数据库连接只要失败一次, 之后就一直失败, 日志打印的错误堆栈都是相同的，一直在"重复"一开始的错误, 即使此时数据库连接已经恢复正常,
导致应用无法连接数据库，除非重启应用才会恢复正常.

这个情况在开发和测试环境都出现过几次, 在本地也可以稳定重现, 重现方式：关闭再开启数据库.

环境:
SpringBoot: 2.0.3.RELEASE, druid-spring-boot-starter: 1.1.10

数据库: MySQL

配置文件:

# DataSource

spring.datasource.type=com.alibaba.druid.pool.DruidDataSource
spring.datasource.driver-class-name=com.mysql.jdbc.Driver
spring.datasource.url=jdbc:mysql://***/***?useUnicode=true&characterEncoding=UTF-8&useSSL=false
spring.datasource.username=***
spring.datasource.password=***

# JPA

spring.jpa.database=MYSQL
spring.jpa.properties.hibernate.enable_lazy_load_no_trans=true
spring.jpa.hibernate.naming.physical-strategy=org.hibernate.boot.model.naming.PhysicalNamingStrategyStandardImpl

# Database Connection Pool (Druid)

spring.datasource.druid.initialSize=5
spring.datasource.druid.minIdle=5
spring.datasource.druid.maxActive=20
spring.datasource.druid.maxWait=8000
spring.datasource.druid.breakAfterAcquireFailure=true
spring.datasource.druid.timeBetweenEvictionRunsMillis=60000
spring.datasource.druid.minEvictableIdleTimeMillis=300000
spring.datasource.druid.validationQuery=SELECT 'x'
spring.datasource.druid.testWhileIdle=true
spring.datasource.druid.poolPreparedStatements=true
spring.datasource.druid.maxPoolPreparedStatementPerConnectionSize=20
spring.datasource.druid.filters=stat,wall,slf4j
spring.datasource.druid.connectionProperties=druid.stat.mergeSql=true;druid.stat.slowSqlMillis=5000
spring.datasource.druid.stat-view-servlet.url-pattern=/druid/*
spring.datasource.druid.stat-view-servlet.login-username=***
spring.datasource.druid.stat-view-servlet.login-password=***