code: public Controller keepPara() {
  		Map<String, String[]> map = request.getParameterMap();
  		for (Entry<String, String[]> e: map.entrySet()) {
  			String[] values = e.getValue();
  			if (values.length == 1)
  				request.setAttribute(e.getKey(), values[0]);
  			else
  				request.setAttribute(e.getKey(), values);
  		}
  		return this;
  	}
docs: Keep all parameter's value except model value