今天在试验事务隔离级别demo的时候，出现的这个问题。
org.springframework.dao.CannotAcquireLockException:
### Error updating database. Cause: com.mysql.cj.jdbc.exceptions.MySQLTransactionRollbackException: Lock wait timeout exceeded; try restarting transaction
### The error may exist in file [E:\Java_Gitee\mysql-mybatis-demo\target\classes\config\StockMapper.xml]
### The error may involve com.example.dao.StockMapper.updateStock-Inline
### The error occurred while setting parameters
### SQL: update stock set num = num - 1 where id = ?
### Cause: com.mysql.cj.jdbc.exceptions.MySQLTransactionRollbackException: Lock wait timeout exceeded; try restarting transaction使用的是mysql数据库，在事务中我使用线程休眠了60秒，我先后启动了两个单元测试去调用同一个事务，所以出现了上边的情况。这明显是锁等待时间超时，所以需要调整innodb_lock_wait_timeout这个参数。
远程登陆后，操作如下：
[root@iZm5e9c4pjvz19yded1r6bZ ~]# mysql -uroot -pmysql@118
Welcome to the MariaDB monitor. Commands end with ; or \g.
Your MariaDB connection id is 1573
Server version: 5.5.68-MariaDB MariaDB Server
Copyright (c) 2000, 2018, Oracle, MariaDB Corporation Ab and others.
Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.
MariaDB [(none)]> show variables like 'innodb_lock_wait_timeout';
+--------------------------+-------+
| Variable_name | Value |
+--------------------------+-------+
| innodb_lock_wait_timeout | 50 |
+--------------------------+-------+
1 row in set (0.00 sec)
MariaDB [(none)]> set global innodb_lock_wait_timeout=120;
Query OK, 0 rows affected (0.00 sec)
MariaDB [(none)]> quit
Bye重新进入一下，看下修改结果：
[root@iZm5e9c4pjvz19yded1r6bZ ~]# mysql -uroot -pmysql5290@118
Welcome to the MariaDB monitor. Commands end with ; or \g.
Your MariaDB connection id is 1576
Server version: 5.5.68-MariaDB MariaDB Server
Copyright (c) 2000, 2018, Oracle, MariaDB Corporation Ab and others.
Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.
MariaDB [(none)]> show variables like 'innodb_lock_wait_timeout';
+--------------------------+-------+
| Variable_name | Value |
+--------------------------+-------+
| innodb_lock_wait_timeout | 120 |
+--------------------------+-------+
1 row in set (0.00 sec)再测试，没有出现此问题。
