Dependency Injection

Introduction

Dependency Injection (DI) is Spring's superpower. Instead of new-ing collaborators inside your class, you declare them as constructor parameters and Spring wires them in. The result: testable, swappable, decoupled code.

Understanding the topic

Three flavours of injection — only one is correct in 2026:

• ✅ Constructor injection — required, immutable, easy to test. Use this.
• ⚠️ Setter injection — for optional deps only.
• ❌ Field injection (@Autowired private Foo foo) — hides dependencies, breaks final, hostile to testing. Avoid.

Informative example

Constructor injection (no @Autowired needed since Spring 4.3):

@Service
public class OrderService {
  private final OrderRepository repo;
  private final PricingClient pricing;

  public OrderService(OrderRepository repo, PricingClient pricing) {
    this.repo = repo;
    this.pricing = pricing;
  }

  public Order place(NewOrder cmd) {
    int total = pricing.quote(cmd.items());
    return repo.save(new Order(cmd.userId(), total));
  }
}

Best practices

• Always use constructor injection; mark fields final.
• Prefer Lombok's @RequiredArgsConstructor to remove boilerplate.
• If a class has 6+ dependencies, it's doing too much — split it.