异常解决：Caused by: com.mysql.jdbc.exceptions.jdbc4.CommunicationsException: Communications link failure
这个异常通常有如下信息:
com.mysql.jdbc.exceptions.jdbc4.CommunicationsException: Communications link failure
The last packet successfully received from the server was 59,977 milliseconds ago. The last packet sent successfully to the server was 1 milliseconds ago.
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
at java.lang.reflect.Constructor.newInstance(Constructor.java:423)
at com.mysql.jdbc.Util.handleNewInstance(Util.java:404)
at com.mysql.jdbc.SQLError.createCommunicationsException(SQLError.java:988)
at com.mysql.jdbc.MysqlIO.reuseAndReadPacket(MysqlIO.java:3552)
at com.mysql.jdbc.MysqlIO.reuseAndReadPacket(MysqlIO.java:3452)
at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:3893)
at com.mysql.jdbc.MysqlIO.sendCommand(MysqlIO.java:2526)
at com.mysql.jdbc.MysqlIO.sqlQueryDirect(MysqlIO.java:2673)
at com.mysql.jdbc.ConnectionImpl.execSQL(ConnectionImpl.java:2549)
at com.mysql.jdbc.PreparedStatement.executeInternal(PreparedStatement.java:1861)
at com.mysql.jdbc.PreparedStatement.executeQuery(PreparedStatement.java:1962)
at com.alibaba.druid.pool.DruidPooledPreparedStatement.executeQuery(DruidPooledPreparedStatement.java:227)
at org.springframework.jdbc.core.JdbcTemplate$1.doInPreparedStatement(JdbcTemplate.java:692)
at org.springframework.jdbc.core.JdbcTemplate.execute(JdbcTemplate.java:633)
... 20 common frames omitted
原因分析
当数据库重启或数据库空闲连接超过设置的最大timemout时间，数据库会强行断开已有的链接，最大timeout时间可以通过一下命令查询：
show global variables like "wait_timeout";
解决办法
为了避免空闲时间过长超过最大空闲时间而被断开，我们设置三个配置：
validationQuery: SELECT 1
testWhileIdle: true
timeBetweenEvictionRunsMillis: 28000
其中timeBetweenEvictionRunsMillis需要小于mysql的wait_timeout
但是这种方法无法避免重启的情况，不过一般数据库不会频繁重启，影响不大，如果非得频繁重启，可以通过设置testOnBorrow，即申请连接的时候先试一试连接是否可用，不过带来的影响就是性能降低，需要根据实际需求合理取舍