How do I use Reactor's StepVerifier to verify a Mono is empty?

Question

I am using StepVerifier to test values:

@Test
public void testStuff() {
    Thing thing = new Thing();
    Mono<Thing> result = Mono.just(thing);
    StepVerifier.create(result).consumeNextWith(r -> {
        assertEquals(thing, r);
    }).verifyComplete();
}

What I'd like to do now is test for the absence of an item in the Mono. Like this:

@Test
public void testNoStuff() {
    Mono<Thing> result = Mono.empty();
    StepVerifier.create(result)... // what goes here?
}

I want to test that the Mono is in fact empty. How do I do that?

Howl

Answer 1

Simply use verifyComplete(). If the Mono emits any data, it will fail the stepverifier as it doesn't expect an onNext signal at this point.

Overestimate

Answer 2

here it is checked that onNext is not called

 StepVerifier.create(result).expectNextCount(0).verifyComplete()

Marshamarshal

Please explain the difference to the other, older, upvoted answer. It seems that you recommend the same thing, just without any explanation – Petersen

expectNextCount will be keeping track of how many elements are pending to iterate. If there is no elements in the flux/mono, it will match with 0 & the test case will pass – Escobar