Spring Boot 配置MySQL数据库重连

使用jdbc连接MySQL，如果连接失效，可能会报类似的错误：

com.mysql.jdbc.exceptions.jdbc4.CommunicationsException: The last packet successfully received from the server was 84,371,623 milliseconds ago.
The last packet sent successfully to the server was 78,860,631 milliseconds ago. is longer than the server configured value of 'wait_timeout'.
You should consider either expiring and/or testing connection validity before use in your application, increasing the server configured values for client timeouts, or using the Connector/J connection property 'autoReconnect=true' to avoid this problem.

如错误提示，可以在连接的url上添加autoReconnect=true来解决。

需要注意的是：mysql是不推荐使用autoReconnect配置，因为如果没有合适处理SQLException的话，它会带来一些数据一致性的副作用，可以参考：https://dev.mysql.com/doc/connector-j/5.1/en/connector-j-reference-configuration-properties.html中的autoReconect部分。

spring boot 1.4+需要看使用的是什么数据库连接池库，支持的连接池包括：tomcat, hikari, dbcp（1.5+废弃）, dbcp2。

tomcat

spring.datasource.tomcat.test-on-borrow=true
spring.datasource.tomcat.validation-query=SELECT 1

dbcp2

spring.datasource.dbcp2.test-on-borrow=true
spring.datasource.dbcp2.validation-query=SELECT 1