@Autowired là gì và hoạt động thế nào?

@Autowired kích hoạt tự động inject dependency — Spring tự resolve và inject bean phù hợp.

Đặt được trên: constructor, setter, field.

Resolution mặc định: by type. Nếu nhiều bean cùng type → ambiguity, fix bằng @Qualifier("beanName") hoặc @Primary.

@Service
class OrderService {
  // Spring 4.3+: constructor duy nhất không cần @Autowired
  private final UserRepository repo;
  public OrderService(UserRepository repo) { this.repo = repo; }
}

// Khi có nhiều bean cùng type
@Service
class NotificationService {
  public NotificationService(@Qualifier("smtp") MailSender sender) {}
}

// Optional dependency
@Autowired(required = false)
private Cache cache;

Quy tắc 2026:
- ✅ Constructor injection — rõ ràng, final, dễ test.
- ❌ Field injection — hidden dependency, cần reflection để mock.
- Setter injection cho optional dependency.

Lombok @RequiredArgsConstructor giúp viết constructor injection gọn — tự sinh constructor cho final field.

@Autowired enables automatic dependency injection — Spring resolves and injects the right bean.

Can be placed on: constructors, setters, fields.

Default resolution: by type. Multiple beans of the same type → ambiguity, fix with @Qualifier("beanName") or @Primary.

@Service
class OrderService {
  // Spring 4.3+: a single constructor needs no @Autowired
  private final UserRepository repo;
  public OrderService(UserRepository repo) { this.repo = repo; }
}

// When multiple beans of the same type exist
@Service
class NotificationService {
  public NotificationService(@Qualifier("smtp") MailSender sender) {}
}

// Optional dependency
@Autowired(required = false)
private Cache cache;

2026 rules:
- ✅ Constructor injection — explicit, final, test-friendly.
- ❌ Field injection — hidden dependency, requires reflection to mock.
- Setter injection for optional dependencies.

Lombok's @RequiredArgsConstructor makes constructor injection concise — generates a constructor for final fields.