/** writes safely (to temp file, then rename) */
  public static void saveBinaryFile(String fileName, byte[] contents) throws IOException {
    File file = new File(fileName);
    File parentFile = file.getParentFile();
    if (parentFile != null)
      parentFile.mkdirs();
    String tempFileName = fileName + "_temp";
    FileOutputStream fileOutputStream = new FileOutputStream(tempFileName);
    fileOutputStream.write(contents);
    fileOutputStream.close();
    if (file.exists() && !file.delete())
      throw new IOException("Can't delete " + fileName);

    if (!new File(tempFileName).renameTo(file))
      throw new IOException("Can't rename " + tempFileName + " to " + fileName);
  }