How do I return a byte array as response using spring boot and ajax?

Question

I am trying to create a page that will download a file from a data stream. I am able to get the byte stream and I want to return this to client side to save it as file.

I tried the code below however, the file that is being saved is corrupted. The download process also is too quick that I think it is not downloading properly..

Java code:

@RequestMapping(value = /download, method=RequestMethod.POST)
public ResponseEntity<ByteArrayResource> download() {
        byte b[] = <retrieving byte data array here>;
        ByteArrayResource resource = new ByteArrayResource(b);
        
        ResponseEntity<ByteArrayResource> r = ResponseEntity.ok()
                .header("Content-type", "application/octet-stream")
                .header("Content-disposition", "attachment; filename=\"test.mp4\"")
                .contentLength(resource.contentLength())
                .body(resource);

    return r;
} catch (IOException e) {
    // TODO Auto-generated catch block
    e.printStackTrace();
}
    return null;

}

Javascript code:

$('#download').click(function() {
$.ajax({
  type: "POST",
  url: "/download",
  data: {}
}).done(function(data) {
  var a = document.createElement("a");
  document.body.appendChild(a);
  a.style = "display: none";

  var blob = new Blob([data], {type: "octet/stream"});
  var url = window.URL.createObjectURL(blob);
  a.href = url;
  a.download = "test.mp4";
  a.click();
}
}).fail(function(data) {
});
});

Hope you guys could help me.