Testing

A Java backend engineer who cannot write good tests is considered junior regardless of other skills. This domain covers the full testing pyramid: fast unit tests with JUnit 5 and Mockito, integration tests with Spring Boot test slices, and realistic end-to-end tests using Testcontainers to spin up real databases in CI.

What You'll Find Here

Notes are being added. Planned topics:

Topic	Description
JUnit 5	@Test, @ParameterizedTest, @BeforeEach, lifecycle, assertions, assumptions.
Mockito	@Mock, @InjectMocks, when(...).thenReturn(...), verify, ArgumentCaptor.
Spring Boot Test Slices	@WebMvcTest, @DataJpaTest, @JsonTest — fast partial-context tests.
Integration Tests	@SpringBootTest with TestRestTemplate; full-context tests.
Testcontainers	Running real PostgreSQL, MySQL, Redis, Kafka in Docker containers during tests.
MockMvc & WebTestClient	HTTP-level controller testing without a running server.

Learning Path

1. JUnit 5 — the basics (@Test, @BeforeEach, assertThrows) are required before everything else.
2. Mockito — @Mock stub setup and verify interaction testing are the day-to-day tools.
3. MockMvc — test Spring MVC controllers without a real server; use with @WebMvcTest for speed.
4. @DataJpaTest — test repositories with an in-memory H2 database (or Testcontainers for realism).
5. Testcontainers — the current gold standard for integration tests; Docker-based environments that match production.

Related Domains

• Spring Boot — Spring Boot test slices are the primary testing tool.
• Databases — Testcontainers integrates with all major databases for realistic integration tests.
• Docker — Testcontainers uses Docker under the hood.