Transactions

Introduction

Spring's @Transactional wraps methods in a database transaction — commit on success, rollback on runtime exception. Understanding propagation, isolation and rollback rules prevents subtle data corruption bugs.

Informative example

Transactional service method:

@Service
public class TransferService {
    @Transactional
    public void transfer(Long fromId, Long toId, BigDecimal amount) {
        Account from = accountRepo.findById(fromId).orElseThrow();
        Account to   = accountRepo.findById(toId).orElseThrow();
        from.debit(amount);
        to.credit(amount);
        // both saves commit together — or both roll back
    }

    @Transactional(readOnly = true)
    public List<Account> listAll() {
        return accountRepo.findAll();
    }

    @Transactional(propagation = REQUIRES_NEW)
    public void auditLog(String action) { /* separate tx */ }
}

Best practices

• Put @Transactional on service layer, not controllers or repositories.
• Self-invocation bypasses the proxy — call through another bean.
• Use readOnly=true for queries — Hibernate skips dirty checking.

Purpose of this lesson

Master Transactions so you can apply it confidently in production Java code, technical interviews, and code reviews.

Step-by-step explanation

1. Understand the core idea behind Transactions.
2. Walk through the runnable example and tweak it in the playground.
3. Apply the pattern in a small Spring Boot or CLI exercise of your own.
4. Re-read the common mistakes and interview Q&A to lock the concept in.

Interactive workflow diagram

Debugging tips

• Changes not persisted? Method is not @Transactional or self-invocation bypassed the proxy.
• Rollback not happening on checked exception? Add rollbackFor = Exception.class.
• LazyInitializationException after return? Transaction closed — fetch inside the tx or use @EntityGraph.

Optimization strategies

• Profile before optimizing — JFR (Java Flight Recorder) and async-profiler reveal real hotspots.
• Prefer immutable data and stream pipelines over hand-rolled loops when readability matters.
• Reach for the right JDK collection (ArrayList vs LinkedList vs ArrayDeque) before writing custom data structures.

Enterprise example

A payment service we debugged had @Transactional on the controller — moving it to the service layer fixed phantom partial commits.

Interview questions & answers

Summary

In this lesson you learned Transactions — the concept, syntax, a runnable example, and the production pitfalls to avoid. Apply it in the playground before moving on.