Back in 2025, a developer tackled a 20‑year‑old JSP stack, moving it into a microservice architecture. The original code lived in scattered JSP files, bloated with static utilities and deep inheritance. Copy‑pasting the old structure into the new services only amplified the mess, turning a simple request into a nightmare for the team to understand.

SonarQube analysis revealed a bugfest: SQL injection spots, duplicated blocks, and naming violations. The cleanup cut nearly 2,000 unused classes and trimmed 30% of duplication, shrinking the codebase to 900 classes and 55,000 lines. The real challenge was reaching 80% test coverage, forcing the team to write tests that exposed hidden complexity in the legacy.

Writing tests became a diagnostic tool; deep nesting forced the author to refactor with guard clauses and Java streams, replacing 10‑level if‑chains with concise logic. Switching from Lombok’s @Data to explicit @Getter/@Setter removed hidden branches, boosting coverage. The lesson: fear the rewrite, and let tests guide clean‑code evolution for future maintainers and quality in production.