Auto-Configuration

Spring Boot inspects what's on the classpath and automatically configures beans you'd otherwise wire up by hand.

@EnableAutoConfiguration (bundled inside @SpringBootApplication) scans a list of auto-configuration classes, each guarded by @Conditional checks — e.g. 'only configure a DataSource if a JDBC driver is on the classpath'.

@ConditionalOnClass / @ConditionalOnMissingBean drive most auto-config decisions
You can always override any auto-configured bean by defining your own @Bean
spring.factories / AutoConfiguration.imports registers which classes to consider

Tip

Mention @ConditionalOnMissingBean specifically — it's how your own beans always win over defaults.