Mysql 系列文章主页

===============

刚开始学习 Mysql 锁的时候,觉得 Mysql 使用的是行锁,再加上其默认的可重复读的隔离级别,那就应该能够自动解决并发事务更新的问题。可事实上,并不能解决!

可是,为什么不能解决呢?

带着问题,自己写了个简单Demo,来测试并分析其中道理:

Demo 代码路径:https://github.com/cyhbyw/cyh_Spring_IsolationConcurrencyTransaction

Demo 工程名称:usingMybatis

0 准备工作

工程中所有文件如下图所示;

代码运行前,注意设置数据库的用户名和密码,同时,需要运行 test.sql 文件中的脚本用以初始化数据;

1 测试

1.1 业务代码

     @Transactional(rollbackFor = RuntimeException.class)
public void increaseMoney(Integer id) {
Employee employee = employeeMapper.findById(id);
final Integer oldMoney = employee.getMoney();
LOGGER.info("oldMoney: {}", oldMoney);
employee.setMoney(oldMoney + 1);
employeeMapper.updateEmployee(employee);
}

代码说明:

  • 第1行,添加事务
  • 第3行,取出数据
  • 第4,5行,取出旧的Money值并记录日志
  • 第6行,为 Employee 对象设置新的值(就是+1操作)
  • 第7行,执行Sql更新

1.2 测试代码

     private void increaseMoney() {
int threadCount = 100;
while (threadCount-- > 0) {
new Thread(() -> employeeService.increaseMoney(1)).start();
}
}

代码说明:

  • 启了 100 根线程去调用业务方法
  • 每个线程将 Money+1,理论上,最后的Money=100(初始值为0)

1.3 测试结果

预期结果:100,实际结果:22

提示:22这个值不固定(因为是多线程访问嘛,而线程的执行是由CPU决定的,具有一定波动性)

