code: public String toQueryString() {
          String encoding = request.getCharacterEncoding();
          try {
              return WebUtils.toQueryString(this, encoding);
          }
          catch (UnsupportedEncodingException e) {
              throw new ControllerExecutionException("Unable to convert parameter map [" + this +
                   "] to a query string: " + e.getMessage(), e);
          }
      }
docs: Converts this parameter map into a query String. Note that this will flatten nested keys separating them with the
  . character and URL encode the result
  
  @return A query String starting with the ? character