How do you validate Spring Boot configuration at startup?

For configuration shared across applications, I version property names and support migrations deliberately because deployment manifests are consumers of that contract. I distinguish absence from an intentionally empty value and document precedence and units. Startup validation reduces ambiguous incidents, but I still expose sanitized effective configuration and health evidence so operators can distinguish a bad setting from an unavailable dependency. That keeps a local contract failure actionable without pretending validation can prove the remote system is healthy.