What is auto-configuration? How does Spring Boot decide what to configure?

Auto-configuration is Spring Boot automatically registering beans you'd otherwise declare by hand, based on what's on the classpath, what beans already exist, and what properties are set. It is not magic and it is not reflection-scanning your code — it's a fixed list of @Conditional-guarded @Configuration classes that Boot evaluates at startup.

How the decision is made

@EnableAutoConfiguration (inside @SpringBootApplication) tells Boot to load every auto-configuration class it can find and evaluate it. Each class carries @Conditional annotations that act as gates:

• @ConditionalOnClass — only configure if a type is on the classpath (e.g. DataSource.class). This is how "add the jar and it just works" happens.
• @ConditionalOnMissingBean — only configure if you haven't defined that bean. This is why your own @Bean always wins.
• @ConditionalOnProperty — only when a property has a given value.
• @ConditionalOnWebApplication — only in a servlet/reactive web context.

If every condition on a class passes, it contributes its beans; if any fails, the class is silently skipped.

The order of operations

1. The application context starts and processes @EnableAutoConfiguration.
2. Boot reads the candidate class names from META-INF/spring/org.springframework.boot.autoconfigure.AutoConfiguration.imports in every jar on the classpath.
3. Candidates are ordered (@AutoConfiguration(before=..., after=...), @AutoConfigureOrder) and importantly evaluated after your own @Component/@Configuration beans, so user beans exist when @ConditionalOnMissingBean runs.
4. Each surviving class registers its beans.

@SpringBootApplication
public class App {
    public static void main(String[] args) {
        SpringApplication.run(App.class, args);
    }
    // No @Bean DataSource here, yet Boot wires one because
    // spring-boot-starter-data-jpa is on the classpath and
    // spring.datasource.* properties are set.
}

The mental model to state

Auto-configuration = "a curated set of @Configuration classes, each saying I'll configure X only if these conditions hold." The conditions are classpath presence, absence of a user-defined bean, and property values. Because of @ConditionalOnMissingBean, every default is overridable — you opt out simply by defining the bean yourself.