Haken Java Application Engineer - Fintech & APIs | Tokyo

Join a leading company within Japan's fintech and digital commerce ecosystem as a Mid-Level Java Application Engineer, developing reliable, high-performance web applications and backend services that support digital payments and online merchant operations.

Based in Tokyo, this contract opportunity focuses on Java web application development, backend engineering, payment processing systems, merchant APIs and scalable transaction platforms used by businesses across Japan.

You will contribute to the design and development of mission-critical payment workflows, ensuring that consumer transactions are processed efficiently, securely and reliably. The role requires careful attention to system performance, fault tolerance and relevant security standards, including PCI DSS.

A key responsibility will be building and maintaining merchant-facing APIs, administrative consoles and backend operational management systems. These applications help online merchants manage their payment services and support efficient day-to-day platform operations.

You will also develop and improve batch processing and real-time data aggregation pipelines that handle large volumes of transaction records, supporting the accuracy and scalability of the company's payment infrastructure.

Another important focus is enhancing merchant onboarding and self-service registration systems, making it easier for new e-commerce businesses to access and manage digital payment services.

Alongside new development, you will help optimise, refactor and stabilise existing Java applications, improving maintainability and performance while introducing new fintech capabilities.

Working closely with local engineering teams and other technical stakeholders, you will contribute to practical technical decisions, troubleshoot application issues and identify opportunities for continuous improvement.

This is an attractive opportunity for a Java Developer, Java Backend Engineer, Web Application Engineer or Fintech Software Engineer looking to gain experience in large-scale payment systems and secure transactional platforms.

Please note: This position is open only to candidates currently residing in Japan.

Key Responsibilities

• Design, develop and maintain Java-based web applications and backend services supporting digital payment and merchant platform operations.
• Build and improve consumer payment workflows, prioritising performance, fault tolerance and secure transaction processing.
• Develop applications with consideration for payment security and compliance requirements, including relevant PCI DSS standards.
• Design, implement and maintain merchant-facing APIs used to support payment-related business processes.
• Develop and enhance administrative consoles and backend operational management systems for internal and merchant-facing activities.
• Build scalable batch processing and real-time data aggregation workflows for large volumes of transaction records.
• Improve merchant onboarding systems and self-service registration processes to support efficient account creation and platform adoption.
• Analyse application performance, identify bottlenecks and implement SQL and application-level performance improvements.
• Refactor and stabilise existing Java services and legacy application components to improve reliability and maintainability.
• Collaborate with local engineers and cross-functional technical teams to investigate issues and deliver effective solutions.
• Contribute to the introduction of new fintech features and digital payment capabilities while maintaining service stability.
• Recommend practical improvements to software architecture, development workflows and application quality.

Required Skills and Qualifications

Experience:

• 3+ years of professional web application development experience using Java.
• 2+ years of hands-on experience developing consumer-facing web services.
• Strong working knowledge of Linux or Unix operating systems and fundamental development or troubleshooting practices.
• 1+ year of practical experience with SQL-based application development and performance tuning.
• Experience developing, maintaining or enhancing web applications and backend services in a professional engineering environment.
• Understanding of application performance, reliability and maintainability in production-oriented software systems.
• Ability to investigate technical issues and implement practical improvements to application functionality or performance.
• Experience collaborating with software engineers and technical stakeholders throughout application development and maintenance activities.
• Current residence in Japan is required for this contract opportunity.

Soft Skills:

• Strong situational judgement and analytical problem-solving abilities when addressing software development challenges.
• Proactive approach to identifying technical issues and recommending pragmatic improvements.
• Ability to communicate effectively and build trust within a local Japanese engineering team.
• Collaborative mindset and willingness to support team-based software development and continuous improvement.
• Strong attention to detail when working with payment transactions, application stability and data accuracy.
• Organised, quality-focused approach to delivering reliable and maintainable software solutions.

Language Requirements:

• Japanese: Business-level proficiency is indicated in the position details, supporting effective communication and collaboration with the local engineering team.
• English: Conversational proficiency, approximately equivalent to TOEIC 600 or above, is required.

Preferred Skills & Qualifications

• Practical development experience with Spring Framework or Struts.
• Experience with web user interface development, including HTML, CSS and JavaScript.
• Familiarity with web application server technologies such as Apache HTTP Server and Apache Tomcat.
• Experience deploying or operating applications using Docker, Kubernetes or public cloud environments.
• Hands-on knowledge of CI/CD and software build tools, including Git, Jenkins and Maven.
• Previous experience developing or maintaining digital payment, fintech, e-commerce or transaction processing systems.

About the Company

Our client is a leading fintech and digital payment technology company operating at the centre of Japan's major internet and digital commerce ecosystem.

The organisation provides essential payment infrastructure to more than 9,000 e-commerce merchants across Japan, supporting secure and efficient digital transactions.

Its technology platforms operate at the intersection of large-scale online retail activity and secure financial transaction processing, making application reliability, system performance and security essential priorities.

The engineering organisation develops and maintains payment applications, merchant services, backend systems and transaction processing capabilities that support the company's growing digital commerce network.

The company promotes a fast-moving, technology-driven engineering culture, encouraging developers to improve system scalability, strengthen application quality and support international security compliance requirements.

Joining this team offers the opportunity to work on high-impact fintech systems, gain exposure to large-scale transaction processing and develop valuable expertise in Java backend engineering.

The organisation also offers potential long-term career development through internal employee conversion opportunities, making this contract role attractive to engineers interested in building a future within Japan's fintech sector.

Why You'll Love Working Here

• Work on digital payment infrastructure supporting more than 9,000 e-commerce merchants across Japan.
• Develop high-performance Java applications and backend services used in real-world payment operations.
• Gain experience with secure transaction processing, payment workflows and PCI DSS-related requirements.
• Build and maintain merchant APIs, operational platforms and scalable transaction data processing systems.
• Strengthen your skills in Java, SQL, Linux and modern backend development practices.
• Contribute to both new fintech features and the modernisation of existing application systems.
• Collaborate with experienced local engineers in a dynamic, technology-focused environment.
• Explore potential internal employee conversion and longer-term career development opportunities.
• Enjoy complimentary daily meals at the company's on-site employee food hall.
• Work comfortably in a modern, casual dress environment.
• Benefit from a flexible hybrid working arrangement, supporting a healthy work-life balance.

Don't Miss Out - Apply Now!