錯誤：
org.mybatis.spring.MyBatisSystemException: nested exception is org.apache.ibatis.exceptions.PersistenceException:
### Error querying database. Cause: org.springframework.jdbc.CannotGetJdbcConnectionException: Could not get JDBC Connection; nested exception is java.sql.SQLException: An attempt by a client to checkout a Connection has timed out.
### The error may exist in file [B:\maven_workspace\seckill\target\classes\mapper\SeckillDao.xml]
### The error may involve org.seckill.dao.SeckillDao.queryById
### The error occurred while executing a query
### Cause: org.springframework.jdbc.CannotGetJdbcConnectionException: Could not get JDBC Connection; nested exception is java.sql.SQLException: An attempt by a client to checkout a Connection has timed out.
Caused by: org.springframework.jdbc.CannotGetJdbcConnectionException: Could not get JDBC Connection; nested exception is java.sql.SQLException: An attempt by a client to checkout a Connection has timed out.
原因：
mysql版本問題！！！
mysql5.x 版本
driver=com.mysql.jdbc.Driver
url=jdbc:mysql://127.0.0.1:3306/seckill?useUnicode=true&characterEncoding=utf8
mysql8.x版本
driver=com.mysql.cj.jdbc.Driver
url=jdbc:mysql://localhost:3306/AppStore?useUnicode=true&characterEncoding=utf8&useSSL=false&serverTimezone=UTC&rewriteBatchedStatements=true
解決方法：
有兩種解決方法：
1.退回到5.x版本,其他配置不變
2.修改jdbc.properties文件和pom.xml文件更改mysql版本與主機對應
