Senior Java Developer

PROJECT INFORMATION: Industry: Fintech Mode: full-time, hybrid work from Warsaw (3 days in the office, the rest remotely) Project language: English Start: December 2026 Assignment type: B2B Project length: minimum 6 months + possible extensions We are looking for an experienced Senior Java Integration Developer to support the development of enterprise integration layers for payment-related initiatives.

combines Java / Spring Boot development, REST API design, asynchronous messaging and enterprise integration , connecting modern digital solutions with backend and mainframe systems. This is a hands-on position with end-to-end technical ownership, from API contract and implementation through integration, deployment and knowledge transfer. Your responsibilities Designing and developing REST APIs using Java and Spring Boot .

Working in a contract-first API development model using OpenAPI / Swagger. Translating API contracts into reliable production-ready implementations. Building integration and transport services connecting digital applications with backend and mainframe systems.

Developing integrations using IBM MQ or comparable messaging technologies . Implementing reliable asynchronous message exchange. Designing data mappings and transformation logic between systems.

Implementing validation, error handling and appropriate failure scenarios. Supporting deployment within containerised / PaaS environments. Supporting potential migration from a single-cluster to a multi-