docs: private void checkId(FieldDescriptorDef fieldDef, String checkLevel) throws ConstraintException
      {
          if (CHECKLEVEL_NONE.equals(checkLevel))
          {
              return;
          }
  
          String id = fieldDef.getProperty(PropertyHelper.OJB_PROPERTY_ID);
  
          if ((id != null) && (id.length() > 0))
          {
              try
              {
                  Integer.parseInt(id);
              }
              catch (NumberFormatException ex)
              {
                  throw new ConstraintException("The id attribute of field "+fieldDef.getName()+" in class "+fieldDef.getOwner().getName()+" is not a valid number");
              }
          }
      }