Senior Java Developer
Summary
Senior Java integration developer for payment-related initiatives at a fintech client: designs contract-first REST APIs with Java/Spring Boot and OpenAPI, builds asynchronous messaging integrations (IBM MQ) connecting digital apps to backend and mainframe systems, supports containerised/PaaS deployment, and mentors teammates. Hybrid from Warsaw (3 days on-site), B2B, 6+ months.
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. The role 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-cluster platform setup.
Troubleshooting integration and production issues.
Taking technical ownership of solutions from design through implementation and deployment.
Sharing technical knowledge with other developers.
Mentoring and training team members.
Preparing clear technical documentation and structured handover materials.
Must-have experience
Strong hands-on experience with Java and Spring Boot.
Senior-level backend or integration development experience.
Strong experience designing and implementing REST APIs.
Very good knowledge of OpenAPI / Swagger and API contracts.
Practical experience with IBM MQ or a comparable enterprise messaging technology.
Good understanding of asynchronous communication and enterprise integration patterns.
Experience with data mapping, validation and error handling.
Experience deploying applications in containerised or PaaS environments.
Strong troubleshooting and analytical capabilities.
Ability to work independently and take technical ownership.
Willingness and ability to mentor developers and transfer knowledge.
Good English communication skills.
Nice to have
Experience with COBOL and mainframe development.
Experience integrating Java applications with IBM mainframe environments.
Knowledge of z/OS, CICS, DB2 or related mainframe technologies.
Previous experience within banking, payments or financial services.
Experience with container orchestration and multi-cluster environments.
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