How to mock Spring WebFlux WebClient?

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