1.4 程序日志

 2018-04-25 11:32:05,416  INFO [Thread-83] (EmployeeService.java:27) - oldMoney: 0
2018-04-25 11:32:05,416 INFO [Thread-18] (EmployeeService.java:27) - oldMoney: 0
2018-04-25 11:32:05,416 INFO [Thread-19] (EmployeeService.java:27) - oldMoney: 0
2018-04-25 11:32:05,416 INFO [Thread-54] (EmployeeService.java:27) - oldMoney: 0
2018-04-25 11:32:05,416 INFO [Thread-15] (EmployeeService.java:27) - oldMoney: 0
2018-04-25 11:32:05,416 INFO [Thread-52] (EmployeeService.java:27) - oldMoney: 0
2018-04-25 11:32:05,416 INFO [Thread-17] (EmployeeService.java:27) - oldMoney: 0
2018-04-25 11:32:05,416 INFO [Thread-55] (EmployeeService.java:27) - oldMoney: 0
2018-04-25 11:32:05,416 INFO [Thread-53] (EmployeeService.java:27) - oldMoney: 0
2018-04-25 11:32:05,416 INFO [Thread-20] (EmployeeService.java:27) - oldMoney: 0
2018-04-25 11:32:05,416 INFO [Thread-43] (EmployeeService.java:27) - oldMoney: 0
2018-04-25 11:32:05,416 INFO [Thread-2] (EmployeeService.java:27) - oldMoney: 0
2018-04-25 11:32:05,416 INFO [Thread-56] (EmployeeService.java:27) - oldMoney: 0
2018-04-25 11:32:05,416 INFO [Thread-8] (EmployeeService.java:27) - oldMoney: 0
2018-04-25 11:32:05,416 INFO [Thread-16] (EmployeeService.java:27) - oldMoney: 0
2018-04-25 11:32:05,462 INFO [Thread-14] (EmployeeService.java:27) - oldMoney: 1
2018-04-25 11:32:05,527 INFO [Thread-27] (EmployeeService.java:27) - oldMoney: 2
2018-04-25 11:32:05,534 INFO [Thread-26] (EmployeeService.java:27) - oldMoney: 2
2018-04-25 11:32:05,534 INFO [Thread-22] (EmployeeService.java:27) - oldMoney: 2
2018-04-25 11:32:05,535 INFO [Thread-23] (EmployeeService.java:27) - oldMoney: 2
2018-04-25 11:32:05,534 INFO [Thread-24] (EmployeeService.java:27) - oldMoney: 2
2018-04-25 11:32:05,538 INFO [Thread-29] (EmployeeService.java:27) - oldMoney: 3
2018-04-25 11:32:05,538 INFO [Thread-25] (EmployeeService.java:27) - oldMoney: 3
2018-04-25 11:32:05,540 INFO [Thread-32] (EmployeeService.java:27) - oldMoney: 3
2018-04-25 11:32:05,541 INFO [Thread-30] (EmployeeService.java:27) - oldMoney: 3
2018-04-25 11:32:05,540 INFO [Thread-28] (EmployeeService.java:27) - oldMoney: 3
2018-04-25 11:32:05,542 INFO [Thread-31] (EmployeeService.java:27) - oldMoney: 3
2018-04-25 11:32:05,542 INFO [Thread-33] (EmployeeService.java:27) - oldMoney: 3
2018-04-25 11:32:05,548 INFO [Thread-37] (EmployeeService.java:27) - oldMoney: 4
2018-04-25 11:32:05,550 INFO [Thread-21] (EmployeeService.java:27) - oldMoney: 4
2018-04-25 11:32:05,550 INFO [Thread-34] (EmployeeService.java:27) - oldMoney: 4
2018-04-25 11:32:05,575 INFO [Thread-39] (EmployeeService.java:27) - oldMoney: 5
2018-04-25 11:32:05,575 INFO [Thread-38] (EmployeeService.java:27) - oldMoney: 5
2018-04-25 11:32:05,576 INFO [Thread-41] (EmployeeService.java:27) - oldMoney: 5
2018-04-25 11:32:05,575 INFO [Thread-40] (EmployeeService.java:27) - oldMoney: 5
2018-04-25 11:32:05,577 INFO [Thread-42] (EmployeeService.java:27) - oldMoney: 5
2018-04-25 11:32:05,577 INFO [Thread-44] (EmployeeService.java:27) - oldMoney: 6
2018-04-25 11:32:05,578 INFO [Thread-45] (EmployeeService.java:27) - oldMoney: 6
2018-04-25 11:32:05,602 INFO [Thread-35] (EmployeeService.java:27) - oldMoney: 7
2018-04-25 11:32:05,603 INFO [Thread-13] (EmployeeService.java:27) - oldMoney: 7
2018-04-25 11:32:05,604 INFO [Thread-36] (EmployeeService.java:27) - oldMoney: 7
2018-04-25 11:32:05,604 INFO [Thread-12] (EmployeeService.java:27) - oldMoney: 8
2018-04-25 11:32:05,605 INFO [Thread-10] (EmployeeService.java:27) - oldMoney: 8
2018-04-25 11:32:05,606 INFO [Thread-9] (EmployeeService.java:27) - oldMoney: 9
2018-04-25 11:32:05,606 INFO [Thread-6] (EmployeeService.java:27) - oldMoney: 9
2018-04-25 11:32:05,606 INFO [Thread-1] (EmployeeService.java:27) - oldMoney: 9
2018-04-25 11:32:05,644 INFO [Thread-47] (EmployeeService.java:27) - oldMoney: 10
2018-04-25 11:32:05,644 INFO [Thread-48] (EmployeeService.java:27) - oldMoney: 10
2018-04-25 11:32:05,644 INFO [Thread-46] (EmployeeService.java:27) - oldMoney: 10
2018-04-25 11:32:05,649 INFO [Thread-49] (EmployeeService.java:27) - oldMoney: 10
2018-04-25 11:32:05,650 INFO [Thread-75] (EmployeeService.java:27) - oldMoney: 10
2018-04-25 11:32:05,651 INFO [Thread-76] (EmployeeService.java:27) - oldMoney: 10
2018-04-25 11:32:05,651 INFO [Thread-50] (EmployeeService.java:27) - oldMoney: 10
2018-04-25 11:32:05,686 INFO [Thread-79] (EmployeeService.java:27) - oldMoney: 11
2018-04-25 11:32:05,686 INFO [Thread-78] (EmployeeService.java:27) - oldMoney: 11
2018-04-25 11:32:05,686 INFO [Thread-77] (EmployeeService.java:27) - oldMoney: 11
2018-04-25 11:32:05,693 INFO [Thread-80] (EmployeeService.java:27) - oldMoney: 11
2018-04-25 11:32:05,693 INFO [Thread-100] (EmployeeService.java:27) - oldMoney: 11
2018-04-25 11:32:05,693 INFO [Thread-81] (EmployeeService.java:27) - oldMoney: 11
2018-04-25 11:32:05,694 INFO [Thread-51] (EmployeeService.java:27) - oldMoney: 11
2018-04-25 11:32:05,694 INFO [Thread-82] (EmployeeService.java:27) - oldMoney: 11
2018-04-25 11:32:05,737 INFO [Thread-59] (EmployeeService.java:27) - oldMoney: 12
2018-04-25 11:32:05,738 INFO [Thread-58] (EmployeeService.java:27) - oldMoney: 12
2018-04-25 11:32:05,739 INFO [Thread-57] (EmployeeService.java:27) - oldMoney: 12
2018-04-25 11:32:05,746 INFO [Thread-60] (EmployeeService.java:27) - oldMoney: 12
2018-04-25 11:32:05,746 INFO [Thread-99] (EmployeeService.java:27) - oldMoney: 12
2018-04-25 11:32:05,746 INFO [Thread-62] (EmployeeService.java:27) - oldMoney: 12
2018-04-25 11:32:05,748 INFO [Thread-61] (EmployeeService.java:27) - oldMoney: 12
2018-04-25 11:32:05,786 INFO [Thread-63] (EmployeeService.java:27) - oldMoney: 13
2018-04-25 11:32:05,786 INFO [Thread-66] (EmployeeService.java:27) - oldMoney: 13
2018-04-25 11:32:05,786 INFO [Thread-64] (EmployeeService.java:27) - oldMoney: 13
2018-04-25 11:32:05,792 INFO [Thread-65] (EmployeeService.java:27) - oldMoney: 13
2018-04-25 11:32:05,792 INFO [Thread-97] (EmployeeService.java:27) - oldMoney: 13
2018-04-25 11:32:05,792 INFO [Thread-98] (EmployeeService.java:27) - oldMoney: 13
2018-04-25 11:32:05,792 INFO [Thread-68] (EmployeeService.java:27) - oldMoney: 13
2018-04-25 11:32:05,792 INFO [Thread-96] (EmployeeService.java:27) - oldMoney: 13
2018-04-25 11:32:05,839 INFO [Thread-95] (EmployeeService.java:27) - oldMoney: 14
2018-04-25 11:32:05,840 INFO [Thread-92] (EmployeeService.java:27) - oldMoney: 14
2018-04-25 11:32:05,839 INFO [Thread-93] (EmployeeService.java:27) - oldMoney: 14
2018-04-25 11:32:05,860 INFO [Thread-94] (EmployeeService.java:27) - oldMoney: 14
2018-04-25 11:32:05,862 INFO [Thread-67] (EmployeeService.java:27) - oldMoney: 14
2018-04-25 11:32:05,861 INFO [Thread-89] (EmployeeService.java:27) - oldMoney: 14
2018-04-25 11:32:05,867 INFO [Thread-91] (EmployeeService.java:27) - oldMoney: 14
2018-04-25 11:32:05,887 INFO [Thread-90] (EmployeeService.java:27) - oldMoney: 15
2018-04-25 11:32:05,890 INFO [Thread-88] (EmployeeService.java:27) - oldMoney: 16
2018-04-25 11:32:05,893 INFO [Thread-85] (EmployeeService.java:27) - oldMoney: 17
2018-04-25 11:32:05,893 INFO [Thread-86] (EmployeeService.java:27) - oldMoney: 17
2018-04-25 11:32:05,894 INFO [Thread-84] (EmployeeService.java:27) - oldMoney: 17
2018-04-25 11:32:05,894 INFO [Thread-87] (EmployeeService.java:27) - oldMoney: 18
2018-04-25 11:32:05,895 INFO [Thread-5] (EmployeeService.java:27) - oldMoney: 18
2018-04-25 11:32:05,896 INFO [Thread-4] (EmployeeService.java:27) - oldMoney: 19
2018-04-25 11:32:05,935 INFO [Thread-71] (EmployeeService.java:27) - oldMoney: 20
2018-04-25 11:32:05,935 INFO [Thread-70] (EmployeeService.java:27) - oldMoney: 20
2018-04-25 11:32:05,936 INFO [Thread-73] (EmployeeService.java:27) - oldMoney: 20
2018-04-25 11:32:05,941 INFO [Thread-69] (EmployeeService.java:27) - oldMoney: 20
2018-04-25 11:32:05,941 INFO [Thread-74] (EmployeeService.java:27) - oldMoney: 20
2018-04-25 11:32:05,941 INFO [Thread-72] (EmployeeService.java:27) - oldMoney: 20
2018-04-25 11:32:05,941 INFO [Thread-7] (EmployeeService.java:27) - oldMoney: 20
2018-04-25 11:32:05,978 INFO [Thread-11] (EmployeeService.java:27) - oldMoney: 21
2018-04-25 11:32:05,978 INFO [Thread-3] (EmployeeService.java:27) - oldMoney: 21

