Q:

Spring REST controller behaves differently in unit tests

The problem
I am new to Spring and trying to write some unit tests for my REST controller. Testing manually with httpie or curl works well, however, with @WebMvcTest, strange things happen.
Here is what happens when I PUT a new user by curl:
$ curl -v -H'Content-Type: application/json' -d@- localhost:8080/api/users <john_smith.json                                                                                                                                  
*   Trying 127.0.0.1:8080...
* Connected to localhost (127.0.0.1) port 8080 (#0)
> POST /api/users HTTP/1.1
> Host: localhost:8080
> User-Agent: curl/7.69.1
> Accept: */*
> Content-Type: application/json
> Content-Length: 102
> 
* upload completely sent off: 102 out of 102 bytes
* Mark bundle as not supporting multiuse
< HTTP/1.1 200 
< Content-Type: application/json
< Transfer-Encoding: chunked
< Date: Sat, 18 Apr 2020 22:29:43 GMT
< 
* Connection #0 to host localhost left intact
{"id":1,"firstName":"John","lastName":"Smith","email":"john.smith@example.com","password":"l33tp4ss"}

As you can see, the Content-Type header is there in the response and the body is indeed the new User.
Below is how I am trying to test the same automatically:
@RunWith(SpringRunner.class)
@WebMvcTest(UserController.class)
public class UserControllerTest {

    @Autowired
    private MockMvc mvc;

    @MockBean
    private UserService service;

    private final User john = new User("John", "Smith",
                                       "john.smith@example.com",
                                       "s3curep4ss");

    @Test
    public void givenNoUser_whenCreateUser_thenOk()
    throws Exception
    {
        given(service.create(john)).willReturn(john);

        mvc.perform(post("/users")
                    .contentType(APPLICATION_JSON)
                    .content(objectToJsonBytes(john)))
        .andExpect(status().isOk())
        .andExpect(content().contentType(APPLICATION_JSON))
        .andExpect(jsonPath("$.id", is(0)))
        .andDo(document("user"));
    }

}

But what I get is this:
$ mvn test
[...]
MockHttpServletRequest:                                                                                                                
      HTTP Method = POST    
      Request URI = /users                   
       Parameters = {}                                                                                                                 
          Headers = [Content-Type:"application/json", Content-Length:"103"]                                                                                                                                                                                     
             Body = {"id":0,"firstName":"John","lastName":"Smith","email":"john.smith@example.com","password":"s3curep4ss"}
    Session Attrs = {}                                                                                                                                                                                                                                                        

Handler:                
             Type = webshop.controller.UserController
           Method = webshop.controller.UserController#create(Base)                                                                     

Async:                                        
    Async started = false                                                                                                              
     Async result = null                      

Resolved Exception:                       
             Type = null                                           

ModelAndView:                                                                                                                                                                                                                                                                 
        View name = null                                                                                                                                                                                                                                                      
             View = null                                                                                                                                                                                                                                                      
            Model = null                                                                                                               

FlashMap:                                                                                                                                                                                                                                                                     
       Attributes = null                                           

MockHttpServletResponse:    
           Status = 200                      
    Error message = null                                                                                                               
          Headers = []                                                                                                                                                                                                                                                        
     Content type = null                                                                                                               
             Body =                                                                                                                                                                                                                                                           
    Forwarded URL = null                                                                                                               
   Redirected URL = null
          Cookies = []                               
[ERROR] Tests run: 6, Failures: 1, Errors: 0, Skipped: 0, Time elapsed: 11.271 s <<< FAILURE! - in webshop.UserControllerTest          
[ERROR] givenNoUser_whenCreateUser_thenOk  Time elapsed: 0.376 s  <<< FAILURE!                                                         
java.lang.AssertionError: Content type not set
        at webshop.UserControllerTest.givenNoUser_whenCreateUser_thenOk(UserControllerTest.java:70)

What is happening? Where is the body from the MockHttpServletResponse? I must be missing something, as it seems to act completely differently.

Other code in case it is needed
My generic controller class:
public class GenericController<T extends Base>
implements IGenericController<T> {

    @Autowired
    private IGenericService<T> service;

    @Override
    @PostMapping(consumes = APPLICATION_JSON_VALUE,
                 produces = APPLICATION_JSON_VALUE)
    public T create(@Valid @RequestBody T entity)
    {
        return service.create(entity);
    }

    /* ... Other RequestMethods ... */

}

The actual User controller:
@RestController
@RequestMapping(path="/users")
public class UserController extends GenericController<User> { }

UPDATE 2020-04-22
As suggested, I took generics out of the equation, but it did not help.

A:

Seems like the @WebMvcTest annotation is configuring a UserService bean that is using the real implementation, and your bean is somehow ignored.
We can try to create the UserService bean differently
@RunWith(SpringRunner.class)
@WebMvcTest(UserController.class)
@Import(UserControllerTest.Config.class)
public class UserControllerTest {

    @TestConfiguration
    static class Config {

        @Primary
        @Bean
        UserService mockedUserService() {
            UserService service = Mockito.mock(UserService.class);
            given(service.create(john)).willReturn(UserControllerTest.john());
            return service;
        }
    }

    static User john() {
        return new User("John", "Smith", "john.smith@example.com", "s3curep4ss");
    }

    ...
}

You could also move the stubbing to a @Before method in your tests
@Configuration
public class CommonTestConfig {
   @Primary
   @Bean
   UserService mockedUserService() {
      return Mockito.mock(UserService.class)
   }
}

@RunWith(SpringRunner.class)
@WebMvcTest(UserController.class)
@Import(CommonTestConfig.class)
public class Test1 {
   @Autowired
   private UserService userService;

   @Before
   public void setup() {
      given(userService.create(any())).willReturn(user1());
   }
}

@RunWith(SpringRunner.class)
@WebMvcTest(UserController.class)
@Import(CommonTestConfig.class)
public class Test2 {
   @Autowired
   private UserService userService;

   @Before
   public void setup() {
      given(userService.create(any())).willReturn(user2());
   }
}

A:

Apparently, the problem was with this line:
given(service.create(john)).willReturn(john);

When I change the first john (which is a User object) to e.g. any(), the test passes just fine.

Could somebody please shed some light for me as to why this is? Swapping john with any() works, but feels somewhat hacky. The controller passes the JSON deserialized john object to its service. Is it simply that that deserialized john is obviously not the same object as the one I create in the test class?

