Senior Software Engineer

Senior Software Engineer – Core Java - Java 21 monolith - Remote within Germany Our client is a data integration platform serving more than 2,000 customers, helping them connect business systems, automate workflows, and exchange data across cloud and on-premise environments. Their platform powers supply chains through real-time integration, automation, and intelligent data processing. The codebase is a Java 21 monolith with no microservices and no Spring Boot but rather homegrown framework built on top of libraries such as Netty, Jetty, and JPA. This role sits in a team, where the focus is not distributed systems or load balancing but using AI to modernize and clean up legacy tech debt + improve internal AI workflows within the codebase. What You'll Do Design, develop, test, and maintain production-grade backend software in Java Work through legacy code to identify and resolve tech debt, using AI-assisted engineering tools as a core part of the workflow Contribute to technical design discussions and code reviews Debug and optimize complex production systems Collaborate closely with Product, QA, and fellow engineers to deliver high-quality releases Take ownership of features from implementation through production What We're Looking For 5+ years of backend Java development Strong Core Java skills, including the ability to write Java without the Spring Boot framework Genuine, hands-on experience using AI tools to refactor, clean up, or modernize legacy codebases (not just AI-assisted feature work) Experience with concurrency and multithreading Experience with JUnit, Mockito, or similar testing frameworks Experience with PostgreSQL, Oracle, MySQL, SQL Server, or MongoDB Comfortable working in a large, mature monolithic codebase, with curiosity about legacy decisions rather than a desire to rewrite from scratch