@Test
@WithMockUser(username="user@example.com", password="Test@777", roles="Manager")
public void getDataSuccess() throws Exception {

    mockMvc.perform(get("URL" ))
            .andDo(print())
            .andExpect(status().isInternalServerError());
}

MyUser user = (MyUser) SecurityContextHolder.getContext().getAuthentication().getPrincipal();
Long uId = Long.valueOf(user.getId());

import org.springframework.security.core.GrantedAuthority;
import org.springframework.security.core.userdetails.UserDetails;

public class MyUser implements UserDetails {

    // properties

    public MyUser ( MyJwtUserDetails userDetails , String userToken , Collection<? extends GrantedAuthority> authorities) {
        this..
}

java.lang.ClassCastException: org.springframework.security.core.userdetails.User cannot be cast to com.example.MyUser