推荐链接：
    总结——》【Java】
    总结——》【Mysql】
    总结——》【Spring】
    总结——》【SpringBoot】
    总结——》【MyBatis、MyBatis-Plus】

1、操作

正常查询数据库

2、现象

通信异常：通信链路故障
从服务器成功接收的最后一个数据包是66734毫秒前。最后一个成功发送到服务器的数据包是66732毫秒前。

2022-11-16 17:59:05.129 ERROR [10.122.139.93] [http-nio-8917-exec-9] [house-cms-0a7a8b5d-463497-1034] com.alibaba.druid.filter.logging.Slf4jLogFilter.statementLogError [line=149 ] : {conn-10020, pstmt-34465} execute error. SELECT  id,title,dept_name,emp_code,emp_name,emp_phone,password,emp_mail,status,sort,type,remark,login_ip,login_time,is_delete,update_user,create_user,version,create_time,update_time  FROM tb_user 
 WHERE  is_delete=0

AND (emp_code = ?) limit 0,1
com.mysql.jdbc.exceptions.jdbc4.CommunicationsException: Communications link failure

The last packet successfully received from the server was 66,734 milliseconds ago.  The last packet sent successfully to the server was 66,732 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:425)
        at com.mysql.jdbc.SQLError.createCommunicationsException(SQLError.java:990)
        at com.mysql.jdbc.MysqlIO.reuseAndReadPacket(MysqlIO.java:3562)
        at com.mysql.jdbc.MysqlIO.reuseAndReadPacket(MysqlIO.java:3462)
        at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:3905)
        at com.mysql.jdbc.MysqlIO.sendCommand(MysqlIO.java:2530)
        at com.mysql.jdbc.MysqlIO.sqlQueryDirect(MysqlIO.java:2683)
        at com.mysql.jdbc.ConnectionImpl.execSQL(ConnectionImpl.java:2495)
        at com.mysql.jdbc.PreparedStatement.executeInternal(PreparedStatement.java:1903)
        at com.mysql.jdbc.PreparedStatement.execute$original$Qo3qQCn7(PreparedStatement.java:1242)
        at com.mysql.jdbc.PreparedStatement.execute$original$Qo3qQCn7$accessor$m73OheMY(PreparedStatement.java)
        at com.mysql.jdbc.PreparedStatement$auxiliary$qLH27S5e.call(Unknown Source)
        at org.apache.skywalking.apm.agent.core.plugin.interceptor.enhance.InstMethodsInter.intercept(InstMethodsInter.java:86)
        at com.mysql.jdbc.PreparedStatement.execute(PreparedStatement.java)
        at com.alibaba.druid.filter.FilterChainImpl.preparedStatement_execute(FilterChainImpl.java:3461)
        at com.alibaba.druid.filter.FilterEventAdapter.preparedStatement_execute(FilterEventAdapter.java:440)
        at com.alibaba.druid.filter.FilterChainImpl.preparedStatement_execute(FilterChainImpl.java:3459)
        at com.alibaba.druid.filter.FilterEventAdapter.preparedStatement_execute(FilterEventAdapter.java:440)
        at com.alibaba.druid.filter.FilterChainImpl.preparedStatement_execute(FilterChainImpl.java:3459)
        at com.alibaba.druid.wall.WallFilter.preparedStatement_execute(WallFilter.java:626)
        at com.alibaba.druid.filter.FilterChainImpl.preparedStatement_execute(FilterChainImpl.java:3459)
        at com.alibaba.druid.proxy.jdbc.PreparedStatementProxyImpl.execute(PreparedStatementProxyImpl.java:167)
        at com.alibaba.druid.pool.DruidPooledPreparedStatement.execute(DruidPooledPreparedStatement.java:497)

3、原因

Mysql的连接等待时间（wait_timeout）默认8小时。在wait_timeout时间内，mysql的连接（connection）处于等待状态，一旦超过这个时间，mysql就会关闭。但是在我们的项目中的连接池中仍存在合法的connection，当你的程序需要再次访问数据库时，就会出现以上异常，但是再重新访问，又能正常从数据库中获取到数据。

# 连接等待时间，单位为秒
# 默认8小时 = 28800s，windows最大24天，linux最大365天
show global variables like 'wait_timeout';

4、解决

1）执行sql调大wait_timeout的值

# mysql重启之后又会恢复成8小时
set global wait_timeout=1814400;

2）修改/etc/my.cnf调大wait_timeout的值

# 重启mysql服务之后生效，大约21天
wait_timeout=1814400