DataSnap - Streaming JPEG Files via TStream From Server To Client (XE2)


I've written a DataSnap server method that returns a TStream object to transfer a file. The client application calls the method and reads the stream to download the file. The server method is very simple :

function TServerMethods.DownloadFile(sFilePath: string): TStream;
  strFileStream: TFileStream;
  strFileStream := TFileStream.Create(sFilePath, fmOpenRead);
  Result := strFileStream;

It works fine downloading many file types (PDF, GIF, BMP, ZIP, EXE) but it doesn't work when downloading JPG files. On the client side the stream object returned from the method call is always 0 in size with JPGs. I can successfully stream JPG files locally on my PC, so it must be something to do with DataSnap. I've done some research which suggests DataSnap converts the stream to JSON behind the scenes and there could be a problem with this when it comes to JPG files - can anybody confirm this? On the cl
ient side I'm using the TDSRESTConnection to call the server method. I realise I could ZIP the JPG files before streaming, but would rather not have to do this.

Any help appreciated.

2/6/2012 5:49:18 PM
embarcadero.datasnap 643 articles. 1 followers. Follow

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Thought I'd update the thread on my attempts to resolve this.  I never found a way to transfer a JPEG file over DataSnap using TStream, but have done it by converting the stream to a TJSONArray and passing this back instead.  So my server method now looks as follows :

function TServerMethods.DownloadJPEGFile(sFilePath: string): TJSONArray;
  strFileStream: TFileStream;
  strFileStream := TFileStream.Create(sFilePath, fmOpenRead);
  Result := TDBXJSONTools.StreamToJSON(strFileStream, 0, strFileStream.Size);

then at the client end I convert back to a TStream with :

strFileStream := TDBXJSONTools.JSONToStream(JSONArray);

I have created this as a new server method call purely for downloading JPEGs, as I've found transferring the files using TJSONArray instead of TStream is as much as 4 times slower, so I use my original method for all other file types.

2/9/2012 4:54:26 PM
As a further update, I've found this problem is related to regional settings on the client PC.  My system locale is set to "English (United Kingdom)" and the error also occurs with "English (United States)".  If I change the locale to "Japan (Japanese)" then the errors disappear.  The problem also affects Microsoft Word (.DOC) files as well as JPEGs.  I've submitted a QC report to Embarcadero :

3/27/2012 8:11:19 AM
This error (which also affects .DOC files) can be fixed by editing one of the DataSnap framework files :

1 .copy '...\RAD Studio\9.0\source\data\datasnap\Datasnap.DSClientRest.pas' to your DataSnap Client project folder
2. add the .pas file to the project
3. modify Line#1288 as below

//  LResponseJSON := TJSONObject.ParseJSONValue(BytesOf(LResponseText.StringValue), 0);
  LResponseJSON := TJSONObject.ParseJSONValue(BytesOf(UTF8String(LResponseText.StringValue)), 0);

4. rebuild DataSnap REST Client project
5. run it with REST Server
4/3/2012 12:33:47 PM

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