Mock class inside REST controller with Mockito

I have a spring-boot application which exposes a REST interface via a controller. This is an example of my controller:

@RestController
public class Controller {

  @Autowired
  private Processor processor;

  @RequestMapping("/magic")
  public void handleRequest() {

    // process the POST request
    processor.process();

  }   
}

I am trying to write unit tests for this class and I have to mock the processor (since the processing takes very long time and I am trying to avoid this step during testing the controller behavior). Please note, that the provided example is simplified for the sake of this question.

I am trying to use the mockito framework for this task:

@RunWith(SpringJUnit4ClassRunner.class)
@SpringApplicationConfiguration(classes = App.class)
@WebAppConfiguration
@ActiveProfiles("test")
public class ControllerTest {

  @Autowired
  private WebApplicationContext wac;

  private MockMvc mockMvc;

  @Before
  public void setUp() throws Exception {

    this.mockMvc = MockMvcBuilders.webAppContextSetup(this.wac).build();

    Processor processor = Mockito.mock(Processor.class);
    ReflectionTestUtils.setField(Controller.class, "processor", processor);
  }

  @Test
  public void testControllerEmptyBody() throws Exception {

    this.mockMvc.perform(post("/magic")).andExpect(status().isOk());

  }
}

However, this fails with

java.lang.IllegalArgumentException: Could not find field [processor] of type [null] on target [class org.company.Controller]
    at org.springframework.test.util.ReflectionTestUtils.setField(ReflectionTestUtils.java:112)
    ...

Could please someone give me a hint, how this mock could be injected in my controller?