openPrefetchingReadChannel無法在Google雲端存儲客戶端API中工作

我正嘗試使用openPrefetchingReadChannelGCSInputChannel從存儲桶中獲取對象。由於Google developer tutorial says：openPrefetchingReadChannel無法在Google雲端存儲客戶端API中工作

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.

那麼這不是發生在我身上。請在我的片段看看：

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(); 
    } 
    }

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

上面的代碼應該從上傳的對象數據的傳遞1KB但它返回的對象即8.4KB的全部數據。請看截圖：

Check Network tab of Inspect element