Spring transaction management và @Transactional là gì?

@Transactional — declarative transaction: Spring tự động begin → commit → rollback.

Không cần viết connection.commit() thủ công.

@Service
class OrderService {
  @Transactional
  public void placeOrder(Order order) {
    orderRepo.save(order);
    inventoryService.reserve(order);   // nếu throw exception → rollback ALL
    paymentService.charge(order);
  }
}

Attribute chính:
- propagation — REQUIRED (default, join hoặc tạo mới), REQUIRES_NEW (luôn tạo mới, suspend cái cũ), NESTED (savepoint).
- isolation — READ_COMMITTED (default), REPEATABLE_READ, SERIALIZABLE.
- readOnly = true — hint cho ORM tắt dirty checking, DB route sang replica.
- rollbackFor — chỉ định exception class nào trigger rollback.
- timeout — giây trước khi force rollback.

Default rollback rule:
- Rollback khi unchecked exception (RuntimeException, Error).
- KHÔNG rollback khi checked exception (IOException...) → muốn rollback phải khai báo rollbackFor = Exception.class.

Gotcha lớn nhất — proxy bypass: Spring tạo proxy bao quanh bean. Gọi method @Transactional từ bên trong cùng class sẽ bypass proxy → không có transaction:

class Service {
  public void a() { b(); }              // ❌ bypass proxy, b() không có transaction
  @Transactional public void b() {}
}

Fix: tách b() sang class khác, hoặc self-inject (AopContext.currentProxy()).

@Transactional — declarative transactions: Spring handles begin → commit → rollback automatically.

No manual connection.commit() calls.

@Service
class OrderService {
  @Transactional
  public void placeOrder(Order order) {
    orderRepo.save(order);
    inventoryService.reserve(order);   // any exception → rollback EVERYTHING
    paymentService.charge(order);
  }
}

Key attributes:
- propagation — REQUIRED (default, join or create), REQUIRES_NEW (always new, suspends current), NESTED (savepoint).
- isolation — READ_COMMITTED (default), REPEATABLE_READ, SERIALIZABLE.
- readOnly = true — hints the ORM to skip dirty checking; the DB may route to a read replica.
- rollbackFor — exception classes that trigger rollback.
- timeout — seconds before forced rollback.

Default rollback rule:
- Rollback on unchecked exceptions (RuntimeException, Error).
- Does NOT rollback on checked exceptions (IOException...) → use rollbackFor = Exception.class if needed.

Biggest gotcha — proxy bypass: Spring wraps the bean in a proxy. Calling a @Transactional method from inside the same class bypasses the proxy → no transaction:

class Service {
  public void a() { b(); }              // ❌ bypasses proxy, b() has no transaction
  @Transactional public void b() {}
}

Fix: extract b() into another class, or self-inject (AopContext.currentProxy()).