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

// org.springframework.boot.SpringApplication
public static ConfigurableApplicationContext run(Class<?>[] primarySources, String[] args) {return new SpringApplication(primarySources).run(args);
}public ConfigurableApplicationContext run(String... args) {···省略···try {···省略···context = createApplicationContext();exceptionReporters = getSpringFactoriesInstances(SpringBootExceptionReporter.class,new Class[] { ConfigurableApplicationContext.class }, context);// 真正启动应用程序上下文前的一些准备动作// 这里会去将Application.class，注册到org.springframework.context.annotation.AnnotatedBeanDefinitionReader// 也就是去把Application.class注册成一个BeanDefinition实例// 不过Application必须要是一个@ComponentprepareContext(context, environment, listeners, applicationArguments, printedBanner);// 刷新上下文，这个过程中主要就是Bean的实例化和属性赋值绑定// 如果是Web环境，涉及到Web相关的一些东西，但是本质上还是各种Bean的实例化// 和Bean之间依赖关系的处理，代理Bean的生成(涉及到AspectJ的Advice处理)等等refreshContext(context);}return context;
}

@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 {···省略···
}

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import(AutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {···省略···
}

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@Import(AutoConfigurationPackages.Registrar.class)
public @interface AutoConfigurationPackage {}

// org.springframework.boot.autoconfigure.AutoConfigurationImportSelector
public class AutoConfigurationImportSelector implements DeferredImportSelector, BeanClassLoaderAware,ResourceLoaderAware, BeanFactoryAware, EnvironmentAware, Ordered {@Overridepublic String[] selectImports(AnnotationMetadata annotationMetadata) {if (!isEnabled(annotationMetadata)) {return NO_IMPORTS;}// 为了加载spring-boot-autoconfiguration包下的配置文件META-INF/spring-autoconfigure-metadata.properties// 这里配置的主要是一些SpringBoot启动时用到的一些@ConditionOnClass的配置AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader.loadMetadata(this.beanClassLoader);// 这里的AutoConfigurationEntry，就包含了所有的导入的需要被实例化的BeanAutoConfigurationEntry autoConfigurationEntry = getAutoConfigurationEntry(autoConfigurationMetadata,annotationMetadata);// 返回这些被导入Bean的类全限定名数组return StringUtils.toStringArray(autoConfigurationEntry.getConfigurations());}protected AutoConfigurationEntry getAutoConfigurationEntry(AutoConfigurationMetadata autoConfigurationMetadata,AnnotationMetadata annotationMetadata) {if (!isEnabled(annotationMetadata)) {return EMPTY_ENTRY;}··· 省略 ···// 获取所有的需要导入的Bean，这些被导入的Bean就是各个组件需要自动化配置的启动点List<String> configurations = getCandidateConfigurations(annotationMetadata, attributes);··· 省略 ···return new AutoConfigurationEntry(configurations, exclusions);}protected List<String> getCandidateConfigurations(AnnotationMetadata metadata, AnnotationAttributes attributes) {// 使用SpringFactoriesLoader#loadFactoryNames方法，从所有的包及classpath目录下，// 查找META-INF/spring.factories文件，且名称为org.springframework.boot.autoconfigure.EnableAutoConfiguration的配置List<String> configurations = SpringFactoriesLoader.loadFactoryNames(getSpringFactoriesLoaderFactoryClass(),getBeanClassLoader());Assert.notEmpty(configurations, "No auto configuration classes found in META-INF/spring.factories. If you "+ "are using a custom packaging, make sure that file is correct.");return configurations;}
}