How do you test a Spring service that uses a database and calls another API?

Test the domain logic with plain JUnit and no Spring context, since constructor injection makes that trivial. Test persistence against a real database in a container rather than an in memory substitute, because dialects differ and migrations need testing too. Use test slices rather than booting the whole application for controller or repository tests, and stub the external API at the HTTP layer so your client code, retries and parsing all run.

Most of the logic gets plain unit tests with no framework at all: constructor injection means I can instantiate the class with test doubles for its collaborators and it runs in milliseconds. For persistence I use a real database of the same engine and version in a container, with migrations applied as part of the test setup, so the migration itself is verified on every run. An in memory database is faster but it lies about dialects and constraints, and I have shipped a query that worked in tests and failed in production because of it. For the web layer I use a slice that loads just the controller and the serialization, not the whole application, because booting everything for every test is what makes suites take twenty minutes. The external API is stubbed at the HTTP level with a mock server, so my client configuration, timeouts, retry logic and JSON mapping are exercised for real. I also reuse containers across the run and roll back each test in a transaction, which keeps things isolated and parallelizable.