How to mock Spring WebClient in Unit Test

Question

We wrote a small Spring Boot REST application, which performs a REST request on another REST endpoint.

@RequestMapping("/api/v1")
@SpringBootApplication
@RestController
@Slf4j
public class Application
{
    @Autowired
    private WebClient webClient;

    @RequestMapping(value = "/zyx", method = POST)
    @ResponseBody
    XyzApiResponse zyx(@RequestBody XyzApiRequest request, @RequestHeader HttpHeaders headers)
    {
        webClient.post()
            .uri("/api/v1/someapi")
            .accept(MediaType.APPLICATION_JSON)
            .contentType(MediaType.APPLICATION_JSON)
            .body(BodyInserters.fromObject(request.getData()))
            .exchange()
            .subscribeOn(Schedulers.elastic())
            .flatMap(response ->
                    response.bodyToMono(XyzServiceResponse.class).map(r ->
                    {
                        if (r != null)
                        {
                            r.setStatus(response.statusCode().value());
                        }

                        if (!response.statusCode().is2xxSuccessful())
                        {
                            throw new ProcessResponseException(
                                    "Bad status response code " + response.statusCode() + "!");
                        }

                        return r;
                    }))
            .subscribe(body ->
            {
                // Do various things
            }, throwable ->
            {
                // This section handles request errors
            });

        return XyzApiResponse.OK;
    }
}

We are new to Spring and are having trouble writing a Unit Test for this small code snippet.

Is there an elegant (reactive) way to mock the webClient itself or to start a mock server that the webClient can use as an endpoint?

Answer 1

I think the build-in spring support for this is still under way - https://jira.spring.io/browse/SPR-15286

I really like wiremock to (integration-)test such scenarios. Especially because you test the whole serialization and deserialization with this. With wiremock you start a server that serves your requests using predefined stubs.

Answer 2

You can use MockWebServer by the OkHttp team. Basically, the Spring team uses it for their tests too (at least how they said here). Here is an example, using code from this blog post:

Let's consider that we have the following service

class ApiCaller {

    private WebClient webClient;

    ApiCaller(WebClient webClient) {
        this.webClient = webClient;
    }

    Mono<SimpleResponseDto> callApi() {
        return webClient.put()
                .uri("/api/resource")
                .contentType(MediaType.APPLICATION_JSON)
                .header("Authorization", "customAuth")
                .syncBody(new SimpleRequestDto())
                .retrieve()
                .bodyToMono(SimpleResponseDto.class);
    }
}

then the test could be designed in such an eloquent way:

class ApiCallerTest {

    private final MockWebServer mockWebServer = new MockWebServer();
    private final ApiCaller apiCaller = new ApiCaller(WebClient.create(mockWebServer.url("/").toString()));

    @AfterEach
    void tearDown() throws IOException {
        mockWebServer.shutdown();
    }

    @Test
    void call() throws InterruptedException {
        mockWebServer.enqueue(
                new MockResponse()
                        .setResponseCode(200)
                        .setHeader(HttpHeaders.CONTENT_TYPE, MediaType.APPLICATION_JSON_VALUE)
                        .setBody("{\"y\": \"value for y\", \"z\": 789}")
        );
        SimpleResponseDto response = apiCaller.callApi().block();
        assertThat(response, is(not(nullValue())));
        assertThat(response.getY(), is("value for y"));
        assertThat(response.getZ(), is(789));

        RecordedRequest recordedRequest = mockWebServer.takeRequest();
        //use method provided by MockWebServer to assert the request header
        recordedRequest.getHeader("Authorization").equals("customAuth");
        DocumentContext context = JsonPath.parse(recordedRequest.getBody().inputStream());
        //use JsonPath library to assert the request body
        assertThat(context, isJson(allOf(
                withJsonPath("$.a", is("value1")),
                withJsonPath("$.b", is(123))
        )));
    }
}

Answer 3

With the following method it was possible to mock the WebClient with Mockito for calls like this:

webClient
.get()
.uri(url)
.header(headerName, headerValue)
.retrieve()
.bodyToMono(String.class);

or

webClient
.get()
.uri(url)
.headers(hs -> hs.addAll(headers));
.retrieve()
.bodyToMono(String.class);

Mock method:

private static WebClient getWebClientMock(final String resp) {
    final var mock = Mockito.mock(WebClient.class);
    final var uriSpecMock = Mockito.mock(WebClient.RequestHeadersUriSpec.class);
    final var headersSpecMock = Mockito.mock(WebClient.RequestHeadersSpec.class);
    final var responseSpecMock = Mockito.mock(WebClient.ResponseSpec.class);

    when(mock.get()).thenReturn(uriSpecMock);
    when(uriSpecMock.uri(ArgumentMatchers.<String>notNull())).thenReturn(headersSpecMock);
    when(headersSpecMock.header(notNull(), notNull())).thenReturn(headersSpecMock);
    when(headersSpecMock.headers(notNull())).thenReturn(headersSpecMock);
    when(headersSpecMock.retrieve()).thenReturn(responseSpecMock);
    when(responseSpecMock.bodyToMono(ArgumentMatchers.<Class<String>>notNull()))
            .thenReturn(Mono.just(resp));

    return mock;
}