apollo-core/src/main/java/com/ctrip/framework/foundation/internals/ServiceBootstrap.java/ServiceBootstrap/loadFirst
class ServiceBootstrap:  public static <S> S loadFirst(Class<S> clazz) {
    Iterator<S> iterator = loadAll(clazz);
    if (!iterator.hasNext()) {
      throw new IllegalStateException(String.format(
          "No implementation defined in /META-INF/services/%s, please check whether the file exists and has the right implementation class!",
          clazz.getName()));
    }
    return iterator.next();
  }