日志分析:

  1. 日志中第1到15行,它们的 oldMoney 全部等于0
  2. 说明对应的15根线程在同一时间读到了相同的值0
  3. 这15根线程给Money设置的新值都是 0+1=1
  4. 这15根线程再分别执行Sql更新操作,期间可能会竞争Mysql的行锁

2 不能解决的原因分析

现在来分析一下为什么Mysql的行锁加上可重复读不能解决并发事务的更新问题

可重复读:表示的是在同一个事务内,所有的读取操作返回相同的数据,也就是不感知外部事务对数据的变更。假设事务A是当前事务,事务B是外部事务,事务A现在读取到 Money=10,接下来,事务B也读到值为10同时将其更新为11,但是对于事务A来说,它不能感知到外部事务对数据的变更,所以,事务A还是错误地认为当前 Money=10 并将其更新为11,于是,事务A覆盖了事务B的提交,从而造成更新丢失。

至于行锁,这只是用来保证并发事务更新时的先后顺序。比如,事务B在将Money=10更新为11时,如果正在这个更新期间(假设更新时间比较长)事务A也想做更新操作,则事务A将被阻塞,直到事务B更新完成后事务A才能更新,它与数据本身(事务A的、事务B的)没有任何关系。

由此可见,虽然 Mysql 是行锁且默认是可重复读,但是对于并发事务的更新操作,仍然会出现更新丢失问题,解决办法是——锁。

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