異常解決：Caused by: com.mysql.jdbc.exceptions.jdbc4.CommunicationsException: Communications link failure
這個異常通常有如下信息:
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
當數據庫重啟或數據庫空閑連接超過設置的最大timemout時間，數據庫會強行斷開已有的鏈接，最大timeout時間可以通過一下命令查詢：
show global variables like "wait_timeout";
解決辦法
為了避免空閑時間過長超過最大空閑時間而被斷開，我們設置三個配置：
validationQuery: SELECT 1
testWhileIdle: true
timeBetweenEvictionRunsMillis: 28000
其中timeBetweenEvictionRunsMillis需要小於mysql的wait_timeout
但是這種方法無法避免重啟的情況，不過一般數據庫不會頻繁重啟，影響不大，如果非得頻繁重啟，可以通過設置testOnBorrow，即申請連接的時候先試一試連接是否可用，不過帶來的影響就是性能降低，需要根據實際需求合理取舍