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. 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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