SpringBoot 常用注解的原理和使用 _生活百科

@AutoConfiguration读取所有jar包下的 /META-INF/spring.factories 并追加到一个 LinkedMultiValueMap 中。每一个url中记录的文件路径如下：

file:/C:/Users/wangchao/apache-maven-3.5.0/repo/com/baomidou/mybatis-plus-boot-starter/3.5.1/mybatis-plus-boot-starter-3.5.1.jar!/META-INF/spring.factories

按照如下路径查看

// 1. @EnableAutoConfiguration@Target(ElementType.TYPE)@Retention(RetentionPolicy.RUNTIME)@Documented@Inherited@AutoConfigurationPackage@Import(AutoConfigurationImportSelector.class)public @interface EnableAutoConfiguration {}// 2. AutoConfigurationImportSelector.class#selectImports()public class AutoConfigurationImportSelector implements DeferredImportSelector, BeanClassLoaderAware,ResourceLoaderAware, BeanFactoryAware, EnvironmentAware, Ordered {	@Override	public String[] selectImports(AnnotationMetadata annotationMetadata) {if (!isEnabled(annotationMetadata)) {return NO_IMPORTS;}AutoConfigurationEntry autoConfigurationEntry = getAutoConfigurationEntry(annotationMetadata);return StringUtils.toStringArray(autoConfigurationEntry.getConfigurations());	}}// 3. AutoConfigurationImportSelector.class#getAutoConfigurationEntry()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> exclusions = getExclusions(annotationMetadata, attributes);	checkExcludedClasses(configurations, exclusions);	configurations.removeAll(exclusions);	configurations = getConfigurationClassFilter().filter(configurations);	fireAutoConfigurationImportEvents(configurations, exclusions);	return new AutoConfigurationEntry(configurations, exclusions);}// 4. AutoConfigurationImportSelector.class#getCandidateConfigurations()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;}// 5. org.springframework.core.io.support.SpringFactoriesLoader#loadFactoryNames()public static List<String> loadFactoryNames(Class<?> factoryType, @Nullable ClassLoader classLoader) {	String factoryTypeName = factoryType.getName();	return loadSpringFactories(classLoader).getOrDefault(factoryTypeName, Collections.emptyList());}

最终使用 loadSpringFactories(@Nullable ClassLoader classLoader) 方法读取所有配置文件。

// 6. org.springframework.core.io.support.SpringFactoriesLoader#loadSpringFactories()private static Map<String, List<String>> loadSpringFactories(@Nullable ClassLoader classLoader) {MultiValueMap<String, String> result = cache.get(classLoader);if (result != null) {return result;}try {Enumeration<URL> urls = (classLoader != null ?classLoader.getResources(FACTORIES_RESOURCE_LOCATION) :ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));result = new LinkedMultiValueMap<>();while (urls.hasMoreElements()) {// 读取所有jar包下的 /META-INF/spring.factoriesURL url = urls.nextElement();UrlResource resource = new UrlResource(url);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());}}}cache.put(classLoader, result);return result;}catch (IOException ex) {throw new IllegalArgumentException("Unable to load factories from location [" +FACTORIES_RESOURCE_LOCATION + "]", ex);}	}

tomcat的自动配置内置于springboot的autoconfiguration中。参考tomcat的自动配置 https://www.cnblogs.com/zhaokejin/p/15626392.html
mybatis-plus的配置没有被springboot包括。因此mybatis-stater中包含一个包mybatis-spring-boot-autoconfigure，这其中配置了需要自动配置的类。
因此我们也可以在自己的项目下新建 /META-INF/spring.factories，并配置自动配置类。
@Import【SpringBoot 常用注解的原理和使用】@Import 用于导入配置类或需要前置加载的类。被导入的类会注册为Bean，可直接作为Bean被引用。它的 value 属性可以支持三种类型：