Unit versus integration boundaries, mocking strategy, Testcontainers, and suite design.
3 questions in Java & Spring.
Mostly into starting Spring application contexts. Context caching reuses one context per distinct test configuration, so each unique mock bean, property set, profile or @DirtiesContext forces a fresh start. Count contexts from the cache statistics log, converge the configurations, and move narrow tests to slices.
Make time and scheduling injectable. Take a java.time.Clock as a dependency so a test can fix it, and replace a fixed sleep with a bounded poll on the observable outcome, so a fast machine does not wait and a slow one does not fail.
Draw the line by what the test is allowed to touch - a unit test stays in-process, an integration test crosses a real boundary. Mock collaborators, never the component under test. Use Testcontainers for real dependencies and keep the suite fast by reusing the Spring context and the containers.