Spring Webflux Mockito - mock the response of a Webclient call

Question

Small question regarding how to "force" or "mock" the response of a Webclient http call during unit test phase please.

I have a very straightforward method which does:

public String question() {
  String result = getWebClient().mutate().baseUrl(someUrlVariable).build().post().uri("/test").retrieve().bodyToMono(String).block();
   if (result == null) {
     doSomething1();
   }
 if (result.equals("")) {
     doSomething2();
   }
 if (result.equals("foo")) {
     doSomething3();
   }

As you can see, the complex part of this method is the Webclient call. It has (in this example) 7.method() like.mutate(), .post(). etc...

In my use case, I am not that interested testing this Webclient, at all.

What I would like to have with Mockito, is somehow the equivalent of:

public String question() {
  // it is just unit test. Mockito, please just return me the string I tell you to return please. Don't even execute this next line if possible, just return me this dummy response
  String result = the-thing-I-tell-mockito-to-return;
   if (result == null) {
     doSomething1();
   }
 if (result.equals("")) {
     doSomething2();
   }
 if (result.equals("foo")) {
     doSomething3();
   }

So far, I tried Mockito doNothing(), or Mockito.when(getWebclient()... ) of the entire line plus.thenReturn, but no luck.

How to achieve such please?

Thank you

Answer 1

You have to first ensure that getWebclient() returns a mock. Based on your existing code example I can't tell if that's coming for a different class or is a private method (it might make sense to inject the WebClient or WebClient.Builder via the constructor).

Next, you have to mock the whole method chain with Mockito. This includes almost copy/pasting your entire implementation:

when(webClient.mutate()).thenReturn(webClient);
when(webClient.baseUrl(yourUrl)).thenReturn(...);
// etc. 

Mockito can return deep stubs (check the documentation and search for RETURN_DEEP_STUBS ) that could simplify this stubbing setup.

However, A better solution would be to spawn a local HTTP server for your WebClient test and mock the HTTP responses. This involves less Mockito ceremony and also allows testing error scenarios (different HTTP responses, slow responses, etc.),

Answer 2

I would like to avoid those copy/pasting of when()

Well you have designed your code so that the only way to test it is by copy pasting of when .

So how have you designed it? well you have mixed API-code with logic which is something you should not do. The first thing you need to think about when writing tests is " What is it i want to test? " and the answer is usually Business logic .

If we look at your code:

public String question() {
    // This is api code, we dont want to test this, 
    // spring has already tested this for us.
    String result = getWebClient()
                     .mutate()
                     .baseUrl(someUrlVariable)
                     .build()
                     .post()
                     .uri("/test")
                     .retrieve()
                     .bodyToMono(String)
                     .block();

    // This is logic, this is want we want to test
    if (result == null) {
        doSomething1();
    }
    if (result.equals("")) {
         doSomething2();
    }
    if (result.equals("foo")) {
        doSomething3();
    }
}

When we design an application, we divide it into layers, usually a front facing api (RestController), then the business logic in the middle (Controllers) and lastly different resources that call other apis (repositories, resources etc.)

So when it comes to your application i would redesign it, split up the api and the logic:

@Bean
@Qualifier("questionsClient") 
public WebClient webclient(WebClient.Builder webClient) {
    return webClient.baseUrl("https://foobar.com")
                .build();
}

// This class responsibility is to fetch, and do basic validation. Ensure
// That whatever is returned from its functions is a concrete value.
// Here you should handle things like basic validation and null.
@Controller
public class QuestionResource {

    private final WebClient webClient;
    
    public QuestionResource(@Qualifier("questionsClient") WebClient webClient) {
        this.webClient = webClient;
    }


    public String get(String path) {
        return webClient.post()
                     .uri(path)
                     .retrieve()
                     .bodyToMono(String)
                     .block();
    }
}


// In this class we make business decisions on the values we have.
// If we get a "Foo" we do this. If we get a "Bar" we do this.
@Controller
public class QuestionHandler {

    private final QuestionResource questionResource;
    
    public QuestionResource(QuestionResource questionResource) {
        this.questionResource = questionResource;
    }

    public String get() {
        final String result = questionResource.get("/test");
        
        // also i dont see how the response can be null.
        // Null should never be considered a value and should not be let into the logic. 
        // Because imho. its a bomb. Anything that touches null will explode (NullPointerException). 
        // Null should be handled in the layer before.
        if (result == null) {
            return doSomething1();
        }
        if (result.equals("")) {
             return doSomething2();
        }
        if (result.equals("foo")) {
             return doSomething3();
        }
    }
}

Then in your test:

@Test
public void shouldDoSomething() {
    final QuestionResource questionResourceMock = mock(QuestionResource.class);
    when(questionResourceMock.get("/test")).thenReturn("");
    
    final QuestionHandler questionHandler = new QuestionHandler(questionResourceMock);
    final String something = questionHandler.get();
    
    // ...
    // assert etc. etc.
}

Also, i suggest you don't mutate webclients, create one webclient for each api because it gets messy fast.

This is written without an IDE, so there might be compile errors etc. etc.