是什么导致了CommunicationsException, druid version 1.1.16
最近有个已经上线很久的项目，日志突然开始频繁报如下两段错误：
CommunicationsException, druid version 1.1.16, jdbcUrl : jdbc:mysql://rm-xxxxxxxxxxx.mysql.rds.aliyuncs.com:3306/db_xxxxx_prod?useSSL=false&useAffectedRows=true&useUnicode=true&characterEncoding=UTF-8, testWhileIdle true, idle millis 59435, minIdle 1, poolingCount 1, timeBetweenEvictionRunsMillis 60000, lastValidIdleMillis 59435, driver com.mysql.cj.jdbc.Driver, exceptionSorter com.alibaba.druid.pool.vendor.MySqlExceptionSorter
com.mysql.cj.exceptions.CJCommunicationsException: Communications link failure
The last packet successfully received from the server was 59,436 milliseconds ago. The last packet sent successfully to the server was 59,437 milliseconds ago.
观察了下日志发现，是中午12点开始报错的，找到第一条报错日志观察附件执行的程序，发现是一个上线很久没怎么用过的定时任务开始跑了，业务很简单是读取某个邮件中的附件，然后解析数据入库，因为这个错误导致很多其他业务操作数据库失败，因此产生的日志较多。
经过一波翻找日志，发现日志中有一段巨长的sql，是一个批量update，具体多长呢，看了下附件大概有5000多条数据，下这个sql拼接5000多差不多是1.5M吧。。。。这就很尴尬了，居然没有分段执行。
update t_xxxxx_xxxx a join ( select ? as id, ? as product_code, ? as 'status', ? as color_cn , ? as color_en, ? as style, ? as source, ? as confirm_time UNION select ? as id, ? as product_code, ? as 'status', ? as color_cn , ? as color_en, ? as style, ? as source, ? as confirm_time UNION select ? as id, ? as product_code, ? as 'status', ? as color_cn , ? as color_en, ? as style, ? as source, ? as confirm_time UNION select ? as id, ? as product_code, ? as 'status', ? as color_cn , ? as color_en, ? as style, ? as source, ? as confirm_time............................................................
综上，初步判断sql太长，传输 + 解析建立连接时间超长，导致连接被回收了，因此底层抛出通信链路故障的异常，所以大家在写sql的时候一定要注意，for循环拼接的时候一定要考虑限制长度，因为你也不知道未来别人会怎么用这段sql，入参到底有多大。
当然也可以考虑调整参数来解决，但一版这些参数默认就好，默认就是比较合理的时间参数了，因此最好还是从自己程序去调整。