在SpringBoot中，EnableAutoConfiguration注解用于开启自动装配功能。

本文将详细分析该注解的工作流程。

EnableAutoConfiguration注解

启用SpringBoot自动装配功能，尝试猜测和配置可能需要的组件Bean。

自动装配类通常是根据类路径和定义的Bean来应用的。例如，如果类路径上有tomcat-embedded.jar，那么可能需要一个TomcatServletWebServerFactory(除非已经定义了自己的Servlet WebServerFactory Bean)。

自动装配试图尽可能地智能化，并将随着开发者定义自己的配置而取消自动装配相冲突的配置。开发者可以使用exclude()排除不想使用的配置，也可以通过spring.autoconfig.exclude属性排除这些配置。自动装配总是在用户定义的Bean注册之后应用。

用@EnableAutoConfiguration注解标注的类所在包具有特定的意义，通常用作默认扫描的包。通常建议将@EnableAutoConfiguration(如果没有使用@SpringBootApplication注解)放在根包中，以便可以搜索所有子包和类。

自动装配类是普通的Spring @Configuration类，使用SpringFactoriesLoader机制定位。通常使用@Conditional方式装配，最常用的是@ConditionalOnClass和@ConditionalOnMissingBean注解。

@AutoConfigurationPackage
@Import(AutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {/*** Exclude specific auto-configuration classes such that they will never be applied.*/Class<?>[] exclude() default {};/*** Exclude specific auto-configuration class names such that they will never be* applied.* 当类路径下没有指定的类时，可以使用这个属性指定排除的类*/String[] excludeName() default {};
}

该注解Import了AutoConfigurationImportSelector类，AutoConfigurationImportSelector类实现了DeferredImportSelector接口。

Import注解和DeferredImportSelector接口在之前的"Spring @Import注解源码分析"中详细分析过，此处在介绍它们，只分析AutoConfigurationImportSelector的工作流程。

AutoConfigurationImportSelector类

DeferredImportSelector接口

A variation of ImportSelector that runs after all @Configuration beans have been processed. This type of selector can be particularly useful when the selected imports are @Conditional.

Implementations can also extend the org.springframework.core.Ordered interface or use the org.springframework.core.annotation.Order annotation to indicate a precedence against other DeferredImportSelectors.

Implementations may also provide an import group which can provide additional sorting and filtering logic across different selectors.

AutoConfigurationGroup类

AutoConfigurationImportSelector的getImportGroup方法返回了AutoConfigurationGroup类。

private static class AutoConfigurationGroup implements DeferredImportSelector.Group, BeanClassLoaderAware, BeanFactoryAware, ResourceLoaderAware {private final Map<String, AnnotationMetadata> entries = new LinkedHashMap<>();private final List<AutoConfigurationEntry> autoConfigurationEntries = new ArrayList<>();// ... 略@Overridepublic void process(AnnotationMetadata annotationMetadata,DeferredImportSelector deferredImportSelector) {// AutoConfigurationEntry类使用List保存Configuration类AutoConfigurationEntry autoConfigurationEntry =((AutoConfigurationImportSelector) deferredImportSelector).getAutoConfigurationEntry(annotationMetadata);this.autoConfigurationEntries.add(autoConfigurationEntry);for (String importClassName : autoConfigurationEntry.getConfigurations()) {this.entries.putIfAbsent(importClassName, annotationMetadata);}}@Overridepublic Iterable<Entry> selectImports() {// 查找排除的配置类Set<String> allExclusions = this.autoConfigurationEntries.stream().map(AutoConfigurationEntry::getExclusions).flatMap(Collection::stream).collect(Collectors.toSet());// 所有配置类Set<String> processedConfigurations = this.autoConfigurationEntries.stream().map(AutoConfigurationEntry::getConfigurations).flatMap(Collection::stream).collect(Collectors.toCollection(LinkedHashSet::new));// 将排除的配置类移除掉processedConfigurations.removeAll(allExclusions);// 排序return sortAutoConfigurations(processedConfigurations, getAutoConfigurationMetadata()).stream().map((importClassName) -> new Entry(this.entries.get(importClassName), importClassName)).collect(Collectors.toList());}// ... 略
}

从上面的代码可以看出，查找自动装配类的逻辑在getAutoConfigurationEntry方法中。

getAutoConfigurationEntry方法

从META-INF/spring.factories文件解析EnableAutoConfiguration配置。

META-INF/spring.factories文件示例：

protected AutoConfigurationEntry getAutoConfigurationEntry(AnnotationMetadata annotationMetadata) {AnnotationAttributes attributes = getAttributes(annotationMetadata);// 查找自动装配类List<String> configurations = getCandidateConfigurations(annotationMetadata, attributes);// 以下几行为查找排除类、过滤等操作configurations = removeDuplicates(configurations);Set<String> exclusions = getExclusions(annotationMetadata, attributes);checkExcludedClasses(configurations, exclusions);configurations.removeAll(exclusions);// 这里的Filter是从META-INF/spring.factories文件解析出来的configurations = getConfigurationClassFilter().filter(configurations);// 触发事件fireAutoConfigurationImportEvents(configurations, exclusions);return new AutoConfigurationEntry(configurations, exclusions);
}protected List<String> getCandidateConfigurations(AnnotationMetadata metadata, AnnotationAttributes attributes) {// 从META-INF/spring.factories文件查找EnableAutoConfiguration配置List<String> configurations = SpringFactoriesLoader.loadFactoryNames(getSpringFactoriesLoaderFactoryClass(), getBeanClassLoader());return configurations;
}

SpringFactoriesLoader类loadFactoryNames方法

Load the fully qualified class names of factory implementations of the given type from “META-INF/spring.factories”, using the given class loader.

public static List<String> loadFactoryNames(Class<?> factoryType, ClassLoader classLoader) {String factoryTypeName = factoryType.getName();return loadSpringFactories(classLoader).getOrDefault(factoryTypeName, Collections.emptyList());
}private static Map<String, List<String>> loadSpringFactories(ClassLoader classLoader) {MultiValueMap<String, String> result = cache.get(classLoader);if (result != null) {return result;}try {// FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories"// 从类路径下查找META-INF/spring.factories文件Enumeration<URL> urls = (classLoader != null ?classLoader.getResources(FACTORIES_RESOURCE_LOCATION) :ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));result = new LinkedMultiValueMap<>();while (urls.hasMoreElements()) {URL url = urls.nextElement();UrlResource resource = new UrlResource(url);// 获取properties配置Properties properties = PropertiesLoaderUtils.loadProperties(resource);for (Map.Entry<?, ?> entry : properties.entrySet()) {String factoryTypeName = ((String) entry.getKey()).trim();for (String factoryImplementationName :StringUtils.commaDelimitedListToStringArray((String) entry.getValue())) {result.add(factoryTypeName, factoryImplementationName.trim());}}}// 把配置添加缓存cache.put(classLoader, result);return result;} catch (IOException ex) {throw new IllegalArgumentException("Unable to load factories from location [" +FACTORIES_RESOURCE_LOCATION + "]", ex);}
}