1、@SpringBootApplication注解
启动类注解，该注解包含下面三个注解：
@SpringBootConfiguration
@EnableAutoConfiguration
@ComponentScan
@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
}2、@SpringBootConfiguration
几乎和@Configuration注解没有区别，标识这是一个配置类。
3、@EnableAutoConfiguration
从classpath中搜寻所有的META-INF/spring.factories配置文件，并将其中
org.springframework.boot.autoconfigure.EnableAutoConfiguration对应的配置项通过反射实例化为对应的标注了@Configuration的lOC容器配置类，然后汇总为一个，并加载到loC容器。
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import(AutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {
/**
* Environment property that can be used to override when auto-configuration is
* enabled.
*/
String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";
/**
* Exclude specific auto-configuration classes such that they will never be applied.
* @return the classes to exclude
*/
Class<?>[] exclude() default {};
/**
* Exclude specific auto-configuration class names such that they will never be
* applied.
* @return the class names to exclude
* @since 1.3.0
*/
String[] excludeName() default {};
}该注解包含下面两个个注解：
@AutoConfigurationPackage：
@Import(AutoConfigurationImportSelector.class)：
@EnableAutoConfiguration注解导入了AutoConfigurationImportSelector类，它负责查找所有的配置类。AutoConfigurationImportSelector类中的selectImports方法会查询META-INF/spring.factories文件，这个文件列出了所有Spring Boot的自动配置类【原理参考扩展中的5.1】。对于每一个自动配置类，Spring会检查
@Conditional注解，只有满足条件的配置类才会被实例化。
#AutoConfigurationImportSelector源码：
public class AutoConfigurationImportSelector implements DeferredImportSelector, BeanClassLoaderAware,
ResourceLoaderAware, BeanFactoryAware, EnvironmentAware, Ordered {
@Override
public String[] selectImports(AnnotationMetadata annotationMetadata) {
if (!isEnabled(annotationMetadata)) {
return NO_IMPORTS;
}
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> ex