重寫BeanUtils，不拷貝空值到對象

/**
	 * 拷貝屬性，但不適用null替換原屬性
	 * @param source
	 * @return
	 */
	public static String[] getNullPropertyNames (Object source) {
		final BeanWrapper src = new BeanWrapperImpl(source);
		java.beans.PropertyDescriptor[] pds = src.getPropertyDescriptors();

		Set<String> emptyNames = new HashSet<String>();
		for(java.beans.PropertyDescriptor pd : pds) {
			Object srcValue = src.getPropertyValue(pd.getName());
			if (srcValue == null) emptyNames.add(pd.getName());
		}
		String[] result = new String[emptyNames.size()];
		return emptyNames.toArray(result);
	}

	public static void copyPropertiesIgnoreNull(Object src, Object target){
		BeanUtils.copyProperties(src, target, getNullPropertyNames(src));
	}