如何进行自动装配？

完成Spring Boot约定的组件自动装配需要包含三个因素：

激活@EnableAutoConfiguration注解（全流程管理）

配置/META-INF/spring.factories（约定，是否将组件下某个class加入自动装配管理）

实现xxxAutoConfiguration组件配置类（符合什么条件才加入自动装配）

整体流程是@EnableAutoConfiguration中实现类扫描“classpath下即pom依赖”中所有包含/META-INF/spring.factories的包，并根据xxxAutoConfiguration和/META-INF/spring-autoconfigure-metadata.properties中的条件判断来决定是否进行自动装配。

SpringBootApplication是一组注解的组合，其中包含@EnableAutoConfiguration注解。

@Target(ElementType.TYPE)

@Retention(RetentionPolicy.RUNTIME)

@Documented

@Inherited

@SpringBootConfiguration

@EnableAutoConfiguration

@ComponentScan(excludeFilters = {

@Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),

@Filter(type = FilterType.CUSTOM,

classes = AutoConfigurationExcludeFilter.class) })

public @interface SpringBootApplication {}

而根据@Enable模块驱动设计模式（即Import的类被@Configuration注解或实现ImportSelector接口，可实现注入）@EnableAutoConfiguration必然Import了“实现ImportSelector接口的实现类”，并实现其SelectImport方法。

// 1. Import了AutoConfigurationImportSelector类

@Target(ElementType.TYPE)

@Retention(RetentionPolicy.RUNTIME)

@Documented

@Inherited

@AutoConfigurationPackage

@Import(AutoConfigurationImportSelector.class) 

public @interface EnableAutoConfiguration {

}

//2. 实现了DeferredImportSelector接口，并重写selectImports方法

public class AutoConfigurationImportSelector

implements DeferredImportSelector, BeanClassLoaderAware, ResourceLoaderAware,

BeanFactoryAware, EnvironmentAware, Ordered {

}

//3. 继承ImportSelector接口

public interface DeferredImportSelector extends ImportSelector {}

整体流程：

获取自动加载配置的元数据：主要是获取classpath下所有META-INF/spring-autoconfigure-metadata。properties中的配置信息，文件里面配置的作用等同于在xxxAutoConfiguration上面的@ConditionalOnClass等注解，这么做的好处就是参考官网说明。（Spring Boot uses an annotation processor to collect the conditions on auto-configurations in a metadata file (META-INF/spring-autoconfigure-metadata.properties). If that file is present, it is used to eagerly filter auto-configurations that do not match, which will improve startup time.）