Is this code, using (Mockito and JUnit) a good way to test a method getById from my @Service?

Question

I am using mockito to write some unit tests and I'm not sure if this test is satisfactory for method getById.

@Mock
private CustomerService customerService;

@Test
public void Customer_Get_By_ID() {
        Customer customer = Customer.builder()
                .firstName("Name")
                .lastName("Last Name")
                .email("name.last@mail.com")
                .build();
        Long customerId = customerService.create(customer);
        assertNotNull(customerId);

        when(customerService.get(customerId)).thenReturn(customer);
        Customer saved = customerService.get(customerId);
        assertEquals(saved.getFirstName(), customer.getFirstName());
        assertEquals(saved.getLastName(), customer.getLastName());
        verify(customerService, times(1)).get(customerId);
}

Is this test correct? Any advice or another way to write this test?

Answer 1

The idea of mocking is that the method you want to test of your SUT (System under Test) depends on a collaborator that you can't set up for the test. So you mock this collaborator. Let

• S be the SUT
• m() be a method of SUT which you want to test
• t be a test method that creates an instance of S and calls m() on it
• C be a collaborator that S depends on
• n() be aa method on C that ist called inside m() .

Now let's assume that it is difficult to set up C so you want to mock it.

interface C {
  String n();
}

@Mock
private C cMock;

For your test method you instruct cMock to answer in a way that makes m() behave in a way you want to test.

@Test
public void t() {
  when(cMock.n()).thenReturn("you've called n()");

  // S depends on a C.
  S sut = new S(cMock);

  // Execute the method of the SUT you want to test.
  String result = sut.m();

  // Verify the result.
  assertThat(result).isEqualTo("C said: you've called n()");

  // Optional: Verify that n() was called as expected.
  verify(cMock).n();
}

Since you are doing TDD (you are, aren't you?), you can now start to implement m() .

public class S {
  C c;

  public S(C c) {
    this.c = c;
  }

  public String m() {
    // TODO implement me by calling c.n()
    return null;
  }
}