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

通过 SpringApplication.run 可以进入方法得知 spring 底层自动扫描所有在META-INF/spring.factories中默认配置好的自动装配类，然后根据条件按需加载组件（bean）到 spring 容器中。

private static Map<String, List<String>> loadSpringFactories(@Nullable ClassLoader classLoader) {

   try {
      Enumeration<URL> urls = (classLoader != null ?
            classLoader.getResources(FACTORIES_RESOURCE_LOCATION) :
            ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));
	}
}

package cc.voox.demo;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.ConfigurableApplicationContext;

@SpringBootApplication
public class DemoApplication {
}

@SpringBootApplication 是由多个注解组合而成。

@SpringBootConfiguration
@EnableAutoConfiguration
@ComponentScan(excludeFilters = { @Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),
		@Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })
public @interface SpringBootApplication {
}

@SpringBootConfiguration 和@Configuration 功能一样

@Configuration
public @interface SpringBootConfiguration {}

@SpringBootConfiguration is a class-level annotation that is part of the Spring Boot framework. It indicates that a class provides application configuration. Spring Boot favors Java-based configuration. As a result, the @SpringBootConfiguration annotation is the primary source for configuration in applications.

@ComponentScan 包扫描器

@AutoConfigurationPackage
@Import(AutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {
}

自动导入AutoConfigurationImportSelector 组件后，通过selectImports方法封装所有满足的类成AutoConfigurationEntry进行导入

@Override
public String[] selectImports(AnnotationMetadata annotationMetadata) {

	AutoConfigurationEntry autoConfigurationEntry = getAutoConfigurationEntry(annotationMetadata);
	return StringUtils.toStringArray(autoConfigurationEntry.getConfigurations());
}

protected AutoConfigurationEntry getAutoConfigurationEntry(AnnotationMetadata annotationMetadata) {
	if (!isEnabled(annotationMetadata)) {
		return EMPTY_ENTRY;
	}
	AnnotationAttributes attributes = getAttributes(annotationMetadata);
	List<String> configurations = getCandidateConfigurations(annotationMetadata, attributes);
	configurations = removeDuplicates(configurations);
	Set<String> exclusions = getExclusions(annotationMetadata, attributes);
	checkExcludedClasses(configurations, exclusions);
	configurations.removeAll(exclusions);
	configurations = getConfigurationClassFilter().filter(configurations);
	fireAutoConfigurationImportEvents(configurations, exclusions);
	return new AutoConfigurationEntry(configurations, exclusions);
}

利用PackageImports类批量注册组件到容器中。

@Import(AutoConfigurationPackages.Registrar.class)
public @interface AutoConfigurationPackage {
}

static class Registrar implements ImportBeanDefinitionRegistrar, DeterminableImports {

		@Override
		public void registerBeanDefinitions(AnnotationMetadata metadata, BeanDefinitionRegistry registry) {
			register(registry, new PackageImports(metadata).getPackageNames().toArray(new String[0]));
		}
}