From COBOL to Java: How Developers Transition 60 Years of Legacy into Modern Java Services

COBOL systems written decades ago still power some of the most critical parts of modern banking. But how do you modernize systems serving more than 50 million customers without disrupting daily operations?

In this workshop, we share our modernization journey within the German Sparkasse Finance Group, where mission-critical mainframe applications continue to process core banking workloads every day.

Faced with hundreds of interconnected COBOL programs, limited documentation, retired domain experts, and years of temporary solutions turned permanent features, we first had to answer a fundamental question: What does the system actually do?

Through concrete examples, we show how we reverse engineered business capabilities from legacy systems, analyzed data models and processing flows, identified the logic worth keeping, and deliberately discarded complexity that no longer created value.

You’ll see how we transformed monolithic applications into modular services, introduced shared and configurable business capabilities, and designed a target architecture focused on maintainability, scalability, and developer productivity.

We also demonstrate how AI helped developers understand legacy code, uncover hidden dependencies, and accelerate modernization efforts.

Beyond technology, we explain how we used developer experience as a lever for transformation. By improving developer experience, automating repetitive tasks, and reducing friction in daily work, teams were able to focus on innovation instead of maintenance.

This workshop is for developers and architects facing large legacy environments and looking for practical strategies to modernize systems without simply rebuilding yesterday’s architecture with today’s technology.