異常解決：Caused by: com.mysql.jdbc.exceptions.jdbc4.CommunicationsException: Communications link failure

異常解決：Caused by: com.mysql.jdbc.exceptions.jdbc4.CommunicationsException: Communications link failure

這個異常一般有以下信息:

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

緣由分析

當數據庫重啓或數據庫空閒鏈接超過設置的最大timemout時間，數據庫會強行斷開已有的連接，最大timeout時間能夠經過一下命令查詢：

show global variables like "wait_timeout";

解決辦法

爲了不空閒時間過長超過最大空閒時間而被斷開，咱們設置三個配置：

validationQuery: SELECT 1
testWhileIdle: true
timeBetweenEvictionRunsMillis: 28000

其中timeBetweenEvictionRunsMillis須要小於mysql的wait_timeout

可是這種方法沒法避免重啓的狀況，不過通常數據庫不會頻繁重啓，影響不大，若是非得頻繁重啓，能夠經過設置testOnBorrow，即申請鏈接的時候先試一試鏈接是否可用，不過帶來的影響就是性能下降，須要根據實際需求合理取捨