项目里面为什么不加上@ComponentScan("com.yym.*")注解，也能加载到子目录里面的Controller，Service，Component的bean呢？

启动类没有@ComponentScan注解

@SpringBootApplication
public class BootStrap {public static void main(String[] args) {SpringApplication.run(BootStrap.class, args);}
}

原因：

spring boot 启动类加上@SpringBootApplication会自动扫描当前目录，及子目录下的Controller，Service，Component注解的bean。

查看@SpringBootApplication注解源码，里面有@ComponentScan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

查看Controller，Service注解源码都有Component注解

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Component
public @interface Controller {/*** The value may indicate a suggestion for a logical component name,* to be turned into a Spring bean in case of an autodetected component.* @return the suggested component name, if any (or empty String otherwise)*/@AliasFor(annotation = Component.class)String value() default "";}

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Component
public @interface Service {/*** The value may indicate a suggestion for a logical component name,* to be turned into a Spring bean in case of an autodetected component.* @return the suggested component name, if any (or empty String otherwise)*/@AliasFor(annotation = Component.class)String value() default "";}

原理：

##AnnotationConfigServletWebServerApplicationContext构造器初始化AnnotatedBeanDefinitionReader、ClassPathBeanDefinitionScanner

##AnnotatedBeanDefinitionReader构造器初始化调用AnnotationConfigUtils.registerAnnotationConfigProcessors静态方法注册ConfigurationClassPostProcessor.class

##注册ConfigurationClassPostProcessor.class的BeanDefinition

##解析启动配置类

##看到ConfigurationClassParser解析ComponentScans.class, ComponentScan.class注解

##ComponentScanAnnotationParser的parse方法解析

##包名为空，添加启动类所在的包

##找到启动类所在包，及子包所有的bean候选者

##至此类被扫描成BeanDefinition并注册到DefaultListableBeanFactory的beanDefinitionMap