I am trying to write a unit test for the following controller:

@RestController
@RequestMapping(value = ENDPOINT_URL)
public class MainController extends RestControllerBase {
    MainService mainService;

    public MainController(MainService mainService) {
        this.mainService = mainService;
    }

    @GetMapping
    public ResponseEntity<List<MainDataDto>> getDashboardData() {
        checkPermission();
        List<MainDataDto> result = mainService.getData();
        return ResponseEntity.ok().body(result);
    }
}

This is what I have so far in terms of a unit test:

class MainControllerTest {
    MainController mainController;

    @Test
    public void test_getDashboardData_shouldReturn200Response() {
        MainService mainService = Mockito.mock(MainService.class);

        mainController = new MainController(mainService);

        List<MainDataDto> mockData = List.of(new MainDataDto());
        when(mainService.getData()).thenReturn(mockData);

        Assertions.assertEquals(HttpStatus.OK, mainController.getDashboardData().getStatusCode());
}

When I run this unit test, I get the following exception:

java.lang.NullPointerException: Cannot invoke "org.springframework.security.core.Authentication.getPrincipal()" because the return value of "org.springframework.security.core.context.SecurityContext.getAuthentication()" is null

checkPermission is a protected method in the RestControllerBase class. I cannot change the access modifier for this method.

I am new to writing unit tests, so I am trying to do this a small bit at a time so I can understand what I am doing. Because of the exception, I want to “figure out how to deal” with checkPermission and (with my limited understanding) I am thinking that I can/should somehow mock this method, or something along the lines of passing this part of my controller method over.

Being that this method is protected, what is the best way to change my unit test so I do not get the exception above anymore?