code: public static byte[] convertToXmlByteArray(SelectObjectContentRequest selectRequest) {
          XmlWriter xml = new XmlWriter();
          xml.start("SelectObjectContentRequest");
  
          addIfNotNull(xml, "Expression", selectRequest.getExpression());
          addIfNotNull(xml, "ExpressionType", selectRequest.getExpressionType());
          addRequestProgressIfNotNull(xml, selectRequest.getRequestProgress());
          addInputSerializationIfNotNull(xml, selectRequest.getInputSerialization());
          addOutputSerializationIfNotNull(xml, selectRequest.getOutputSerialization());
  
          xml.end();
          return xml.getBytes();
      }
docs: Converts the SelectObjectContentRequest to an XML fragment that can be sent to
  the SelectObjectContent operation of Amazon S3.