Mastering Unit Testing in Spring Boot: How I Achieved 70%+ Coverage for Open Source

Testing is not about finding bugs; it's about gaining the confidence to ship code. Recently, while contributing to an Open Source project for GSSoC 2026, I took on the challenge of writing unit tests for a complex DecisionService. My goal was 70%+ coverage, but the journey was filled with NullPointerExceptions and mocking hurdles. Here is how I solved them. The Challenge The DecisionService I was testing relied on multiple repositories and complex DTO mapping. When I first ran my tests, I kept h