/**

 * Class that can be used to bootstrap and launch a Spring application from a Java main

 * method. By default class will perform the following steps to bootstrap your

 * application:

 *

 * <ul>

 * <li>Create an appropriate {@link ApplicationContext} instance (depending on your

 * classpath)</li>

 * <li>Register a {@link CommandLinePropertySource} to expose command line arguments as

 * Spring properties</li>

 * <li>Refresh the application context, loading all singleton beans</li>

 * <li>Trigger any {@link CommandLineRunner} beans</li>

 * </ul>

 *

 * In most circumstances the static {@link #run(Class, String[])} method can be called

 * directly from your {@literal main} method to bootstrap your application:

 *

 * <pre class="code">

 * @Configuration

 * @EnableAutoConfiguration

 * public class MyApplication  {

 *

 *   // ... Bean definitions

 *

 *   public static void main(String[] args) throws Exception {

 *     SpringApplication.run(MyApplication.class, args);

 *   }

 * }

 * </pre>

 *

 * <p>

 * For more advanced configuration a {@link SpringApplication} instance can be created and

 * customized before being run:

 *

 * <pre class="code">

 * public static void main(String[] args) throws Exception {

 *   SpringApplication application = new SpringApplication(MyApplication.class);

 *   // ... customize application settings here

 *   application.run(args)

 * }

 * </pre>

 *

 * {@link SpringApplication}s can read beans from a variety of different sources. It is

 * generally recommended that a single {@code @Configuration} class is used to bootstrap

 * your application, however, you may also set {@link #getSources() sources} from:

 * <ul>

 * <li>The fully qualified class name to be loaded by

 * {@link AnnotatedBeanDefinitionReader}</li>

 * <li>The location of an XML resource to be loaded by {@link XmlBeanDefinitionReader}, or

 * a groovy script to be loaded by {@link GroovyBeanDefinitionReader}</li>

 * <li>The name of a package to be scanned by {@link ClassPathBeanDefinitionScanner}</li>

 * </ul>

 *

 * Configuration properties are also bound to the {@link SpringApplication}. This makes it

 * possible to set {@link SpringApplication} properties dynamically, like additional

 * sources ("spring.main.sources" - a CSV list) the flag to indicate a web environment

 * ("spring.main.web-application-type=none") or the flag to switch off the banner

 * ("spring.main.banner-mode=off").

 */

private <T> Collection<T> getSpringFactoriesInstances(Class<T> type,

        Class<?>[] parameterTypes, Object... args) {

    ClassLoader classLoader = Thread.currentThread().getContextClassLoader();

    // Use names and ensure unique to protect against duplicates

    Set<String> names = new LinkedHashSet<>(

            SpringFactoriesLoader.loadFactoryNames(type, classLoader));            // 获取指定类型的工厂名字

    List<T> instances = createSpringFactoriesInstances(type, parameterTypes,    // 根据名字、类型创建工厂实例

            classLoader, args, names);

    AnnotationAwareOrderComparator.sort(instances);

    return instances;

}

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));    // classLoader.getResources(FACTORIES_RESOURCE_LOCATION)获取类路径下全部的META-INF/spring.factories的URL

        result = new LinkedMultiValueMap<>();

        while (urls.hasMoreElements()) {                                        // 遍历全部的URL，逐个读取META-INF/spring.factories中的属性

            URL url = urls.nextElement();

            UrlResource resource = new UrlResource(url);

            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);        // 属性全部放入MultiValueMap<String, String> result中，注意result的类型

            }

        }

        cache.put(classLoader, result);                                            // 结果放入缓存，方便下次查找

        return result;

    }

    catch (IOException ex) {

        throw new IllegalArgumentException("Unable to load factories from location [" +

                FACTORIES_RESOURCE_LOCATION + "]", ex);

    }

}