code: public String sign(String content, PrivateKey privateKey) {
  
          if (content == null) {
              return null;
          }
  
          byte[] bytes = content.getBytes(StandardCharsets.UTF_8);
          InputStream input = new ByteArrayInputStream(bytes);
          return sign(input, privateKey);
          // ByteArrayInputStream does not need to be closed.
      }
docs: Generates a digital signature for the given string.
  
  @param content    The string to be digitally signed.
  @param privateKey The {@link PrivateKey} with which the string is to be signed. This can be obtained
  via {@link Keys#newKeyPair()}.
  @return The signature as a base64-encoded string. If the content is null, null is returned.