@Target(ElementType.TYPE)
    @Retention(RetentionPolicy.RUNTIME)
    @Documented
    @Inherited
    @SpringBootConfiguration
    @EnableAutoConfiguration
    @ComponentScan(excludeFilters = { @Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),
            @Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })
    public @interface SpringBootApplication {}

    @Target(ElementType.TYPE)
    @Retention(RetentionPolicy.RUNTIME)
    @Documented
    @Inherited
    @AutoConfigurationPackage
    @Import(AutoConfigurationImportSelector.class)
    public @interface EnableAutoConfiguration {}

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

        protected AutoConfigurationEntry getAutoConfigurationEntry(AnnotationMetadata annotationMetadata) {
            if (!isEnabled(annotationMetadata)) {
                return EMPTY_ENTRY;
            }
            // 獲取注解信息
            AnnotationAttributes attributes = getAttributes(annotationMetadata);
            // 獲取自動配置的信息
            List<String> configurations = getCandidateConfigurations(annotationMetadata, attributes);
            // 去重
            configurations = removeDuplicates(configurations);
            // 獲取需要去除的信息
            Set<String> exclusions = getExclusions(annotationMetadata, attributes);
            // 檢查
            checkExcludedClasses(configurations, exclusions);
            // 去除需要被去除的配置
            configurations.removeAll(exclusions);
            configurations = getConfigurationClassFilter().filter(configurations);
            fireAutoConfigurationImportEvents(configurations, exclusions);
            return new AutoConfigurationEntry(configurations, exclusions);
        }

        protected List<String> getCandidateConfigurations(AnnotationMetadata metadata, AnnotationAttributes attributes) {
            // 重要
            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;
        }

        protected Class<?> getSpringFactoriesLoaderFactoryClass() {
              return EnableAutoConfiguration.class;
        }

        protected ClassLoader getBeanClassLoader() {
              return this.beanClassLoader;
        }

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

   private static Map<String, List<String>> loadSpringFactories(ClassLoader classLoader) {
        // 查詢緩存
      Map<String, List<String>> result = cache.get(classLoader);
      if (result != null) {
         return result;
      }
      result = new HashMap<>();
      try {
            // 1. 獲取類加載器能讀取的所有在META-INF目錄下的spring.factories文件
         Enumeration<URL> urls = classLoader.getResources(FACTORIES_RESOURCE_LOCATION);
         while (urls.hasMoreElements()) {
            // 遍歷路徑
            URL url = urls.nextElement();
            UrlResource resource = new UrlResource(url);
                // 將路徑下的文件數(shù)據(jù)讀取為Properties
            Properties properties = PropertiesLoaderUtils.loadProperties(resource);
            // 遍歷Properties
            for (Map.Entry<?, ?> entry : properties.entrySet()) {
               String factoryTypeName = ((String) entry.getKey()).trim();
               String[] factoryImplementationNames =
                     StringUtils.commaDelimitedListToStringArray((String) entry.getValue());
               for (String factoryImplementationName : factoryImplementationNames) {
                  result.computeIfAbsent(factoryTypeName, key -> new ArrayList<>())
                        .add(factoryImplementationName.trim());
               }
            }
         }
         // Replace all lists with unmodifiable lists containing unique elements
            // 用不可修改列表替換
         result.replaceAll((factoryType, implementations) -> implementations.stream().distinct()
               .collect(Collectors.collectingAndThen(Collectors.toList(), Collections::unmodifiableList)));
            // 加入緩存
         cache.put(classLoader, result);
      }
      catch (IOException ex) {
         throw new IllegalArgumentException("Unable to load factories from location [" +
               FACTORIES_RESOURCE_LOCATION + "]", ex);
      }
      return result;
   }