Issues while File download

I'm trying to download a file from Angular UI, even after I got exception in backed code still I'm getting 200 ok as the response.

Here is the code I have :

public ResponseEntity<Object> downloadDocument(@PathVariable("docId") Long docId,
			HttpServletResponse response) { 

OutPutStream outputStream = null;

try {
outputStream = response.getOutputStream();
docService.downloadDocument(docId,outputStream);
return ResponseEntity.ok().contentType(MediaType.parseMediaType(MediaType.APPLICATION_OCTET_STREAM_VALUE))
					.body("Success");
} catch(Exception e) {
return ResponseEntity.badRequest().body(e.getMessage());
} finally {
if (Objects.nonNull(outputStream)) {
				IOUtils.closeQuietly(outputStream);
			}
}

Can you please help me out what's wring here.

When you open the outputstream, the headers are sent. period.

There's no backtracking from that point on. You can't first open the outputstream and then later go: Oh, wait! No! nevermind! bad request!

Here's how it works - you pick a side and stick to it. You can either:

1. Handle it yourself; use response and the methods available there to set up your response; you can set headers, the return code and message, and you can obtain an outputstream for the response body, and send data that way. If you do this, you can't ALSO return a ResponseEntity!
2. Do NOT even add an HttpServletResponse parameter, and instead return a ResponseEntity object.

You're doing both, which is not allowed.

I'm frankly surprised; spring is a bit broken and ought to be throwing exceptions here, as it cannot possibly serve up what you're asking it to do here.

NB: Note that the type of an exception is usually more informative than the message (many exceptions don't even have a message).

Putting it all together:

public ResponseEntity<?> downloadDocument(@PathVariable("docId") Long docId) { 

  try {
    ByteArrayOutputStream baos = new ByteArrayOutputStream();
    docService.downloadDocument(docId, baos);
    return ResponseEntity.ok()
      .contentType(MediaType.parseMediaType(MediaType.APPLICATION_OCTET_STREAM_VALUE))
      .body(baos.toByteArray());
  } catch(Exception e) {
    return ResponseEntity.badRequest().body(e.getMessage());
  }
}

'toString' defaults to printing its own type if there is no message, and its own type plus the message if there is, so you get e.g.:

NullPointerException

or

NullPointerException: parameter foo

which is what you want (versus a literal blank string in the former case, and just 'parameter foo' in the latter, which isn't particularly insightful either).

messages are generally intended not to necessarily make sense without the context of the type of the exception.

NB: This will cache the entire content of the downloaded document into the memory of the server before sending it onwards. If the document is large, this is a bad idea, but, if you want to 'stream' it, you have a pretty serious problem inherent in the HTTP protocol: Once you start sending, you've already sent the 'error status' (i.e. you already sent 200 OK), so if the document download process throws an exception halfway through, you cannot go back and send an error message. You need some sort of wire protocol where you send the body in chunks and have a 'type' code you send, so that the recipient can scan for these and knows 'this type means there's more data flowing', and 'this type means an error occured and now I can read the error description'. That all gets quite complicated. By caching it, you avoid this mess. But if that document can be very large you're going to have to deal with the above, there are no quick fixes.