The last packet successfully received from the server was xx milliseconds ago. The last packet sent

同组同事遇到的问题。

完成异常信息如下：

The last packet successfully received from the server was xxx milliseconds ago.  The last packet sent successfully to the server was xxx milliseconds ago.
 
### Error querying database.  Cause: com.mysql.jdbc.exceptions.jdbc4.CommunicationsException: Communications link failure
org.springframework.dao.DataAccessResourceFailureException: 
 
 
 ... 203 more
 at com.mysql.jdbc.MysqlIO.reuseAndReadPacket(MysqlIO.java:2952)
 at com.mysql.jdbc.MysqlIO.readFully(MysqlIO.java:2502)
Caused by: java.io.EOFException: Can not read response from server. Expected to read 4 bytes, read 0 bytes before connection was unexpectedly lost.

根据异常信息的描述，之后再次使用此数据库连接就说连接丢失了。应该是mysql服务器端超过了wait_time,然后主动关闭了这个连接。但是客户端还从线程池里拿到了这个已经被关闭的连接。所以报了这个错误。

程序也有相应的配置：

testOnConnect： 在连接初始化时执行连接可用性检查testOnBorrow：在借出连接时检查连接可用性testOnReturn：在客户端归还连接时检查连接可用性testWhileIdle：在检查闲置连接时同时检查连接可用性。

最后通过与 DBA 的沟通，由于之前系统存在慢 SQL，所以 DBA 上了一个脚本，超过 20s 的SQL 都会被强制 kill。故产生了此异常信息。