code: public static List<Field> getColumns(Class<?> clazz)
  			throws NoColumnAnnotationException {
  		if(clazz == null) {
  			throw new NoColumnAnnotationException("class is null");
  		}
  			
  		List<Field> result = _getAnnotationColumns(clazz, Column.class);
  		if (result.isEmpty()) {
  			throw new NoColumnAnnotationException("class " + clazz.getName()
  					+ " does not have any @Column fields");
  		}
  		
  		return result;
  	}
docs: 获得所有有@Column注解的列，包括继承的父类中的，顺序父类先
  
  @param clazz
  @throws NoColumnAnnotationException 当没有一个@Column注解时抛出
  @return 不会返回null