A solid testing strategy for a Spring Boot application layers several kinds of tests, each targeting a different level of confidence and speed.
Key Points: • Unit tests (JUnit + Mockito) verify individual classes in isolation, mocking out their dependencies. • Integration tests check that components work together correctly, often using @SpringBootTest to load a real (or sliced) application context. • Slice tests like @WebMvcTest or @DataJpaTest test one layer -- controller or repository -- without booting the whole context, keeping them fast. • End-to-end tests simulate real user flows against a running instance, catching issues unit and integration tests miss. • Load/stress tests (JMeter, Gatling) validate behavior under heavy concurrent traffic before it reaches production. • Testcontainers is commonly used to run integration tests against a real database or message broker instead of mocks.
Example: A payment service might have unit tests for the fee-calculation logic, a @DataJpaTest for the repository queries, a @SpringBootTest that exercises the full checkout flow, and a Gatling script that simulates Black Friday traffic.
Interview Tip: A concise interview answer is:
"I layer tests: fast unit tests with JUnit and Mockito for isolated logic, slice tests like @WebMvcTest or @DataJpaTest for individual layers, @SpringBootTest or Testcontainers-backed integration tests for how components work together, and end-to-end plus load tests before major releases to validate real behavior under traffic."