[mybatis-异常]:BindingException: Mapper method 'CartMapper.testSum attempted to return null from a met

异常信息:

org.apache.ibatis.binding.BindingException: Mapper method ‘CartMapper.testSum attempted to return null from a method with a primitive return type (int).

// CartMapper
int testSum(Integer userId);

<select id="testSum" resultType="int">
SELECT sum(quantity)
FROM mall_cart
WHERE user_id = #{userId}
</select>

原因:

试图从具有原始返回类型（int）的方法返回null。
数据库函数sum()在找不到记录时, 会返回null

解决方案

<select id="testSum" resultType="int">
SELECT sum(quantity)
FROM mall_cart
WHERE user_id = #{userId}
</select>