Jar package startup failed -mysql modify the default port number / set password free enter

【 Issues an error 】
com.mysql.cj.jdbc.exceptions.CommunicationsException: Communications link failure
Start the site jar The package reported an error ,Mysql Can't connect .

【 Cause analysis 】
There may be two reasons for thinking ：
The first is Mysql Password set , This leads to the need for password connection , The connection fails ;
The second is Mysql The default port number is 3306,jar Configured in the package mysql The port number configured on the server is 3307.

【 resolvent 】
Solution ①： Set up Mysql Secret free entry

1. modify Mysql Configuration file for ：vi /etc/my.cnf
2. Add skip-grant-tables
   log-error=/var/log/mysqld.log
   pid-file=/var/run/mysqld/mysqld.pid
3. Restart after saving Mysql service ：systemctl restart mysqld
4. restart jar package , Still not connected Mysql.

Solution ②： modify Mysql Default port number

1. modify Mysql Configuration file for ：vi /etc/my.cnf
2. Add... After the document port=3307
   datadir=/var/lib/mysql
   socket=/var/lib/mysql/mysql.sock
3. Restart after saving Mysql service systemctl restart mysqld
4. Alibaba cloud server , Security group policy is open 3307 port
5. restart jar package , success ！