解决方案：检查xml中对应的sql语句，标签不是select而是update
错误写法：
<select id="lockSkuStock" resultType="java.lang.Long">
update wms_ware_sku
set stock_locked = stock_locked + #{num}
where sku_id = #{skuId}
and ware_id = #{wareId}
and stock - stock_locked > 0
</select>
控制台输出：null
修改为：
<update id="lockSkuStock">
update wms_ware_sku
set stock_locked = stock_locked + #{num}
where sku_id = #{skuId}
and ware_id = #{wareId}
and stock - stock_locked > 0
</update>