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.）
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