How to properly test a Spring Boot using Mockito

I never did a test and decided I would do my first. I am using Spring Boot and by default the layout is as follows: https://zapodaj.net/c8b65f1644e95.png.html The file that is created on startup looks like this

@RunWith(SpringRunner.class)
@SpringBootTest
public class RestWebServicesApplicationTests {

    @Test
    public void contextLoads() {
    }

}

I do not change anything here, because I do not need it. By contrast, I created a new class

public class CheckUserDataRestControllerTest {

    @Mock
    private UserService userService;

    @InjectMocks
    private CheckUserDataRestController checkUserDataRestController;

    private MockMvc mockMvc;

    @Before
    public void setup() {
        MockitoAnnotations.initMocks(this);

        mockMvc = MockMvcBuilders.standaloneSetup(checkUserDataRestController).build();
    }

    @Test
    public void textExistsUsername() throws Exception {
        mockMvc
                .perform(get("/checkUsernameAtRegistering")
                .param("username", "jonki97"))
                .andExpect(status().isOk());
    }
}

To test the controller method, which should return the status OK.

@GetMapping("/checkUsernameAtRegistering")
public HttpEntity<Boolean> checkUsernameAtRegistering(@RequestParam String username) {
            return ResponseEntity.ok().body(!userService.existsByUsername(username));
}

However, during the test, he throws me out

java.lang.AssertionError: Status 
Expected :200
Actual   :404
 <Click to see difference>

    at org.springframework.test.util.AssertionErrors.fail(AssertionErrors.java:54)
        ...

This is the tested controller: https://pastebin.com/MWW9YwiH

I did not make any special configurations because I did not find out that I needed to do something.

The error 404 means the Mock does not recognize your endpoint to test. The end point should be:

 @Test
    public void textExistsUsername() throws Exception {
        mockMvc
                .perform(get("/checkUserData/checkUsernameAtRegistering")
                .param("username", "jonki97"))
                .andExpect(status().isOk());
    }

Hope this help.