WARN: 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.
问题：出现在mysql数据库连接
解决：jdbc:mysql://localhost:3306/email?useUnicode=true&characterEncoding=UTF-8&useSSL=false
作用：1.存数据时：数据库在存放项目数据的时候会先用UTF-8格式将数据解码成字节码，然后再将解码后的字节码重新使用GBK编码存放到数据库中。2.取数据时：在从数据库中取数据的时候，数据库会先将数据库中的数据按GBK格式解码成字节码，然后再将解码后的字节码重新按UTF-8格式编码数据，最后再将数据返回给客户端。
另：在xml配置文件中配置数据库utl时，要使用&的转义字符也就是&例如：
<property name="url" value="jdbc:mysql://localhost:3306/crm?useUnicode=true&characterEncoding=UTF-8&useSSL=false" />
