Transactions

Introduction

@Transactional wraps a method in a database transaction: all writes commit together or roll back together. It's the single most important annotation for data correctness.

Understanding the topic

The rules nobody tells you:

• @Transactional works via Spring proxies — calling a transactional method from the same class bypasses it.
• Default rollback only on RuntimeException; checked exceptions don't roll back unless you say so.
• readOnly = true hints the DB and skips dirty checking — big perf win for queries.
• propagation = REQUIRES_NEW opens an independent TX — used for audit logs that must commit even on failure.

Informative example

@Service
@Transactional  // class-level default for all methods
public class TransferService {

  @Transactional(readOnly = true)
  public Account get(Long id) { ... }

  @Transactional(rollbackFor = Exception.class)
  public void transfer(Long from, Long to, int amount) {
    var src = repo.findByIdForUpdate(from);     // pessimistic lock
    var dst = repo.findByIdForUpdate(to);
    src.debit(amount);
    dst.credit(amount);
  }
}