Spring FactoriesLoader机制实例详解

public static List<String> loadFactoryNames(Class<?> factoryType, @Nullable ClassLoader classLoader) {
  // 获取包含包名的工厂类名称
  String factoryTypeName = factoryType.getName();
  // 获取所有配置在 META-INF/spring.factories 文件的值
  // 然后获取指定类的实现类名列表
  return loadSpringFactories(classLoader).getOrDefault(factoryTypeName, Collections.emptyList());
}

// 默认的工厂配置路径地址，可以存放在多个 JAR 包下
public static final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories";

private static Map<String, List<String>> loadSpringFactories(@Nullable ClassLoader classLoader) {
  // 判断是否有缓存结果，如果有直接返回
  MultiValueMap<String, String> result = cache.get(classLoader);
  if (result != null) {
    return result;
  }

  try {
    // 扫描 classpath 上所有 JAR 中的文件 META-INF/spring.factories
    Enumeration<URL> urls = (classLoader != null ?
        classLoader.getResources(FACTORIES_RESOURCE_LOCATION) :
        ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));
    result = new LinkedMultiValueMap<>();
    while (urls.hasMoreElements()) {
      // 找到的每个 META-INF/spring.factories 文件都是一个 Properties 文件，将其内容加载到一个 Properties 对象然后处理其中的每个属性
      URL url = urls.nextElement();
      UrlResource resource = new UrlResource(url);
      Properties properties = PropertiesLoaderUtils.loadProperties(resource);
      for (Map.Entry<?, ?> entry : properties.entrySet()) {
        // 获取工厂类名称（接口或者抽象类的全限定名）
        String factoryTypeName = ((String) entry.getKey()).trim();
        // 将逗号分割的属性值逐个取出，然后放到 结果result 中去
        for (String factoryImplementationName : StringUtils.commaDelimitedListToStringArray((String) entry.getValue())) {
          result.add(factoryTypeName, factoryImplementationName.trim());
        }
      }
    }
    // 将结果存放到缓存中
    cache.put(classLoader, result);
    return result;
  }
  catch (IOException ex) {
    throw new IllegalArgumentException("Unable to load factories from location [" +
        FACTORIES_RESOURCE_LOCATION + "]", ex);
  }
}

default V getOrDefault(Object key, V defaultValue) {
  V v;
  return (((v = get(key)) != null) || containsKey(key))
    ? v
    : defaultValue;
}

public static <T> List<T> loadFactories(Class<T> factoryType, @Nullable ClassLoader classLoader) {
  Assert.notNull(factoryType, "'factoryType' must not be null");
  // 如果未指定类加载器，则使用默认的
  ClassLoader classLoaderToUse = classLoader;
  if (classLoaderToUse == null) {
    classLoaderToUse = SpringFactoriesLoader.class.getClassLoader();
  }
  // 获取指定工厂名称列表
  List<String> factoryImplementationNames = loadFactoryNames(factoryType, classLoaderToUse);
  // 如果记录器Trace跟踪激活的话,将工厂名称列表输出
  if (logger.isTraceEnabled()) {
    logger.trace("Loaded [" + factoryType.getName() + "] names: " + factoryImplementationNames);
  }
  // 创建结果集
  List<T> result = new ArrayList<>(factoryImplementationNames.size());
  for (String factoryImplementationName : factoryImplementationNames) {
    // 实例化工厂类,并添加到结果集中
    result.add(instantiateFactory(factoryImplementationName, factoryType, classLoaderToUse));
  }
  // 对结果集列表进行排序
  AnnotationAwareOrderComparator.sort(result);
  return result;
}

private static <T> T instantiateFactory(String factoryImplementationName, Class<T> factoryType, ClassLoader classLoader) {
  try {
    Class<?> factoryImplementationClass = ClassUtils.forName(factoryImplementationName, classLoader);
    if (!factoryType.isAssignableFrom(factoryImplementationClass)) {
      throw new IllegalArgumentException(
          "Class [" + factoryImplementationName + "] is not assignable to factory type [" + factoryType.getName() + "]");
    }
    return (T) ReflectionUtils.accessibleConstructor(factoryImplementationClass).newInstance();
  }
  catch (Throwable ex) {
    throw new IllegalArgumentException(
      "Unable to instantiate factory class [" + factoryImplementationName + "] for factory type [" + factoryType.getName() + "]",
      ex);
  }
}