How to mock Spring WebFlux WebClient?

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

We accomplished this by providing a custom ExchangeFunction that simply returns the response we want to the WebClientBuilder:

 webClient = WebClient.builder()             .exchangeFunction(clientRequest ->                      Mono.just(ClientResponse.create(HttpStatus.OK)                     .header("content-type", "application/json")                     .body("{ \"key\" : \"value\"}")                     .build())             ).build();  myHttpService = new MyHttpService(webClient);  Map<String, String> result = myHttpService.callService().block();  // Do assertions here      

If we want to use Mokcito to verify if the call was made or reuse the WebClient accross multiple unit tests in the class, we could also mock the exchange function:

@Mock private ExchangeFunction exchangeFunction;  @BeforeEach void init() {     WebClient webClient = WebClient.builder()             .exchangeFunction(exchangeFunction)             .build();      myHttpService = new MyHttpService(webClient); }  @Test void callService() {     when(exchangeFunction.exchange(any(ClientRequest.class)))    .thenReturn(buildMockResponse());     Map<String, String> result = myHttpService.callService().block();      verify(exchangeFunction).exchange(any());      // Do assertions here }      

Note: If you get null pointer exceptions related to publishers on the when call, your IDE might have imported Mono.when instead of Mockito.when.

Sources:

• WebClient javadoc
• WebClient.Builder javadoc
• ExchangeFunction javadoc

Answer 2

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; }