Mock Principal for Spring Rest controller

I have the following REST controller:

@RequestMapping(path = "", method = RequestMethod.GET)
public ExtendedGetUserDto getCurrentUser(Principal principal) {
    CustomUserDetails userDetails = userDetailsService.loadByUsername(principal.getName())
    // .....
}

CustomUserDetails has a number of fields, including username and password

I want to mock principal in the controller method (or pass from test to the controller method). How should I do that ? I read many posts, but none of them actually answered this question.

Edit 1

@Test
public void testGetCurrentUser() throws Exception {

    RequestBuilder requestBuilder = MockMvcRequestBuilders.get(
            USER_ENDPOINT_URL).accept(MediaType.APPLICATION_JSON);

    MvcResult result = mockMvc.perform(requestBuilder).andReturn();

    MockHttpServletResponse response = result.getResponse();
    int status = response.getStatus();
    Assert.assertEquals("response status is wrong", 200, status);
}