What is Spring Boot auto-configuration (and what are starters)?

Deep dive

Spring Boot auto-configuration is a set of `@Configuration` classes enabled by `@SpringBootApplication` (via `@EnableAutoConfiguration`). Boot decides what to configure based on:

• what’s on the **classpath**,
• your **application properties**,
• and `@Conditional...` rules (e.g., `@ConditionalOnClass`, `@ConditionalOnMissingBean`).

Starters (like `spring-boot-starter-web`, `spring-boot-starter-security`, `spring-boot-starter-data-jpa`) are curated dependency sets that:

• pull in common libraries for a feature,
• keep versions aligned through Boot’s dependency management.

How to customize

• Prefer properties first (stable and easy).
• Define your own beans to override defaults (many auto-configs back off if a user bean exists).
• Exclude an auto-config when you don’t want it:

@SpringBootApplication(exclude = DataSourceAutoConfiguration.class)

Common pitfalls

• Adding a starter and being surprised by default behavior (security is a classic).
• Overriding beans accidentally and breaking auto-configured wiring.
• Different classpaths between environments causing different auto-config behavior.