java - openPrefetchingReadChannel 在 Google 云存储客户端 API 中不起作用

Question

我正在尝试使用 openPrefetchingReadChannel 从存储桶中获取对象GCSInputChannel。正如 Google 开发者教程所说:

GcsInputChannel readChannel = gcsService.openPrefetchingReadChannel(fileName, 0, 1024 * 1024);

Prefetching provides is a major performance advantage for most applications, because it 
allows processing part of the file while more data is being downloaded in the background 
in parallel.The third parameter is the size of the prefetch buffer, set in the example 
to 1 megabyte.

好吧，这不会发生在我身上。请看看我的代码片段:

  GcsInputChannel readChannel = gcsService.openPrefetchingReadChannel(fileName, 0, 1024);
  copy(Channels.newInputStream(readChannel), resp.getOutputStream());

   private void copy(InputStream input, OutputStream output) throws IOException {
    try {
      byte[] buffer = new byte[BUFFER_SIZE];
      int bytesRead = input.read(buffer);
      while (bytesRead != -1) {
        output.write(buffer, 0, bytesRead);
        bytesRead = input.read(buffer);
      }
    } finally {
      input.close();
      output.close();
    }
  }

引用:https://code.google.com/p/appengine-gcs-client/source/browse/trunk/java/example/src/com/google/appengine/demos/GcsExampleServlet.java

上面的代码应该从上传的对象中传递 1KB 的数据，但它返回对象的全部数据，即 8.4KB。请看截图:

我不确定发生了什么。需要你们的帮助

Answer 1

openPrefetchingReadChannel 的第三个参数不是要读取(或限制)的最大大小。是预取的内部缓冲区大小。在您的情况下，您可能想要跟踪多少您阅读并继续写作，直到达到所需的限制