Notes

select @@global.tx_isolation,@@tx_isolation;查看全局隔离级别和会话隔离级别
Read Committed级别出现两次查询结果不同：
	1.事务A开启，查询id=1的记录
	2.事务B开启，修改id=1的记录
	3.事务A查询id=1的记录已经改变
Repeatable Read（可重读）：一直读的都是开启事务之前的数据。
SET [SESSION | GLOBAL] TRANSACTION ISOLATION LEVEL {READ UNCOMMITTED | READ COMMITTED | REPEATABLE READ | SERIALIZABLE}
mysql> set global transaction isolation level read committed; //全局的
mysql> set session transaction isolation level read committed; //当前会话

Error:
### Error updating database.  Cause: com.mysql.cj.jdbc.exceptions.MySQLTransactionRollbackException: Lock wait timeout exceeded; try restarting transaction
### The error may involve com.homo.mapper.UsersMapper.updateByPrimaryKeySelective-Inline
### The error occurred while setting parameters
### SQL: update users      SET name = ?      where id = ?
### Cause: com.mysql.cj.jdbc.exceptions.MySQLTransactionRollbackException: Lock wait timeout exceeded; try restarting transaction
; ]; Lock wait timeout exceeded; try restarting transaction; nested exception is com.mysql.cj.jdbc.exceptions.MySQLTransactionRollbackException: Lock wait timeout exceeded; try restarting transaction

Spring事务传播级别实现：通过不同的conn对象连接处理。有超时风险。
conn1开启事务，更新某一行，取得行锁
conn2开启事务更新同一行会抛超时异常。