How to test Spring WebClient retry when?

I need to implement following behavior:

• Make a REST post request
• If response returns with a status 429 Too many requests, retry up to 3 times with a delay of 1 second
• If the third retry fails or any other error occurs, log and write something to the database
• If the request was successful (http status 200), log some information

I would like to use Spring WebClient for this purpose and came up with this code:

Mono<ClientResponse> response = webClient.post()
            .uri(URI.create("/myuri"))
            .body(BodyInserters.fromObject(request))
            .retrieve()
            .onStatus(httpStatus -> httpStatus.equals(HttpStatus.TOO_MANY_REQUESTS), 
                      response -> Mono.error(new TooManyRequestsException("System is overloaded")))
            .bodyToMono(ClientResponse.class)
            .retryWhen(Retry.anyOf(TooManyRequestsException.class)
                                          .fixedBackoff(Duration.ofSeconds(1)).retryMax(3))
            .doOnError(throwable -> saveToDB(some_id, throwable))
            .subscribe(response -> logResponse(some_id, response));

Now I would like to test if the retry mechanism and error handling works as I expect. May be I could use StepVerifier for this purpose, but I just cannot figure out how to use it in my case. Any useful hints?

I think that you might be able to test this with a mock web server, e.g. MockWebServer.

@Test
public void testReactiveWebClient() throws IOException
{
    MockWebServer mockWebServer = new MockWebServer();

    String expectedResponse = "expect that it works";
    mockWebServer.enqueue(new MockResponse().setResponseCode(429));
    mockWebServer.enqueue(new MockResponse().setResponseCode(429));
    mockWebServer.enqueue(new MockResponse().setResponseCode(429));
    mockWebServer.enqueue(new MockResponse().setResponseCode(200)
                                  .setBody(expectedResponse));

    mockWebServer.start();

    HttpUrl url = mockWebServer.url("/mvuri");
    WebClient webClient = WebClient.create();

    Mono<String> responseMono = webClient.post()
            .uri(url.uri())
            .body(BodyInserters.fromObject("myRequest"))
            .retrieve()
            .onStatus(
                    httpStatus -> httpStatus.equals(HttpStatus.TOO_MANY_REQUESTS),
                    response -> Mono.error(new TestStuff.TooManyRequestsException("System is overloaded")))
            .bodyToMono(String.class)
            .retryWhen(Retry.anyOf(TestStuff.TooManyRequestsException.class)
                               .fixedBackoff(Duration.ofSeconds(1)).retryMax(3));

    StepVerifier.create(responseMono)
            .expectNext(expectedResponse)
            .expectComplete().verify();

    mockWebServer.shutdown();
}

If you enqueue another MockResponse with a statuscode 429, the verification will fail, same with e.g. errorcode 500.