code: private void checkImageValidation() throws IOException {
      try {
        imageValidator.join();
      } catch (InterruptedException ie) {
        throw (IOException) new InterruptedIOException().initCause(ie);
      } 
      if (!imageValidator.succeeded) {
        throw new IOException("Image file validation failed",
            imageValidator.error);
      }
    }
docs: Checks the status of image validation during checkpoint.
  @throws IOException