
I have tried to throw an exception by using Mockito
doThrow(CriticalException.class)
.when(myService).myMethod(object);
This is my unit test:
StepVerifier.create(
Mono.just(
webTestClient.post()
.uri("/endpoint")
.accept(MediaType.APPLICATION_JSON)
.body(BodyInserters.fromObject(requestJson)) //Set the body of the request to the given synchronous Object
//Returns:a Mono with the response
//Act
.exchange() //Perform the exchange
//Assert
.expectStatus().isOk()
.expectBody(Agreement.class)
.returnResult()
.getResponseBody()))
.expectNextMatches(agreementResponse -> {
assertNotNull(agreementResponse.getAgreementParticipant());
return true;
})
.expectComplete()
.verify();
This is my controller:
return Mono.fromCallable(() -> {
myService.myMethod(object);
return object;
}).log().subscribeOn(Schedulers.elastic())
.map(p -> ResponseEntity.ok(p))
.defaultIfEmpty(ResponseEntity.notFound().build())
.doOnError(e -> {
LOGGER.error(LOG_FORMAT, e.getMessage(), e.getStackTrace());
});
Mockito is not returning exception while myService.myMethod(object) is been called.
Please suggest proper way to write test for .defaultIfEmpty() and .doOnError() blocks.

Answer 1

Instead of throwing CriticalException.class while mocking your myService.myMethod(object) return an exception wrapped in a Mono
For eg :
Mockito.doReturn(Mono.error(Exception::new)).when(service).callableMethod();
Find the sample code snippet below
import org.junit.Test;
import org.mockito.Mock;
import org.mockito.Mockito;
import reactor.core.publisher.Mono;
import reactor.test.StepVerifier;
class Service {
public Mono<String> callableMethod() {
return Mono.just("1");
}
}
class Controller {
private Service service;
public Controller(Service service) {
this.service = service;
}
public Mono<String> endpoint() {
return service.callableMethod().doOnError(throwable -> {
System.out.println("throwable = " + throwable);
});
}
}
public class TestClass {
#Mock
private Service service = Mockito.mock(Service.class);
#Test
public void controllerTest() {
Mockito.doReturn(Mono.error(Exception::new)).when(service).callableMethod();
StepVerifier.create(new Controller(service).endpoint()).verifyError();
}
}