NullPointerException while unit testing spring security application

I am new to spring security. I am working on unit testing an application, which uses the following method call at some point in execution.

public static List<String> getUserDetails() {
        List<String> currentUserDetails = new ArrayList<String>();
        CurrentUser currentUser = (CurrentUser) SecurityContextHolder.getContext().getAuthentication().getPrincipal();
        currentUserDetails.add(0, currentUser.getUsername());//currentUser is null
        currentUserDetails.add(1, currentUser.getOrg_name());
        currentUserDetails.add(2, currentUser.getPassword());
        return currentUserDetails;
    }

Here the currentUser is a org.springframework.security.core.userdetails.User. After having looked at the SO question I tried this in my unit test to set a user.

@Test
public void testMyMethod() throws Exception {

    Authentication authentication = Mockito.mock(Authentication.class);
    // Mockito.whens() for your authorization object

    SecurityContext securityContext = Mockito.mock(SecurityContext.class);
    when(securityContext.getAuthentication()).thenReturn(authentication);
    SecurityContextHolder.setContext(securityContext);
    assertNotNull(("Should not be null"), new MyObject().myMethod());
}

The security part of the code is not written by me. So, because of this issue I am not able to unit test my modules.