code: public static String propToColumn(String propName) {
  		char[] arr = propName.toCharArray();
  		for(int i = 0; i < arr.length; i++) {
  			if(Character.isUpperCase(arr[i])) {
  				String s1 = propName.substring(0, i);
  				String s = "_" + Character.toLowerCase(arr[i]);
  				String s2 = propName.substring(i+1);
  				return propToColumn(s1 + s + s2);
  			}
  		}
  		return propName;
  	}
docs: e.g. userName to user_name
  Transform field name to sql column.<br>
  (TODO merge. Same function method is ClassStrUtils.hump2Underline)
  e.g. userName to user_name
  @param propName propName
  @return prop_name