Auto-configuration trong Spring Boot hoạt động thế nào?

Auto-configuration tự động configure bean dựa trên dependency có trong classpath — không cần khai báo thủ công.

Cơ chế:
1. @EnableAutoConfiguration kích hoạt scanning META-INF/spring/org.springframework.boot.autoconfigure.AutoConfiguration.imports (Spring Boot 3) hoặc spring.factories (Boot 2).
2. Mỗi entry là một @Configuration class có @Conditional guards:

@AutoConfiguration
   @ConditionalOnClass(DataSource.class)          // chỉ nếu driver có trên classpath
   @ConditionalOnMissingBean(DataSource.class)    // chỉ nếu user chưa tự định nghĩa
   public class DataSourceAutoConfiguration { ... }

3. Nếu condition pass → bean được tạo. Nếu user tự define bean → @ConditionalOnMissingBean block auto-config.

Debug: thêm --debug flag hoặc debug=true → Spring in auto-config report ra console (pass/fail condition từng class).

Tự viết auto-config: tạo file META-INF/spring/...AutoConfiguration.imports, khai báo config class với @ConditionalOn* phù hợp.

Auto-configuration automatically configures beans based on dependencies on the classpath — no manual declaration needed.

Mechanism:
1. @EnableAutoConfiguration triggers scanning of META-INF/spring/org.springframework.boot.autoconfigure.AutoConfiguration.imports (Spring Boot 3) or spring.factories (Boot 2).
2. Each entry is a @Configuration class guarded by @Conditional checks:

@AutoConfiguration
   @ConditionalOnClass(DataSource.class)          // only if the driver is on the classpath
   @ConditionalOnMissingBean(DataSource.class)    // only if the user has not defined one
   public class DataSourceAutoConfiguration { ... }

3. If conditions pass → bean is created. If the user defines the bean → @ConditionalOnMissingBean blocks auto-config.

Debugging: add --debug flag or debug=true → Spring prints an auto-config report to the console (pass/fail per class).

Writing your own auto-config: create META-INF/spring/...AutoConfiguration.imports, declare a config class with appropriate @ConditionalOn* guards.