1.@EnableAutoConfiguration

1.1作用：通过扫描Classpath下的spring.fatories配置文件，将org.springframework.boot.autoconfig.EnableAutoConfiguration对应的配置项实例化且注入到spring容器；

1.2实现原理

进入@ENableAutoCOnfiguration注解，主要由两个注解组成：

@AutoConfigurationPackage
@Import(AutoConfigurationImportSelector.class)

springboot之所以能自动注入应用程序所需要的bean，靠的就是@import注解，继续向下走，进入

AutoConfigurationImportSelector类。

2.AutoConfigurationImportSelector

getAutoConfigurationEntry()方法 

此时debug发现，

configurations集合已经有很多的类的全限定名了，注意返回的是list集合；接着继续跟踪

getCandidateConfigurations()；

getCandidateConfigurations()方法

 继续跟踪loadFactoryNames()

 这个时候我发现calssloader类加载器对象实例作为形参传入了loadSpringFactories()

loadSpringFactories()

	private static Map<String, List<String>> loadSpringFactories(ClassLoader classLoader) {
		Map<String, List<String>> result = cache.get(classLoader);
		if (result != null) {
			return result;
		}

		result = new HashMap<>();
		try {
        //获取spring.properites配置文件里类的全限定名
		Enumeration<URL> urls = classLoader.getResources(FACTORIES_RESOURCE_LOCATION);
        
			while (urls.hasMoreElements()) {
				URL url = urls.nextElement();
				UrlResource resource = new UrlResource(url);
        //PropertiesLoaderUtils，这个工具类主要是针对Properties文件的加载操作，在Spring    //对.properties文件和.factories文件的操作都有使用到。loadProperties:从一个资源文件加载    //Properties；
				Properties properties = PropertiesLoaderUtils.loadProperties(resource);
				for (Map.Entry<?, ?> entry : properties.entrySet()) {
        //获取到properties中的key值
					String factoryTypeName = ((String) entry.getKey()).trim();
        //获取到value值
					String[] factoryImplementationNames =
							StringUtils.commaDelimitedListToStringArray((String) entry.getValue());
        //遍历所有的value。
					for (String factoryImplementationName : factoryImplementationNames) {
						result.computeIfAbsent(factoryTypeName, key -> new ArrayList<>())
								.add(factoryImplementationName.trim());
					}
				}
			}

			// Replace all lists with unmodifiable lists containing unique elements
			result.replaceAll((factoryType, implementations) -> implementations.stream().distinct()
					.collect(Collectors.collectingAndThen(Collectors.toList(), Collections::unmodifiableList)));
			cache.put(classLoader, result);
		}
		catch (IOException ex) {
			throw new IllegalArgumentException("Unable to load factories from location [" +
					FACTORIES_RESOURCE_LOCATION + "]", ex);
		}
		return result;
	}

/*

https://www.jianshu.com/p/059d6e908807
hashMap.computeIfAbsent("china", key -> getValues(key)).add("liSi");

如果原来有china 的key值，就不会放入新值。

如果原来没有china key，则会放入新的key：value china=xxx;
*/

返回：最后，1.返回HashMap类型的result至loadFactoryNames（）方法

public static List<String> loadFactoryNames(Class<?> factoryType, @Nullable ClassLoader classLoader) {
		ClassLoader classLoaderToUse = classLoader;
		if (classLoaderToUse == null) {
			classLoaderToUse = SpringFactoriesLoader.class.getClassLoader();
		}
		String factoryTypeName = factoryType.getName();
		return loadSpringFactories(classLoaderToUse).getOrDefault(factoryTypeName, Collections.emptyList());
	}

return loadSpringFactories(classLoaderToUse).getOrDefault(factoryTypeName, Collections.emptyList());

getOrDefault() 方法获取指定 key 对应对 value，如果找不到 key ，则返回设置的默认值。getOrDefault() 方法的语法为：hashmap.getOrDefault(Object key, V defaultValue)

2.

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;
	}

读取到的confurgations的值

 3.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.selectImports()

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

结束，整个流程就是这样，写死的代码，所以没有道理可言，只能去理解并且看懂！