Java，转账服务和并发问题答案

【问题标题】：Java, Transfer money service and concurrency issueJava，转账服务和并发问题
【发布时间】：2017-07-20 07:00:22
【问题描述】：

我有这样的代码：

@Transactional(rollbackOn = RuntimeException.class)
    synchronized public void transfer(@NonNull final TransferModel transferModel) {
        log.info("Start processing money transfer from user: {} to receiver: {}. Transfer amount: {}", transferModel.getOwnerId(), transferModel
                .getReceiverId(), transferModel.getAmount());

        final AccountEntity ownerAccount = findByOwner(transferModel.getOwnerId());
        final AccountEntity receiverAccount = findByOwner(transferModel.getReceiverId());

        log.info("Owner balancer before transfer: {}. Receiver balance before transfer {}", ownerAccount.getBalance(), receiverAccount.getBalance());

        if (ownerAccount.getBalance().compareTo(transferModel.getAmount()) < 0) {
            throw new IllegalArgumentException("Owner has insufficient amount of money");
        }

        withdrawFromOwnerAccount(ownerAccount, transferModel.getAmount());
        depositToReceiverAccount(receiverAccount, transferModel.getAmount());

        log.info("Transfer money finished successful");
        log.info("Owner balancer after transfer: {}. Receiver balance after transfer {}", ownerAccount.getBalance(), receiverAccount.getBalance());
    }

简单的方法，从所有者那里获取金钱并传递给接收者。这种解决方案很幼稚，因为它一次不能处理多个传输。如何确保此方法可以处理多个请求并在并发环境中安全？

【问题讨论】：

方法不能处理多个请求。方法被调用。在您的情况下，由于方法是同步的，这会按顺序发生。正如我们对您的环境的确切了解零 - 应该如何理解当您并行执行此方法时究竟会发生什么？抱歉，这个问题太笼统了。

标签： java concurrency transactions synchronized

【解决方案1】：

正如 ghostCat 提到的，我们不熟悉您的环境，所以如果任何“看起来很幼稚”的方法有副作用，我们不会知道。

也就是说，假设这是一个非常简单明了的代码，并且方法名称反映了它们正在做什么，我认为我们不必同步所有方法，而只是关键部分：

withdrawFromOwnerAccount(ownerAccount, transferModel.getAmount());
depositToReceiverAccount(receiverAccount, transferModel.getAmount());

我们可以使用账号同步这部分：

synchronized (ownerAccount) {
    withdrawFromOwnerAccount(ownerAccount, transferModel.getAmount());
}
synchronized (receiverAccount) {
    depositToReceiverAccount(receiverAccount, transferModel.getAmount());
}

通过这样做，我们能够并行执行此方法，并且仅当发送方/接收方已经在执行另一个事务时才锁定线程。

注意：要应用此方法，您必须确保同一帐户没有两个对象！

【讨论】：