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-- 事务 A

BEGIN;

UPDATE account SET balance = balance - 100 WHERE id = 1;

-- 此处暂停，等待事务 B 执行

UPDATE account SET balance = balance + 100 WHERE id = 2;

COMMIT;

-- 事务 B（与 A 并发执行）

BEGIN;

UPDATE account SET balance = balance - 100 WHERE id = 2;

UPDATE account SET balance = balance + 100 WHERE id = 1;

COMMIT;

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public void transfer(long fromId, long toId, BigDecimal amount) {

    long first = Math.min(fromId, toId);

    long second = Math.max(fromId, toId);

    // 固定顺序加锁，避免交叉等待

    accountMapper.lockById(first);

    accountMapper.lockById(second);

    accountMapper.decrease(fromId, amount);

    accountMapper.increase(toId, amount);

}

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@Transactional(timeout = 3)

public void settleOrder(Long orderId) {

    // 事务内只做数据库操作

    orderMapper.updateStatus(orderId, "PAID");

    stockMapper.deductByOrderId(orderId);

    // 通知逻辑放到事务提交后执行

}

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-- 为更新条件建立索引，避免全表扫描扩大锁范围

ALTER TABLE orders ADD INDEX idx_user_status (user_id, status);

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public void executeWithRetry(Runnable action, int maxRetry) {

    for (int i = 0; i < maxRetry; i++) {

        try {

            action.run();

            return;

        } catch (DeadlockLoserDataAccessException e) {

            if (i == maxRetry - 1) {

                throw e;

            }

            // 退避后重试，降低再次冲突概率

            sleepQuietly(50L * (i + 1));

        }

    }

}