[java] How to download a file using a Java REST service and a data stream

I have 3 machines:

1. server where the file is located
   
2. server where REST service is running ( Jersey)
   
3. client(browser) with access to 2nd server but no access to 1st server

How can I directly (without saving the file on 2nd server) download the file from 1st server to client's machine?
From 2nd server I can get a ByteArrayOutputStream to get the file from 1st server, can I pass this stream further to the client using the REST service?

It will work this way?

So basically what I want to achieve is to allow the client to download a file from 1st server using the REST service on 2nd server (since there is no direct access from client to 1st server) using only data streams (so no data touching the file system of 2nd server).

What I try now with EasyStream library:

final FTDClient client = FTDClient.getInstance();

try {
    final InputStreamFromOutputStream <String> isOs = new InputStreamFromOutputStream <String>() {
        @Override
        public String produce(final OutputStream dataSink) throws Exception {
            return client.downloadFile2(location, Integer.valueOf(spaceId), URLDecoder.decode(filePath, "UTF-8"), dataSink);
        }
    };
    try {
        String fileName = filePath.substring(filePath.lastIndexOf("/") + 1);

        StreamingOutput output = new StreamingOutput() {
            @Override
            public void write(OutputStream outputStream) throws IOException, WebApplicationException {
                int length;
                byte[] buffer = new byte[1024];
                while ((length = isOs.read(buffer)) != -1) {
                    outputStream.write(buffer, 0, length);
                }
                outputStream.flush();
            }
        };
        return Response.ok(output, MediaType.APPLICATION_OCTET_STREAM)
            .header("Content-Disposition", "attachment; filename=\"" + fileName + "\"")
            .build();
    }
}

UPDATE2

So my code now with the custom MessageBodyWriter looks simple:

ByteArrayOutputStream baos = new ByteArrayOutputStream(2048) ;
client.downloadFile(location, spaceId, filePath, baos);
return Response.ok(baos).build();

But I get the same heap error when trying with large files.

UPDATE3 Finally managed to get it working ! StreamingOutput did the trick.

Thank you @peeskillet ! Many thanks !

Refer this:

@RequestMapping(value="download", method=RequestMethod.GET)
public void getDownload(HttpServletResponse response) {

// Get your file stream from wherever.
InputStream myStream = someClass.returnFile();

// Set the content type and attachment header.
response.addHeader("Content-disposition", "attachment;filename=myfilename.txt");
response.setContentType("txt/plain");

// Copy the stream to the response's output stream.
IOUtils.copy(myStream, response.getOutputStream());
response.flushBuffer();
}

Details at: https://twilblog.github.io/java/spring/rest/file/stream/2015/08/14/return-a-file-stream-from-spring-rest.html

See example here: Input and Output binary streams using JERSEY?

Pseudo code would be something like this (there are a few other similar options in above mentioned post):

@Path("file/")
@GET
@Produces({"application/pdf"})
public StreamingOutput getFileContent() throws Exception {
     public void write(OutputStream output) throws IOException, WebApplicationException {
        try {
          //
          // 1. Get Stream to file from first server
          //
          while(<read stream from first server>) {
              output.write(<bytes read from first server>)
          }
        } catch (Exception e) {
            throw new WebApplicationException(e);
        } finally {
              // close input stream
        }
    }
}