Which test can prove the behavior?

Goal & mental model verified

Boot supports focused test slices and full-context tests. A slice loads a constrained subset; a full application test exercises broader wiring. Different scopes answer different questions.

Worked example · design exercise synthesis

Test pricing rules with ordinary objects; validate controller errors with an MVC slice; exercise repository mappings against a representative database; reserve full application tests for critical integration flows.

Engineering decision synthesis

Pick the cheapest test that can detect the targeted failure. Real infrastructure tests cost time but can expose SQL, serialization and configuration problems that mocks cannot.

Pitfall & diagnosis synthesis

A mocked repository cannot establish real transaction rollback. Test-managed transactions can also conceal commit-time behavior; choose the test boundary to match the production failure.

Improve & validate synthesis

Build a few high-value scenarios: authorization denial, conflict/rollback, malformed requests and startup misconfiguration. Track failure relevance rather than chasing coverage percentage alone.