code: protected char handleInvalidChar(int c)
          throws IOException
      {
          // First, let's flush any output we may have, to make debugging easier
          flush();
          InvalidCharHandler h = mConfig.getInvalidCharHandler();
          if (h == null) {
              h = InvalidCharHandler.FailingHandler.getInstance();
          }
          return h.convertInvalidChar(c);
      }
docs: Method called to handle invalid character in textual content requested
  to be output. Content may be part of textual events (CHARACTER, CDATA),
  attribute value, COMMENT content or PROCESSING_INSTRUCTION data.
  The default behavior is to just throw an exception, but this can
  be configured via property {@link WstxOutputProperties#P_OUTPUT_INVALID_CHAR_HANDLER}.