Senior Java Backend Engineer · API & Systems Integration · Legacy Migration

Freelance backend engineer with 20 years of professional experience building transactional systems

where correctness matters: secured lending, payments, online auctions, invoicing and tax reporting. I led the

replacement of Unicaja's legacy AS/400 pawn-lending system with a new Java platform, including data and

process migration, and have run the Spanish Montes de Piedad national auction portal since 2018.

Today I build Java 21 / Spring Boot 3 / PostgreSQL services: multi-tenant REST APIs, Stripe billing, signed

webhooks, ERP integrations (Sage X3) and automated security and load testing. I own systems end to end —

domain modelling, API design, data, deployment and production support — and use Claude and Codex daily

with full review of generated code.

What I bring: fast onboarding into complex legacy domains · clean integration of ERPs, payment

gateways and public-administration services · production ownership, not just tickets.