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什么是超卖？假如只剩下一个库存，却被多个订单买到了，简单理解就是库存不够了还能正常下单。

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方案1：数据库行级锁

1. 实体类

@Data
@TableName("product")
public class Product {@TableId(type = IdType.AUTO)private Long id;private String name;private Integer stock;@Version // MyBatis-Plus乐观锁注解private Integer version;
}

2. Mapper接口

public interface ProductMapper extends BaseMapper<Product> {/​**​* 使用悲观锁查询商品* @param id 商品ID* @return 商品实体*/@Select("SELECT * FROM product WHERE id = #{id} FOR UPDATE")Product selectByIdWithLock(Long id);/​**​* 自定义乐观锁更新方法* @param product 商品实体* @param oldVersion 旧版本号* @return 更新影响行数*/@Update("UPDATE product SET stock = #{stock}, version = version + 1 " +"WHERE id = #{id} AND version = #{oldVersion}")int updateWithOptimisticLock(Product product, @Param("oldVersion") int oldVersion);
}

3. Service层实现

@Service
@RequiredArgsConstructor
public class ProductService {private final ProductMapper productMapper;/​**​* 使用行级锁扣减库存* @param productId 商品ID* @param quantity 扣减数量* @return 操作结果*/@Transactional(rollbackFor = Exception.class)public String reduceStock(Long productId, Integer quantity) {try {// 1. 加行级锁查询商品Product product = productMapper.selectByIdWithLock(productId);if (product == null) {return "商品不存在";}// 2. 检查库存是否充足if (product.getStock() < quantity) {return "库存不足";}// 3. 扣减库存product.setStock(product.getStock() - quantity);// 4. 更新库存int result = productMapper.updateById(product);if (result <= 0) {throw new RuntimeException("更新库存失败");}return "扣减库存成功";} catch (Exception e) {// 事务回滚throw new RuntimeException("扣减库存异常: " + e.getMessage());}}/​**​* 使用MyBatis-Plus内置乐观锁机制扣减库存* @param productId 商品ID* @param quantity 扣减数量* @return 操作结果*/@Transactional(rollbackFor = Exception.class)public String reduceStockWithOptimisticLock(Long productId, Integer quantity) {// 1. 查询商品(不加锁)Product product = productMapper.selectById(productId);if (product == null) {return "商品不存在";}// 2. 检查库存if (product.getStock() < quantity) {return "库存不足";}// 3. 扣减库存product.setStock(product.getStock() - quantity);// 4. 乐观锁更新int result = productMapper.updateById(product);if (result <= 0) {// 版本号不一致，说明数据已被修改return "操作失败，请重试";}return "扣减库存成功";}/​**​* 手动实现乐观锁扣减库存（带重试机制）* @param productId 商品ID* @param quantity 扣减数量* @return 操作结果*/@Transactional(rollbackFor = Exception.class)public String reduceStockManual(Long productId, int quantity) {// 1. 查询商品Product product = productMapper.selectById(productId);if (product == null) {return "商品不存在";}// 2. 检查库存是否充足if (product.getStock() < quantity) {return "库存不足";}// 3. 扣减库存product.setStock(product.getStock() - quantity);// 4. 手动乐观锁更新int result = productMapper.updateWithOptimisticLock(product, product.getVersion());if (result > 0) {return "扣减库存成功";}return "操作失败，请重试";}
}

方案2：分布式锁

1. 实体类

@Data
@TableName("product")
public class Product {@TableId(type = IdType.AUTO)private Long id;private String name;private Integer stock;
}

2. Mapper接口

public interface ProductMapper extends BaseMapper<Product> {// 使用MyBatis-Plus自带方法/​**​* 自定义扣减库存方法* @param productId 商品ID* @param quantity 扣减数量* @return 影响行数*/@Update("UPDATE product SET stock = stock - #{quantity} WHERE id = #{productId} AND stock >= #{quantity}")int reduceStock(@Param("productId") Long productId, @Param("quantity") int quantity);
}

3. Service层实现

@Service
@RequiredArgsConstructor
public class ProductService {private final ProductMapper productMapper;private final RedissonClient redissonClient;/​**​* Redisson分布式锁*/public String reduceStockWithLock(Long productId, int quantity) {// 1. 创建分布式锁keyString lockKey = "product:lock:" + productId;RLock lock = redissonClient.getLock(lockKey);try {// 2. 尝试获取锁（等待5秒，锁自动释放时间30秒）boolean locked = lock.tryLock(5, 30, TimeUnit.SECONDS);if (!locked) {return "系统繁忙，请稍后再试";}try {// 3. 查询商品Product product = productMapper.selectById(productId);if (product == null) {return "商品不存在";}// 4. 检查库存是否充足if (product.getStock() < quantity) {return "库存不足";}// 5. 扣减库存int result = productMapper.reduceStock(productId, quantity);if (result <= 0) {return "扣减失败，请重试";}return "扣减成功";} finally {// 6. 释放锁// lock.isLocked()检查锁是否仍然被持有（未被释放）// lock.isHeldByCurrentThread()检查当前线程是否持有该锁if (lock.isLocked() && lock.isHeldByCurrentThread()) {lock.unlock();}}} catch (InterruptedException e) {Thread.currentThread().interrupt();return "系统异常，请重试";}}
}