Spring Boot 启动类上一个 @SpringBootApplication 注解是

• @SpringBootConfiguration
• @EnableAutoConfiguration
• @ComponentScan

三个注解组成的一个复合注解。其中 @SpringBootConfiguration 其实也是和@Configuration 注解组成的一个组合注解，功能也是和 @Configuration 相同；@ComponentScan 这个注解是标注需要扫描的包。今天着重说一下 @EnableAutoConfiguration 注解。

其实 @EnableAutoConfiguration 注解也和其它 @Enable*
注解一脉相乘的，简单说一下就是借助 @Import 的支持，收集和注册特定场景相关的Bean的定义：比如：@EnableAspectJAutoProxy 就是通过@Import 注解动态的将Bean注册到 SpringIoc
容器中，而@EnableAutoConfiguration 是借助 @Import 把所有符合条件的 Bean 加载到
SpringIoc 容器中。

参看上一篇文章 Spring Boot
自动配置之@Enable*与@Import注解

1、@EnableAutoConfiguration 也是一个组合注解

@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 {};

}

其中最重要的是 @Import(AutoConfigurationImportSelector.class)注解。借助AutoConfigurationImportSelector ，@EnableAutoConfiguration 帮助Spring Boot 应用将所有符合条件的 @Configuration 配置加载到当前IoC容器中。而最主要的还是借助于 Spring 框架一的一个工具类：SpringFactoriesLoader 将 META-INF/spring.factories加载配置，spring.factories 文件是一个典型的properties配置文件，配置的格式仍然是Key = Value 的形式，中不过 Key 和 Value 都是Java的完整类名。比如：org.springframework.data.repository.core.support.RepositoryFactorySupport=org.springframework.data.jpa.repository.support.JpaRepositoryFactory

2、AutoConfigurationImportSelector 源码

@Override
    public String[] selectImports(AnnotationMetadata annotationMetadata) {
        if (!isEnabled(annotationMetadata)) {
            return NO_IMPORTS;
        }
        AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader
                .loadMetadata(this.beanClassLoader); //1
        AutoConfigurationEntry autoConfigurationEntry = getAutoConfigurationEntry(
                autoConfigurationMetadata, annotationMetadata); //2
        return StringUtils.toStringArray(autoConfigurationEntry.getConfigurations());
    }

• 1.获取注解信息
• 2.获取所有配置列表

其中 AutoConfigurationMetadataLoader.loadMetadata方法源码

public static AutoConfigurationMetadata loadMetadata(ClassLoader classLoader) {
        return loadMetadata(classLoader, PATH);
    }

    static AutoConfigurationMetadata loadMetadata(ClassLoader classLoader, String path) {
        try {
            Enumeration<URL> urls = (classLoader != null) ? classLoader.getResources(path)
                    : ClassLoader.getSystemResources(path);
            Properties properties = new Properties();
            while (urls.hasMoreElements()) {
                properties.putAll(PropertiesLoaderUtils
                        .loadProperties(new UrlResource(urls.nextElement())));
            }
            return loadMetadata(properties);
        }
        catch (IOException ex) {
            throw new IllegalArgumentException(
                    "Unable to load @ConditionalOnClass location [" + path + "]", ex);
        }
    }

getAutoConfigurationEntry 方法：

protected AutoConfigurationEntry getAutoConfigurationEntry(
            AutoConfigurationMetadata autoConfigurationMetadata,
            AnnotationMetadata annotationMetadata) {
        if (!isEnabled(annotationMetadata)) {
            return EMPTY_ENTRY;
        }
        //1.
        AnnotationAttributes attributes = getAttributes(annotationMetadata);
        //2.
        List<String> configurations = getCandidateConfigurations(annotationMetadata,
                attributes);
        //3.
        configurations = removeDuplicates(configurations);
        //4.
        Set<String> exclusions = getExclusions(annotationMetadata, attributes);
        checkExcludedClasses(configurations, exclusions);
        configurations.removeAll(exclusions);
        configurations = filter(configurations, autoConfigurationMetadata);
        //5.
        fireAutoConfigurationImportEvents(configurations, exclusions);
        return new AutoConfigurationEntry(configurations, exclusions);
    }

• 1.获取注解上所有属性信息
• 2.获取候选配置列表 [核心步骤]

  protected List<String> getCandidateConfigurations(AnnotationMetadata metadata,
          AnnotationAttributes attributes) {
      List<String> configurations = SpringFactoriesLoader.loadFactoryNames(
              getSpringFactoriesLoaderFactoryClass(), 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;
  }
  

  通过SpringFactoriesLoader.loadFactoryNames获取配置信息
• 3.去重
• 4.配置exclude 信息去除不需要的
• 5.派发事件