Introduction

Most Java codebases have plenty of tests and still get burned by production bugs. The problem is rarely quantity — it’s that the tests exercise the wrong things: they assert internal wiring instead of behavior, they mock away every collaborator until nothing real is left, or they cover the happy path ten times and the failure path zero times.

Good tests are fast, deterministic, and tell you something true about behavior. This post covers the practices that make JUnit 5 and Mockito tests actually earn their keep.