SpringBoot的启动原理

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)：

1. @EnableAutoConfiguration注解导入了AutoConfigurationImportSelector类，它负责查找所有的配置类。

2. 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】。

3. 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 {@Overridepublic 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