若执行数据库操作有如下警告：
Establishing SSL connection without server‘s identity verification is not recommended. According to mysql 5.5.45+, 5.6.26+ and 5.7.6+ requirements SSL connection must be established by default if explicit option isn‘t set. For compliance with existing applications not using SSL the verifyServerCertificate property is set to ‘false‘. You need either to explicitly disable SSL by setting useSSL=false, or set useSSL=true and provide truststore for server certificate verification.
原因是 MySQL 5.5.45+, 5.6.26+ and 5.7.6+ 这些版本的数据库需要手动指定SSL是否开启，所以原来的连接字符串：jdbc.url=jdbc:mysql://127.0.0.1:3306/db_book?userUnicode=true&characterEncoding=UTF8就不可以了。
需要在其后附加useSSL=true或false，使用新的连接字符串：jdbc.url=jdbc:mysql://127.0.0.1:3306/db_book?userUnicode=true&characterEncoding=UTF8&useSSL=false问题就解决了。