redis乐观锁与悲观锁的实战

WATCH counter
current_count = GET counter
new_count = current_count + 1
MULTI
SET counter new_count
EXEC

GET counter

WATCH counter
current_count = GET counter
new_count = current_count + 1
MULTI
SET counter new_count
EXEC

@Service
public class OrderService {
  private final RedisTemplate<String, Integer> redisTemplate;

  @Autowired
  public OrderService(RedisTemplate<String, Integer> redisTemplate) {
    this.redisTemplate = redisTemplate;
  }

  public void placeOrder(String sku, int quantity) {
    String stockKey = "stock:" + sku;
    while (true) {
      // 监视商品库存键，以便在事务开始前检测是否有其他客户端修改了库存
      redisTemplate.watch(stockKey);
      // 获取当前库存数量
      int currentStock = redisTemplate.opsForHash().get(stockKey, sku);
      // 检查库存是否足够
      if (currentStock < quantity) {
        // 库存不足，放弃事务并抛出异常
        redisTemplate.unwatch();
        throw new RuntimeException("Out of stock");
      }
      // 计算新的库存数量
      int newStock = currentStock - quantity;
      // 开始事务
      redisTemplate.multi();
      // 更新库存数量
      redisTemplate.opsForHash().put(stockKey, sku, newStock);
      // 提交事务
      List<Object> results = redisTemplate.exec();
      // 如果事务执行成功，则退出循环
      if (results != null) {
        break;
      }
      // 如果事务执行失败，则重试
    }
  }
}

@Service
public class OrderService {
  private final RedisTemplate<String, String> redisTemplate;

  @Autowired
  public OrderService(RedisTemplate<String, String> redisTemplate) {
    this.redisTemplate = redisTemplate;
  }

  public void placeOrder(String sku, int quantity) {
    String lockKey = "lock:" + sku;
    // 尝试获取锁，如果锁已经存在，说明有其他线程正在执行相关操作
    Boolean locked = redisTemplate.opsForValue().setIfAbsent(lockKey, "locked");
    if (!locked) {
      // 获取锁失败，抛出异常
      throw new RuntimeException("Unable to acquire lock");
    }
    // 设置锁的过期时间，防止锁被一直占用
    redisTemplate.expire(lockKey, 10, TimeUnit.SECONDS);
    try {
      // 执行订单创建、扣减库存等操作
    } finally {
      // 释放锁
      redisTemplate.delete(lockKey);
    }
  }
}