SpringBoot是如何完成自动配置的

1，SB的项目启动注解

@SpringBootApplication
public class SpringbootHelloWorldApplication {

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

}

2，@SpringBootApplication注解由@Configuration@ComponentScan@EnableAutoConfiguration 三种注解组成

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

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@documented
@Configuration
@Indexed
public @interface SpringBootConfiguration {
    @AliasFor(
        annotation = Configuration.class
    )
    boolean proxyBeanMethods() default true;
}

3，其中EnablAutoConfiguration还引入了AutoConfigurationimportSelector类

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@documented
@Inherited
@AutoConfigurationPackage
@import({AutoConfigurationimportSelector.class})
public @interface EnableAutoConfiguration {
    String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";

    Class[] exclude() default {};

    String[] excludeName() default {};
}

4，AutoConfigurationimportSelector中扫描了 meta-INF/spring.factories

protected List getCandidateConfigurations(Annotationmetadata metadata, AnnotationAttributes attributes) {
        List configurations = SpringFactoriesLoader.loadFactoryNames(this.getSpringFactoriesLoaderFactoryClass(), this.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;
}