Field Injection and Unit Testing Problem

Direct Answer

Field injection hides required dependencies from the constructor and prevents making those injected fields final. A plain test can still instantiate the class if it has a no-arg constructor, but it cannot supply private dependencies through the public API; it then needs reflection, package-visible mutation, or a Spring context. Constructor injection makes dependencies explicit and usually keeps the test simple.

Code Example

// Field injection - hard to test
@Service
class OrderService {
    @Autowired
    private OrderRepository repo;  // private, can't set in a plain test

    public Order createOrder(String item) {
        return repo.save(new Order(item));
    }
}

// Constructor injection - easy to test
@Service
class OrderServiceCtor {
    private final OrderRepository repo;

    public OrderServiceCtor(OrderRepository repo) { // explicit dependency
        this.repo = repo;
    }

    public Order createOrder(String item) {
        return repo.save(new Order(item));
    }
}

// Unit test with constructor injection - no Spring
class OrderServiceTest {
    @Test
    void testCreateOrder() {
        var mockRepo = Mockito.mock(OrderRepository.class);
        var service = new OrderServiceCtor(mockRepo); // one line, no Spring
        service.createOrder("Laptop");
        verify(mockRepo).save(any());
    }
}

Common Mistakes & Pitfalls

• Using @SpringBootTest for every service test: Freshers slap @SpringBootTest on each test class when using field injection, turning unit tests into slow integration tests that load the whole context to test one method.
• Thinking field injection is cleaner: A single @Autowired field looks shorter than a constructor, but it hides the dependencies and makes testing harder. The constructor shows what the class needs.
• Not knowing ReflectionTestUtils exists: Forced to test field-injected code, some give up on unit tests entirely instead of using ReflectionTestUtils.setField() — ugly, but an option when you inherit such code.