@RestController
public class SimpleController {

    private SimpleService simpleService;

    public SimpleController(SimpleService simpleService) {
        this.simpleService = simpleService;
    }

    @GetMapping(value = "/simple",produces = MediaType.APPLICATION_JSON_VALUE)
    public ResponseEntity<StringJsonObject> simpleResult() {
        return ResponseEntity.ok(simpleService.getText());
    }
}

@Service
public class SimpleServiceImpl implements SimpleService{
    @Override
    public StringJsonObject getText(){
        return new StringJsonObject("Cool!");
    }
}

public class StringJsonObject {

    private String content;

    public StringJsonObject(String content) {
        this.content = content;
    }

    public String getContent() {
        return content;
    }
}

// SpringRunner is an alias of SpringJUnit4ClassRunner
// it's a Spring extension of JUnit that handles the TestContext
@RunWith(SpringRunner.class)

// we test only the SimpleController
@WebMvcTest(SimpleController.class)

public class SimpleControllerTest {

    // we inject the server side Spring MVC test support
    @Autowired
    private MockMvc mockMvc;

    // we mock the service, here we test only the controller
    // @MockBean is a Spring annotation that depends on mockito framework
    @MockBean
    private SimpleService simpleServiceMocked;

    @Test
    public void simpleResult() throws Exception {

        // this is the expected JSON answer
        String responseBody = "{\"content\":\"Hello World from Spring!\"}";

        // we set the result of the mocked service
        given(simpleServiceMocked.getText())
                .willReturn(new StringJsonObject("Hello World from Spring!"));

        // the test is executed:
        // perform: it executes the request and returns a ResultActions object
        // accept: type of media accepted as response
        // andExpect: ResultMatcher object that defines some expectations
        this.mockMvc.perform(get("/simple")
                .accept(MediaType.APPLICATION_JSON))
                .andExpect(status().isOk())
                .andExpect(content().string(responseBody));
    }
}