transaction.start();
try{	
	//先查库存，加锁
	select stock from goods  where goods_id= #{goodsId} for update
	if(stock < amout){
		return "库存不足";
	}
	//查询用户已经买到的商品的数量
	int sumGoodsCount = select sum(goodsCount) from miaosha_order where user_id = #{userId} and goods_id = #{goodsId} ;
	if(sumGoodsCount >= 2){
		return "超过最大数";
	}
	//用户还能购买的数量
	int availbaleCount = Math.min(2-sumGoodsCount,  amount)；
	//修改商品表的库存
	update goods set stock = stock - availbaleCount;
	//生成订单
	insert into miaosha_order（user_id, goods_id, availbaleCount);
	//事务提交
	transaction.commit();
}catch(Exception e){
	//事务回滚
	transaction.rollback();
}
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