1、错误原因
项目测试Swagger的时候，接口请求一直在那转，出不来数据，其他的配置都正常呀，我就很纳闷，今天记录一下。
com.mysql.cj.jdbc.exceptions.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 com.mysql.cj.jdbc.exceptions.SQLError.createCommunicationsException(SQLError.java:174) ~[mysql-connector-java-8.0.20.jar:8.0.20] at com.mysql.cj.jdbc.exceptions.SQLExceptionsMapping.translateException(SQLExceptionsMapping.java:64) ~[mysql-connector-java-8.0.20.jar:8.0.20] at com.mysql.cj.jdbc.ConnectionImpl.createNewIO(ConnectionImpl.java:836) ~[mysql-connector-java-8.0.20.jar:8.0.20] at com.mysql.cj.jdbc.ConnectionImpl.<init>(ConnectionImpl.java:456) ~[mysql-connector-java-8.0.20.jar:8.0.20] at com.mysql.cj.jdbc.ConnectionImpl.getInstance(ConnectionImpl.java:246) ~[mysql-connector-java-8.0.20.jar:8.0.20] at com.mysql.cj.jdbc.NonRegisteringDriver.connect(NonRegisteringDriver.java:197) ~[mysql-connector-java-8.0.20.jar:8.0.20] at com.alibaba.druid.pool.DruidAbstractDataSource.createPhysicalConnection(DruidAbstractDataSource.java:1513) ~[druid-1.1.9.jar:1.1.9] at com.alibaba.druid.pool.DruidAbstractDataSource.createPhysicalConnection(DruidAbstractDataSource.java:1578) ~[druid-1.1.9.jar:1.1.9] at com.alibaba.druid.pool.DruidDataSource$CreateConnectionThread.run(DruidDataSource.java:2466) ~[druid-1.1.9.jar:1.1.9] Caused by: com.mysql.cj.exceptions.CJCommunicationsException: Communications link failure
这个时候我们就要检查一下我们链接数据库的**url、root、password、**驱动是否正确，经过我的排查原来是我的url链接名不是localhost,真尴尬，改成localhost就成功了。
jdbc:mysql://localhost:3306/mangodb
2、 特殊的情况
2.1、在JDBC URL中使用autoReconnect属性，实际测试时使用了autoReconnect=true&failOverReadOnly=false，网上所说的只对mysql 4之前的版本有效。
2.2、修改MySQL的参数，wait_timeout最大为31536000即1年，在my.cnf中加入：
[mysqld] wait_timeout=31536000 interactive_timeout=31536000
重启生效，需要同时修改这两个参数。
