@SpringBootTest — what does it do and when is it the right choice?

@SpringBootTest loads the entire ApplicationContext — every bean, every auto-configuration, your real services and repositories wired together exactly as in production. That's its power and its cost: it's the most faithful test you can write, and the slowest. Reach for it when you're verifying that the whole app wires up and behaves end-to-end, not when you're checking one class in isolation.

What it actually does

It finds your @SpringBootConfiguration (usually via @SpringBootApplication), builds the full context, and makes every bean available for @Autowired injection into the test. Combined with context caching, the first test pays the startup cost and subsequent classes with the same configuration reuse the cached context.

The webEnvironment knob

This decides whether (and how) a servlet container starts:

@SpringBootTest(webEnvironment = WebEnvironment.RANDOM_PORT)
class OrderApiTest {
    @LocalServerPort int port;        // injected real port
    @Autowired TestRestTemplate rest; // talks over real HTTP
}

• MOCK (default) — no real port; a mock servlet environment. Pair with @AutoConfigureMockMvc to drive it via MockMvc.
• RANDOM_PORT — starts the embedded server on a random free port (avoids conflicts in parallel/CI). Inject it with @LocalServerPort.
• DEFINED_PORT — uses the configured port (risky: collisions).
• NONE — no web environment at all, for non-web apps.

When it's the right choice — and when it isn't

Use it for integration tests: REST endpoint through controller → service → repository → real database (via Testcontainers), security filters engaged, message listeners firing. It's the only layer that catches wiring bugs a slice can't see.

Do not reach for it to test a single controller's validation (use @WebMvcTest) or a query (use @DataJpaTest). A common smell is a suite where every test is @SpringBootTest: it's slow, and slices would prove the same things faster and with sharper failure messages.

@SpringBootTest
class OrderServiceIntegrationTest {
    @Autowired OrderService service;     // real bean
    @MockitoBean PaymentGateway gateway; // replace the external edge
}

You can still @MockitoBean the truly external collaborators (payment APIs, third-party clients) inside a full context — keeping the internal graph real while stubbing the network edge.