> For the complete documentation index, see [llms.txt](https://jack80342.gitbook.io/spring-boot/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://jack80342.gitbook.io/spring-boot/iv.-spring-boot-features/46.-creating-your-own-auto-configuration/46.1-understanding-auto-configured-beans.md).

# 46.1 理解自动配置的bean

从底层来讲，自动配置（auto-configuration）是通过标准的`@Configuration`类实现的。此外，`@Conditional`注解用来约束自动配置生效的条件。通常自动配置类需要使用`@ConditionalOnClass`和`@ConditionalOnMissingBean`注解，这是为了确保只有在相关的类被发现及没有声明自定义的`@Configuration`时才应用自动配置，具体查看[`spring-boot-autoconfigure`](https://github.com/spring-projects/spring-boot/tree/v2.0.0.RELEASE/spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure)源码中的`@Configuration`类（[`META-INF/spring.factories`](https://github.com/spring-projects/spring-boot/tree/v2.0.0.RELEASE/spring-boot-project/spring-boot-autoconfigure/src/main/resources/META-INF/spring.factories)文件）。