原因: 数据库的链接默认的超时时间是8个小时，如果在8个小时里没有访问，那么这个连接就会被回收
如下配置tomcat连接池
spring.datasource.tomcat.max-active=100
spring.datasource.tomcat.max-idle=30
spring.datasource.tomcat.max-age=18000000
spring.datasource.tomcat.test-on-borrow=true
spring.datasource.tomcat.num-tests-per-eviction-run=3
spring.datasource.tomcat.validation-query=select 1
spring.datasource.tomcat.validation-query-timeout=10000
spring.datasource.tomcat.test-while-idle=true
spring.datasource.tomcat.initial-size=5
spring.datasource.tomcat.remove-abandoned=true
spring.datasource.tomcat.remove-abandoned-timeout=82800
spring.datasource.tomcat.time-between-eviction-runs-millis=600000
spring.datasource.tomcat.log-abandoned=true