帮我写一个用mybatis 完成批量扣减商品数量 的代码 包括mapper 和 mapper.xml

当你想用MyBatis完成批量扣减商品数量的操作，你需要创建一个Mapper接口、一个对应的XML映射文件以及一个Service层来调用这个Mapper。下面是一个简单的示例：

首先，创建Mapper接口（ItemMapper.java）:

import org.apache.ibatis.annotations.Delete;
import org.apache.ibatis.annotations.Insert;
import org.apache.ibatis.annotations.Options;
import org.apache.ibatis.annotations.Param;

public interface ItemMapper {
    @Insert({
            "BEGIN",
            "UPDATE item SET stock = stock - #{quantity} WHERE id IN ",
            "(SELECT item_id FROM order_items WHERE order_id = #{orderId})",
            "AND stock > 0",
            "COMMIT"
    })
    @Options(useGeneratedKeys = true, keyProperty = "itemId")
    int deductStock(@Param("orderId") Long orderId, @Param("quantity") Integer quantity);

    @Delete("DELETE FROM item WHERE id = #{itemId}")
    void deleteItemById(Long itemId);
}

接着，创建Mapper XML配置文件（itemMapper.xml）:

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.yourpackage.ItemMapper">

    <insert id="deductStock" parameterType="map">
        <!-- SQL 更新语句 -->
        <selectKey resultType="long" order="BEFORE" keyProperty="itemId">
            SELECT LAST_INSERT_ID() AS itemId
        </selectKey>
        ${script}
    </insert>

    <delete id="deleteItemById" parameterType="long">
        DELETE FROM item WHERE id = #{itemId}
    </delete>
</mapper>

注意：这里的`${script}`标签需要替换为之前在Java接口中的那行SQL，即`BEGIN...COMMIT`部分。

最后，在Service层或其他业务处理地方调用Mapper方法：

@Service
public class ProductService {
    private final ItemMapper itemMapper;

    public ProductService(ItemMapper itemMapper) {
        this.itemMapper = itemMapper;
    }

    public void deductStockForOrder(Long orderId, Integer quantity) {
        int affectedRows = itemMapper.deductStock(orderId, quantity);
        // 验证影响行数是否为预期，如果有事务管理，也可以在此检查事务状态
        if (affectedRows <= 0) {
            // 处理异常逻辑
        }
    }
}