Dependency Injection (DI) là gì? Spring cài đặt DI thế nào?

DI: dependency được cung cấp từ bên ngoài thay vì object tự tạo → giảm coupling, tăng testability (mock được).

Không có DI:

class UserService {
  private final UserRepository repo = new UserRepository();  // ❌ tight coupling
}

Có DI:

class UserService {
  private final UserRepository repo;
  public UserService(UserRepository repo) { this.repo = repo; }   // inject từ ngoài
}

3 cách inject trong Spring:

1. Constructor injection — KHUYẾN NGHỊ. Immutable (final), required dependency rõ ràng, dễ test:

@Service
   class UserService {
     private final UserRepository repo;
     public UserService(UserRepository repo) { this.repo = repo; }
   }

Spring 4.3+: 1 constructor → không cần @Autowired.

2. Setter injection — cho optional dependency: void setLogger(Logger l).

3. Field injection — @Autowired UserRepository repo; — đơn giản nhưng anti-pattern: khó test (cần reflection để mock), không thấy dependency trong constructor.

IoC container (ApplicationContext) quản lý lifecycle bean + wiring tự động qua component scan.

DI: dependencies are supplied from outside instead of objects creating them → reduces coupling, improves testability (mockable).

Without DI:

class UserService {
  private final UserRepository repo = new UserRepository();  // ❌ tight coupling
}

With DI:

class UserService {
  private final UserRepository repo;
  public UserService(UserRepository repo) { this.repo = repo; }   // injected externally
}

Three injection styles in Spring:

1. Constructor injection — RECOMMENDED. Immutable (final), required dependencies are explicit, easy to test:

@Service
   class UserService {
     private final UserRepository repo;
     public UserService(UserRepository repo) { this.repo = repo; }
   }

Spring 4.3+: a single constructor needs no @Autowired.

2. Setter injection — for optional dependencies: void setLogger(Logger l).

3. Field injection — @Autowired UserRepository repo; — simple but an anti-pattern: hard to test (reflection needed to mock), dependencies hidden from the constructor.

The IoC container (ApplicationContext) manages bean lifecycle + auto-wires through component scanning.