Testing a service method by Unit Test?

Question

I have no test experience and try to test a method by a Unit Test. All the examples that I have a look at perform operations via uses mock values. I know, I will also use mock values with mockito that I use in my project. Here is my service method that I want to test:

ProductServiceImpl:

public List<ProductDTO> findAllByCategoryUuid(UUID categoryUuid) {

    // code omitted

    return result;
}

Here is my Unit Test class:

ProductServiceImplTest:

// ? @InjectMocks
@Autowired 
ProductServiceImpl productService;

@Mock
ProductRepository productRepository;
  

@Test
public void testFindAllByCategoryUuid() {

    UUID categoryUuid = UUID.randomUUID();

    final List<Product> productList = new ArrayList<>();
    for (int i = 0; i < size; i++) {
        // create product by setting "categoryUuid" and add to productList
    }
    productRepository.saveAll(productList);


    when(productService.findAllByCategoryUuid(categoryUuid)
                .thenReturn(productList);
}

My questions:

1. Is the approach above is correct in order to test the service method? I think I should not deal with inside the service method and just pass categoryUuid and check the result of that method for testing? Is that true?

2. In test class, I used @Autowired to access service method, but I am not sure if I should @Mock. Is there any mistake?

Any help would be appreciated.

Update: I also create unit test using DiffBlue plugin and it generates a test method as shown below. But I think it seems to be as testing repository methods rather than the service method. Is not it?

@Test
public void testFindAllByCategoryUuid() {
    when(this.productRepository.findAllByCategoryUuid((UUID) any()))
        .thenReturn(new ArrayList<Product>());
    assertTrue(this.productService.findAllByCategoryUuid(UUID.randomUUID())
        .isEmpty());
    verify(this.productRepository).findAllByCategoryUuid((UUID) any());
    // ...
}

Answer 1

I am not an expert but I will try to answer you question

The general approach to unit test your method should be to test the output against all possible set of inputs. in your specific case you can test

• input: existing UUID output : NonNull List.
• input: non existing UUID output : Empty List.
• input: null : Empty List.

Now what you have done here is right you need to Autowire the class that you are writing test cases for and mock the dependencies in that class. Only thing wrong is

when(productService.findAllByCategoryUuid(categoryUuid)
                .thenReturn(productList);

should be

when(productRepository.findAllByCategoryUuid(categoryUuid)
                .thenReturn(productList);   

here you are mocking productRepository.findAllByCategoryUuid as your goal is to test the method in service class.

after this just add appropriate assert statements for all the conditions mentioned above.

Also I usually follow a rule whenever bug is logged against some code I try to cover that input and output case using assert in my Junit so that every-time I will test all the possible input and output scenarios.

Answer 2

The important things to remember while writing Junit Tests using Mockito

1. All class level @Runwith()
2. Test class should be with @InjectMocks
3. All tests should be annotated with @Test
4. Any external service should be Mocked with @Mock
5. Any calls going to DB or other services should be mocked and values should be returned accordingly.
6. You should have assertions to test your result.

I would write something like this :

@RunWith(MockitoJUnitRunner.class)
public class ProductServiceImplTest {

@InjectMocks
ProductServiceImpl productService;

@Mock
ProductRepository productRepository;

@Test
public void testFindAllByCategoryUuid() {

    UUID categoryUuid = UUID.randomUUID();

    final List<Product> productList = new ArrayList<>();
    for (int i = 0; i < size; i++) {
        // create product by setting "categoryUuid" and add to productList
    }
    
    when(productRepository.saveAll(ArgumentMatchers.any()).thenReturn(productList); 
// Or below might work for newer version of test cases when we get Null Pointer Exp using older version of Junit test cases

//doReturn(productList).when(productRepository).saveAll(any(List.class));
    
    List<ProductDTO> response = productService.findAllByCategoryUuid(categoryUuid);
    
    Assert.assertNotNull(response);
    Assert.assertEquals("Object Value", response.getXXX());
}

Answer 3

Writing unit tests against Service layer comes with drawbacks:

1. You violate encapsulation of the method-under-test. If it changes because you start to invoke different classes/methods - the test will break. Even though the method may be working correctly.
2. Because you intend to use mocking, partly your tests will be simply checking that your mocks are set up to pass the tests. So basically you'll be testing test logic.

It's usually more productive to move the logic down e.g. to the Model. Those classes could be then unit-tested without mocks. Then you could write a higher-level test (including DB) that checks that everything works together.

Reading:

• Anemic architecture - enemy of testing
• Building Test Pyramid to optimize automated testing