如何Mock接收实体类请求且需调用block()方法的WebClient？

我想要Mock一个接收实体类作为请求参数、且需要调用.block()方法将Mono转换为目标类的WebClient。需要Mock的调用代码如下：

response = webClientSample.post().uri(builder.toUriString())
                .headers(httpHeader -> httpHeader.addAll(httpHeaders))
                .body(BodyInserters.fromValue(SampleRequest)).retrieve().bodyToMono(SampleResponse.class).block();

当前Mock代码运行时，上述调用链的.body(...).retrieve().bodyToMono(...).block()部分抛出空指针异常。我尝试通过以下代码Mock block()方法（populateSampleResponse为返回SampleResponse实例的方法）：

Mockito.when(Mono.just(populateSampleResponse()).block()).thenReturn(populateSampleResponse());

但触发报错：

org.mockito.exceptions.misusing.MissingMethodInvocationException:
when() requires an argument which has to be 'a method call on a mock'.
For example:
when(mock.getArticles()).thenReturn(articles);

你不能直接Mock Mono.block()，因为Mono.just(...)返回的是真实Mono实例，并非Mockito生成的Mock对象，所以会触发方法调用错误。正确做法是按WebClient的调用链，逐步Mock每个环节的返回对象：

1. 初始化WebClient Mock实例
   确保webClientSample是Mock对象，可通过注解或手动初始化：

   WebClient webClientSample = Mockito.mock(WebClient.class);
   // 或使用@Mock注解配合MockitoAnnotations.openMocks(this)
   

2. Mock post()方法的返回值

   WebClient.RequestBodyUriSpec requestBodyUriSpec = Mockito.mock(WebClient.RequestBodyUriSpec.class);
   Mockito.when(webClientSample.post()).thenReturn(requestBodyUriSpec);
   

3. Mock uri()方法的返回值

   Mockito.when(requestBodyUriSpec.uri(Mockito.anyString())).thenReturn(requestBodyUriSpec);
   

4. Mock headers()方法的返回值

   Mockito.when(requestBodyUriSpec.headers(Mockito.any(Consumer.class))).thenReturn(requestBodyUriSpec);
   

5. Mock body()方法的返回值

   WebClient.RequestHeadersSpec requestHeadersSpec = Mockito.mock(WebClient.RequestHeadersSpec.class);
   // 匹配任意BodyInserter，也可根据需要精确匹配SampleRequest参数
   Mockito.when(requestBodyUriSpec.body(Mockito.any(BodyInserter.class))).thenReturn(requestHeadersSpec);
   

6. Mock retrieve()方法的返回值

   WebClient.ResponseSpec responseSpec = Mockito.mock(WebClient.ResponseSpec.class);
   Mockito.when(requestHeadersSpec.retrieve()).thenReturn(responseSpec);
   

7. Mock bodyToMono()及block()方法
   Mock一个Mono对象，让它的block()返回预期的SampleResponse实例：

   Mono<SampleResponse> mockMono = Mockito.mock(Mono.class);
   Mockito.when(responseSpec.bodyToMono(SampleResponse.class)).thenReturn(mockMono);
   Mockito.when(mockMono.block()).thenReturn(populateSampleResponse());
   

如果使用Spring Boot 2.3+，也可以考虑使用WebClientTest切片测试，它会自动配置WebClient的Mock环境，简化Mock代码。

内容的提问来源于stack exchange，提问作者cwf8951