code: static String getExpressValue(final String key) {
      if (isExpression(key)) {
        String keyValue = getSystemProperty(key.substring(2, key.length() - 1));
        return keyValue == null ? key : keyValue;
      } else {
        return key;
      }
    }
docs: Gets the express value by the key.
  
  @param key
  the key where the system property is set.
  @return the expression value or the key itself if the system property is
  not set.