Spring Boot Core, Auto-Configuration, and Configuration

Own the composition root: which starter enters the build, which auto-configuration produces a bean, and which value the Environment finally returns. Get those three wrong and every downstream symptom looks like a framework bug.

When to use

• Choosing between adding a starter, writing an @AutoConfiguration, and declaring a bean by hand.
• A bean is missing at startup, or an auto-configuration backs off unexpectedly.
• Binding @ConfigurationProperties with a record, nested records, collections, maps, or Duration, or a property that resolves to the wrong value.
• Profile activation, profile-specific files, spring.config.import, and SDK, pool, or spring.threads.virtual.enabled wiring.

When not to use

• Request handling, filters, interceptors, and async responses belong to spring-boot-mvc.
• Repository, entity mapping, and transaction wiring belong to spring-boot-data-jpa.
• Filter chains, authentication, and authorization belong to spring-boot-security.
• Build pipelines, image layers, and secret injection belong to spring-boot-ci-cd.
• Test layer selection and slice mechanics belong to spring-boot-integration-testing, and the mechanics inside a test class to spring-boot-junit.