When do you use Spring Boot runners, events, and lifecycle hooks?

In a scaled deployment, startup code may run concurrently on several instances, so one-time work needs database coordination, a deployment job, or an idempotent design. I keep liveness independent of external dependencies and use readiness to avoid receiving traffic before required local initialization completes. Lifecycle components own their threads and resources through shutdown, and tests verify failure, repeated startup, and partial completion rather than only the happy path. Startup work should expose duration and failure context so a slow rollout can be separated from an application that is permanently stuck.