Streaming large files with play framework and third party API

Question

I'm writing a play 2 application and I am struggling with a file streaming problem. I retrieve my files using a third party API with a method having the following signature:

FileMetadata getFile(OutputStream destination, String fileId)

In a traditional Servlet application, if I wanted to send the content to my client I would have done something like:

HttpServletResponse resp;
myService.getFile(resp.getOutpuStream, fileId);

My problem is that in my play 2 Controller class I don't have access to the underlying OuputStream, so the simplest implementation of my controller method would be:

public static downloadFile(String id) {
    ByteArrayOutputStream baos = new BAOS(...);
    myApi.getFile(baos,id); //Load inside temp Array      
    ByteArrayInputStream bais = new BAIS(baos.toByteArray())
    return Ok(bais);
 }

It will work but it requires to load the whole content into memory before serving it so it's not an option (files can be huge).

I was thinking of a solution consisting in:

Defining a ByteArrayOutputStream (baos) inside my controller
Calling the third party API with this baos in parameter
Using the chunk return of the play framework to send the content of the baos as soon as something is written inside by the 3rd party API

Problem is that I don't know if it possible (call to getFile is blocking so it would require multiple threads with a shared OutputStream) nor if it's overkill.

As someone ever faced this kind of problem and found a solution? Could my proposed solution solve my problem?

Any insights will be appreciated.

Thanks

EDIT 1 Based on kheraud suggestion I have managed to have a working, but still not perfect, solution (code below).

Unfortunately if a problem occurs during the call to the getFile method, error is not sent back to the client (because I returned Ok) and the browser waits indefinitely for a file that will never come.

Is there a way to handle this case ?

public static Result downloadFile(String fileId {    
      Thread readerThread = null;
      try {
          PipedOutputStream pos = new PipedOutputStream();
          PipedInputStream pis = new PipedInputStream(pos); 
          //Reading must be done in another thread
          readerThread = new DownloadFileWorker(fileId,pos);
          readerThread.start();

          return ok(pis);
      } catch (Exception ex) {
          ex.printStackTrace();
          return internalServerError(ex.toString());

      }
  }

static class DownloadFileWorker extends Thread{
      String fileId;  
      PipedOutputStream pos;

      public DownloadFileWorker(String fileId, PipedOutputStream pos) {
        super();
        this.fileId = fileId
        this.pos = pos;
    }

    public void run(){
          try {
              myApi.getFile(pos,fileId);
              pos.close();
          } catch (Exception ex) {
              ex.printStackTrace();
          }
      }
}

EDIT 2

I found a way to avoid infinite loading of the page by simply adding a pos.close in the catch() part of the worker thread. Client ends up with a zero KB file but I guess that's better than an infinite waiting.

Streaming large files with play framework and third party API

2 Answers