code: @SuppressWarnings("unchecked")
      public <H extends CompletionHandler> H createAsyncHandler(Class<H> handlerClass) {
          if (handlerClass == null || handlerClass == CompletionHandler.class) return (H) createAsyncHandler();
          return context.loadAsyncHandlerCreator(handlerClass).create(createAsyncHandler());
      }
docs: 创建CompletionHandler子类的实例 <br>
  
  传入的CompletionHandler子类必须是public，且保证其子类可被继承且completed、failed可被重载且包含空参数的构造函数。
  
  @param <H>          泛型
  @param handlerClass CompletionHandler子类
  
  @return CompletionHandler