Writing Unit Tests for Spring Services

Unit tests help you verify that your Spring services work as expected. In Spring Boot, you use the @SpringBootTest and @MockBean annotations to create isolated, repeatable tests for your service classes.

Key Steps to Write Unit Tests for Services

• Identify the class or method you want to test;
• Mock dependencies, such as repositories or other services, to isolate the logic;
• Use assertions to check the output matches your expectations.

Example: Testing a Service with a Mocked Repository

Suppose you have a service class called UserService that depends on a UserRepository:

@Service
public class UserService {
    private final UserRepository userRepository;

    public UserService(UserRepository userRepository) {
        this.userRepository = userRepository;
    }

    public User findUserById(Long id) {
        return userRepository.findById(id).orElse(null);
    }
}

To test UserService without accessing a real database, mock the UserRepository:

@SpringBootTest
public class UserServiceTest {
    @MockBean
    private UserRepository userRepository;

    @Autowired
    private UserService userService;

    @Test
    public void testFindUserById_ReturnsUser() {
        User mockUser = new User(1L, "Alice");
        Mockito.when(userRepository.findById(1L)).thenReturn(Optional.of(mockUser));

        User result = userService.findUserById(1L);
        Assertions.assertNotNull(result);
        Assertions.assertEquals("Alice", result.getName());
    }
}

Key points in the example:

• The @MockBean annotation creates a mock of UserRepository;
• The @Autowired annotation injects the real UserService instance, which uses the mocked repository;
• The Mockito.when(...).thenReturn(...) method sets up the mock behavior;
• The Assertions.assertNotNull and Assertions.assertEquals methods check the result.

Best Practices

• Test only the logic inside your service, not the behavior of dependencies;
• Use clear, descriptive method names for your tests;
• Keep each test focused on one scenario.

Unit testing your Spring services ensures your business logic works and gives you confidence to make future changes.