SpringFactoriesLoader是SpringBoot中专门加载META-INF/spring.factories文件内容的SPI加载工具
    spring.factories文件的内容为KV形式，如：

    1. org.springframework.context.ApplicationContextInitializer=\
    2. org.springframework.boot.autoconfigure.SharedMetadataReaderFactoryContextInitializer,\
    3. org.springframework.boot.autoconfigure.logging.ConditionEvaluationReportLoggingListener

    可以配置某个接口下需要加载哪些实现，多个用逗号隔开。

    SpringFactoriesLoader源码解析

    public abstract class SpringFactoriesLoader {
    // 加载路径及文件
    public static final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories";
    // 缓存
    private static final Map<ClassLoader, MultiValueMap<String, String>> cache = new ConcurrentReferenceHashMap<>();


    public static <T> List<T> loadFactories(Class<T> factoryClass, @Nullable ClassLoader classLoader) {
        Assert.notNull(factoryClass, "'factoryClass' must not be null");
        // 确定加载器
        ClassLoader classLoaderToUse = classLoader;
        if (classLoaderToUse == null) {
            classLoaderToUse = SpringFactoriesLoader.class.getClassLoader();
        }
        // 加载解析得到文件内容
        List<String> factoryNames = loadFactoryNames(factoryClass, classLoaderToUse);
        if (logger.isTraceEnabled()) {
            logger.trace("Loaded [" + factoryClass.getName() + "] names: " + factoryNames);
        }
        // 对加载的类进行实例化
        List<T> result = new ArrayList<>(factoryNames.size());
        for (String factoryName : factoryNames) {
            // 反射实例化
            result.add(instantiateFactory(factoryName, factoryClass, classLoaderToUse));
        }
        // 排序
        AnnotationAwareOrderComparator.sort(result);
        return result;
    }

    // 根据类名加载
    public static List<String> loadFactoryNames(Class<?> factoryClass, @Nullable ClassLoader classLoader) {
        String factoryClassName = factoryClass.getName();
        // 具体的加载过程
        return loadSpringFactories(classLoader).getOrDefault(factoryClassName, Collections.emptyList());
    }

    // 执行加载
    private static Map<String, List<String>> loadSpringFactories(@Nullable ClassLoader classLoader) {
        // 检查缓存
        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));
            // 遍历加载的资源
            result = new LinkedMultiValueMap<>();
            while (urls.hasMoreElements()) {
                URL url = urls.nextElement();
                UrlResource resource = new UrlResource(url);
                // 解析为Properties
                Properties properties = PropertiesLoaderUtils.loadProperties(resource);
                for (Map.Entry<?, ?> entry : properties.entrySet()) {
                    //
                    List<String> factoryClassNames = Arrays.asList(
                            StringUtils.commaDelimitedListToStringArray((String) entry.getValue()));
                    result.addAll((String) entry.getKey(), factoryClassNames);
                }
            }
            // 放入缓存
            cache.put(classLoader, result);
            return result;
        } catch (IOException ex) {
            throw new IllegalArgumentException("Unable to load factories from location [" + FACTORIES_RESOURCE_LOCATION + "]", ex);
        }
    }
// 省略部分代码....
}