How test a void method into Service class, with Mockito?

I need to make pass all my test and to reach a complete code coverage for all classes of my project.

I created a Repository, Service and Controller class.

I need to test all single method into these classes.

I have these classes: 1) ApplicationController with ApplicationControllerTest 2) UserService with UserServiceTest 3) UserRepository with UserRepositoryTest

This is the controller:

 @Controller
    @RequestMapping("/")
    @SessionAttributes("user")
    public class ApplicationWebController {

    @Autowired
    private UserService userService;

    @Autowired
    private FundService fundService;

    @PostMapping("/save-user")
    public String saveUser(User user, Model model) {
        final User presentUsername = 
    userService.getUserByUsername(user.getUsername());
        if (presentUsername == null) {
            userService.insertNewUser(user);
            if((user.getUsername()).equals("admin")) {
                userService.updateRoleToAdmin(user.getId());
            }
    }

Here I insert a new User; if User.username is equal to "admin" then I update the field "role" with userService.updateRoleToAdmin(user.getId())

This is the JUnit test for controller:

@RunWith(SpringRunner.class)
@WebMvcTest(controllers = ApplicationWebController.class)
public class ApplicationWebControllerTest {

@Autowired
private WebClient webClient;

@MockBean
private UserService userService;

@Test
public void test_whenInsertedUserIsAdmin_DoUpdateOnRole() throws 
Exception {

User userInserted = new User(1L, "admin", "password", 1);       

mvc.perform(post("/save-user")
        .param("id", "1")
        .param("username", "admin")
        .param("password", "password")
        .param("role", "1"));

if (userInserted.getUsername() == "admin") {
    verify(userService).updateRoleToAdmin(1L);
}
}
}

this test correctly pass.

Now I have the Service class and the relative test class

This is the Service class

@Service
public class UserService {

private UserRepo userRepo;

public void updateRoleToAdmin(Long id) {
    userRepo.updateRoleToAdmin(id);
}
}

This class also must be tested and this is the JUnit class for the Service

@RunWith(MockitoJUnitRunner.class)
public class UserServiceTest {

@Mock
private UserRepo userRepo;

@InjectMocks
private UserService userService;

@Test
public void test_updateRoleToAdmin() {
//I DON'T KNOW HOW WRITE HERE
        //I NEED TO TEST THE RESPECTIVE METHOD IN SERVICE CLASS
        //THE METHOD IN userRepo IS A VOID METHOD 
}
}

These are my Repository classes (class and JUnit test)

public interface UserRepo extends JpaRepository<User, Long> {

@Modifying(flushAutomatically = true, clearAutomatically = true)
@Transactional
@Query("update User u set u.role = 2 where u.id = :userid")
void updateRoleToAdmin(@Param("userid") Long id);

}

The test for the Repo class:

@Test
public void test_UpdateRoleToAdmin() {
User user = new User(null, "admin", "password", 1);
User saved = userRepo.save(user);
userRepo.updateRoleToAdmin(saved.getId());
User found = userRepo.findByUsername("admin");
assertThat(found.getRole()).isEqualTo(2);
}

also, this test correctly pass

Now I must write a test into UserServiceTest JUnit test class.

I don't know what should I do, because it's a void method and I can't use when(method).thenReturn(..) and the following Assertion

I must have a Code Coverage of 100% for all my classes and without test the method "UserService.updateRoleToAdmin(Long id)" I can't reach it.

Please help, because I don't know how to test a void method there.

Thank you. I need to make pass all my test and to reach a complete code coverage for all classes of my project.

I created a Repository, Service and Controller class.

I need to test all single method into these classes.