download.java

// Downloading a static resource, already saved to disk
@GetMapping(value = "/")
public ResponseEntity download() throws IOException {
  var file = new File("/path/to/file");

  var header = new HttpHeaders();
  header.add(HttpHeaders.CONTENT_DISPOSITION, "attachment; filename=" + file.getName()); // you can use any name you want here
  header.add("Cache-Control", "no-cache, no-store, must-revalidate");
  header.add("Pragma", "no-cache");
  header.add("Expires", "0");

  return ResponseEntity.ok()
      .headers(header)
      .contentLength(file.length())
      .contentType(MediaType.parseMediaType("application/octet-stream"))
      .body(new FileSystemResource(file));
}

// Stream download a resource without saving to disk.
// For example read from database and write to csv
@GetMapping("/")
public ResponseEntity downloadFile(HttpServletResponse response) throws IOException {

  response.setHeader("Cache-Control", "no-cache, no-store, must-revalidate");
  response.setHeader("Pragma", "no-cache");
  response.setHeader(HttpHeaders.EXPIRES, "0");
  response.setHeader(HttpHeaders.CONTENT_DISPOSITION, "attachment; filename=" + fileName);
  response.setContentType("application/octet-stream");

  var formatter = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss Z");
  try (var csvAppender = this.csvWriter.append(
      new OutputStreamWriter(new GZIPOutputStream(response.getOutputStream()),
          StandardCharsets.UTF_8))) {

    csvAppender.appendLine(new String[] {}); // header

    // for each row
    while (collection.hasNext()) {
      var d = collection.next();
      csvAppender.appendLine(d.values().stream().map(o ->
          o instanceof Date ? formatter.format((Date) o) : Objects.toString(o, ""))
          .toArray(String[]::new));
    }
  }
  return ResponseEntity.ok().build();
}