问题描述：
报错如下：
com.mysql.jdbc.exceptions.jdbc4.CommunicationsException: Communications link failure
The last packet sent successfully to the server was 0 milliseconds ago. The driver has not received any packets from the server.
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method) ~[na:1.8.0_275]
at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62) ~[na:1.8.0_275]
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45) ~[na:1.8.0_275]
at java.lang.reflect.Constructor.newInstance(Constructor.java:423) ~[na:1.8.0_275]
at com.mysql.jdbc.Util.handleNewInstance(Util.java:425) ~[mysql-connector-java-5.1.46.jar!/:5.1.46]
at com.mysql.jdbc.SQLError.createCommunicationsException(SQLError.java:990) ~[mysql-connector-java-5.1.46.jar!/:5.1.46]
at com.mysql.jdbc.MysqlIO.<init>(MysqlIO.java:342) ~[mysql-connector-java-5.1.46.jar!/:5.1.46]
at com.mysql.jdbc.ConnectionImpl.coreConnect(ConnectionImpl.java:2188) ~[mysql-connector-java-5.1.46.jar!/:5.1.46]
at com.mysql.jdbc.ConnectionImpl.connectOneTryOnly(ConnectionImpl.java:2221) ~[mysql-connector-java-5.1.46.jar!/:5.1.46]
at com.mysql.jdbc.ConnectionImpl.createNewIO(ConnectionImpl.java:2016) ~[mysql-connector-java-5.1.46.jar!/:5.1.46]
at com.mysql.jdbc.ConnectionImpl.<init>(ConnectionImpl.java:776) ~[mysql-connector-java-5.1.46.jar!/:5.1.46]
at com.mysql.jdbc.JDBC4Connection.<init>(JDBC4Connection.java:47) ~[mysql-connector-java-5.1.46.jar!/:5.1.46]
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method) ~[na:1.8.0_275]
at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62) ~[na:1.8.0_275]
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45) ~[na:1.8.0_275]
at java.lang.reflect.Constructor.newInstance(Constructor.java:423) ~[na:1.8.0_275]
at com.mysql.jdbc.Util.handleNewInstance(Util.java:425) ~[mysql-connector-java-5.1.46.jar!/:5.1.46]
at com.mysql.jdbc.ConnectionImpl.getInstance(ConnectionImpl.java:386) ~[mysql-connector-java-5.1.46.jar!/:5.1.46]
at com.mysql.jdbc.NonRegisteringDriver.connect(NonRegisteringDriver.java:330) ~[mysql-connector-java-5.1.46.jar!/:5.1.46]
...
解决方案：
解决方案1：
如mysql版本是 5.7 驱动使用的是 com.mysql.jdbc.Driver
driver=com.mysql.jdbc.Driver
url=jdbc:mysql://localhost:3306/demo?useSSL=true&useUnicode=true&characterEncoding=UTF-8
原因：useSSL=true时 数据库连接 安全认证不通过
解决方案：将useSSL true改为false
解决方案2：
更换JDBC驱动：使用：com.mysql.cj.jdbc.Driver （需注意：需要指定时区serverTimezone:）
driver=com.mysql.cj.jdbc.Driver
url=jdbc:mysql://localhost:3306/demo?useSSL=true&useUnicode=true&characterEncoding=UTF-8&serverTimezone=UTC
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