Spring Boot自动配置原理

当Spring Boot项目启动时，会加载主配置类，通过@EnableAutoConfiguration开启自动配置功能。@EnableAutoConfiguration会引入AutoConfigurationImportSelector对象，该对象会通过selectImports方法从META-INF/spring.factories文件获取候选组件自动配置类名单。然后，根据获取到的名单扫描项目中是否有该组件的自动配置类，如果有，就对该组件进行配置。

流程

1、启动项目，通过@EnableAutoConfiguration启动自动配置功能。

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

2、@EnableAutoConfiguration会通过@Import注释导入AutoConfigurationImportSelector配置

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

3、调用AutoConfigurationImportSelector中的selectImports方法，获取候选组件自动配置类

public String[] selectImports(AnnotationMetadata annotationMetadata) {
    if (!this.isEnabled(annotationMetadata)) {
        return NO_IMPORTS;
    } else {
        AutoConfigurationImportSelector.AutoConfigurationEntry autoConfigurationEntry = this.getAutoConfigurationEntry(annotationMetadata);
        return StringUtils.toStringArray(autoConfigurationEntry.getConfigurations());
    }
}


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

//从META-INF/spring.factories获取候选组件自动配置类
protected List<String> getCandidateConfigurations(AnnotationMetadata metadata, AnnotationAttributes attributes) {
    List<String> 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;
}

4、根据获取到的组件自动配置类名单，从项目中扫描是否有该组件的自动配置类，如果有，就根据该组件的自动配置类对组件进行配置。