启动SpringBoot应用程序后，在服务器启动的几分钟后出现异常。外部未使用任何HikariPool配置，Spring Boot默认情况下使用HikariPool这是我在控制台中遇到的错误：
2020-02-20 03:16:23 - HikariPool-4 - Failed to validate connection
com.mysql.cj.jdbc.ConnectionImpl@4c4180c8 (No operations allowed after connection closed.).
Possibly consider using a shorter maxLifetime value.
2020-02-20 03:16:28 - HikariPool-4 - Failed to validate connection
com.mysql.cj.jdbc.ConnectionImpl@679c2f50 (No operations allowed after connection closed.).
Possibly consider using a shorter maxLifetime value.
2020-02-20 03:16:33 - HikariPool-4 - Failed to validate connection
com.mysql.cj.jdbc.ConnectionImpl@16083061 (No operations allowed after connection closed.).
Possibly consider using a shorter maxLifetime value.
2020-02-20 03:16:38 - HikariPool-4 - Failed to validate connection
com.mysql.cj.jdbc.ConnectionImpl@4fcaf421 (No operations allowed after connection closed.).
Possibly consider using a shorter maxLifetime value.
2020-02-20 03:16:43 - HikariPool-4 - Failed to validate connection
com.mysql.cj.jdbc.ConnectionImpl@33df5d54 (No operations allowed after connection closed.).
Possibly consider using a shorter maxLifetime value.
2020-02-20 03:16:48 - HikariPool-4 - Failed to validate connection
com.mysql.cj.jdbc.ConnectionImpl@373d288c (No operations allowed after connection closed.).
Possibly consider using a shorter maxLifetime value.
2020-02-20 03:16:48 - SQL Error: 0, SQLState: 08003
2020-02-20 03:16:48 - HikariPool-4 - Connection is not available, request timed out after
30156ms.
2020-02-20 03:16:48 - No operations allowed after connection closed.
2020-02-20 03:16:48 - Servlet.service() for servlet [dispatcherServlet] in context with path
[] threw exception [Request processing failed; nested exception is
org.springframework.dao.DataAccessResourceFailureException: Unable to acquire JDBC
Connection; nested exception is org.hibernate.exception.JDBCConnectionException: Unable to
acquire JDBC Connection] with root cause
问题是spring.datasource.hikari.maxLifetime的默认值（30分钟-https://github.com/brettwooldridge/HikariCP#configuration-knobs-baby）高于数据库的“ wait_timeout”（在我的情况下为10分钟）。因此，您有两个选择，要么将hikari.maxLifetime降低到10分钟以下，要么增加数据库的“ wait_timeout”属性。