后台报com.mysql.cj.jdbc.exceptions.CommunicationsException: Communications link failu问题的解决方案
问题：
今早，发现昨天部署的JAVA-WEB服务，不正常启动了，于是查看后台日志，发现
11:07:05.358 [Druid-ConnectionPool-Create-1102883776] ERROR com.alibaba.druid.pool.DruidDataSource - create connection holder error com.mysql.cj.jdbc.exceptions.CommunicationsException: Communications link failure The last packet successfully received from the server was 1,003 milliseconds ago. The last packet sent successfully to the server was 1,003 milliseconds ago. at com.mysql.cj.jdbc.exceptions.SQLError.createCommunicationsException(SQLError.java:172) ~[mysql-connector-java-8.0.11.jar:8.0.11] at com.mysql.cj.jdbc.exceptions.SQLExceptionsMapping.translateException(SQLExceptionsMapping.java:64) ~[mysql-connector-java-8.0.11.jar:8.0.11] at com.mysql.cj.jdbc.ConnectionImpl.isReadOnly(ConnectionImpl.java:1518) ~[mysql-connector-java-8.0.11.jar:8.0.11] at com.mysql.cj.jdbc.ConnectionImpl.isReadOnly(ConnectionImpl.java:1487) ~[mysql-connector-java-8.0.11.jar:8.0.11] at com.alibaba.druid.pool.DruidConnectionHolder.<init>(DruidConnectionHolder.java:143) ~[druid-1.0.29.jar:1.0.29] at com.alibaba.druid.pool.DruidConnectionHolder.<init>(DruidConnectionHolder.java:83) ~[druid-1.0.29.jar:1.0.29] at com.alibaba.druid.pool.DruidDataSource.put(DruidDataSource.java:1890) [druid-1.0.29.jar:1.0.29] at com.alibaba.druid.pool.DruidDataSource$CreateConnectionThread.run(DruidDataSource.java:2143) [druid-1.0.29.jar:1.0.29] Caused by: com.mysql.cj.exceptions.CJCommunicationsException: Communications link failure
解决：
打开navicat，进行解决
1、选择navicat的"工具-->命令列界面"
2、查看WAIT_TIMEOUT的值
mysql> show global variables like 'wait_timeout';
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| wait_timeout | 28800 |
+---------------+-------+
1 row in set (0.24 sec)
mysql> select variable_name,variable_value from information_schema.session_variables where variable_name in ('interactive_timeout','wait_timeout');
1109 - Unknown table 'SESSION_VARIABLES' in information_schema
mysql> set session WAIT_TIMEOUT=1814400;
Query OK, 0 rows affected (0.01 sec)
mysql> show global variables like 'wait_timeout';
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| wait_timeout | 28800 |
+---------------+-------+
1 row in set (0.03 sec)
mysql> show global variables like 'timeout';
Empty set
mysql> set global WAIT_TIMEOUT=1814400;
Query OK, 0 rows affected (0.00 sec)
mysql> show global variables like 'wait_timeout';
+---------------+---------+
| Variable_name | Value |
+---------------+---------+
| wait_timeout | 1814400 |
+---------------+---------+
1 row in set (0.04 sec)
mysql>
3、再次刷新JAVA-WEB界面，成功
总结：
1. 控制连接最大空闲时长的wait_timeout参数。
2. 对于非交互式连接，类似于jdbc连接，wait_timeout的值继承自服务器端全局变量wait_timeout（set session WAIT_TIMEOUT=1814400;是不起作用的，只有set global WAIT_TIMEOUT=1814400;才行）
对于交互式连接，类似于mysql客户单连接，wait_timeout的值继承自服务器端全局变量interactive_timeout。
3. 判断一个连接的空闲时间，可通过show processlist输出中Sleep状态的时间。
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