code: public static List<Field> getKeyColumns(Class<?> clazz) 
  	    throws NoKeyColumnAnnotationException {
  		List<Field> fields = getColumns(clazz);
  		List<Field> keyFields = new ArrayList<Field>();
  		for(Field field : fields) {
  			Column column = field.getAnnotation(Column.class);
  			if(column.isKey()) {
  				keyFields.add(field);
  			}
  		}
  		if(keyFields.isEmpty()) {
  			throw new NoKeyColumnAnnotationException();
  		}
  		return keyFields;
  	}
docs: 获得字段里面的key字段
  @param clazz
  @return
  @throws NoKeyColumnAnnotationException 如果没有key Column，抛出该异常。