code: public static Flux<ByteBuf> byteBufStreamFromFile(AsynchronousFileChannel fileChannel, int chunkSize, long offset, long length) {
          return new ByteBufStreamFromFile(fileChannel, chunkSize, offset, length);
      }
docs: Creates a {@link Flux} from an {@link AsynchronousFileChannel}
  which reads part of a file into chunks of the given size.
  
  @param fileChannel The file channel.
  @param chunkSize the size of file chunks to read.
  @param offset The offset in the file to begin reading.
  @param length The number of bytes to read from the file.
  @return the Flowable.