Unit vs Integration vs End-to-End Testing

Unit tests verify one class in isolation with mocked dependencies — they run in milliseconds and tell you exactly what broke. Integration tests verify that components work together with real infrastructure — Spring context, database, HTTP. E2E tests verify complete user flows through the entire system.

The pyramid isn't about having "more unit tests" — it's about feedback speed vs confidence. A failing unit test points to the exact method in seconds. A failing E2E test says "something is broken" and takes minutes to diagnose.

The anti-pattern is the ice cream cone — mostly E2E tests, few unit tests. Tests become so slow that developers stop running them locally, and CI becomes the only feedback loop. My rule: if you can test it with a unit test, don't write an integration test. Push testing to the lowest level that still gives you confidence.