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应用遇到异常报警：
Caused by: com.mysql.jdbc.exceptions.jdbc4.CommunicationsException: Communications link failureThe last packet successfully received from the server was 23,579 milliseconds ago. The last packet sent successfully to the server was 0 milliseconds ago. at sun.reflect.GeneratedConstructorAccessor40.newInstance(Unknown Source) at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:27) at java.lang.reflect.Constructor.newInstance(Constructor.java:513) at com.mysql.jdbc.Util.handleNewInstance(Util.java:409) at com.mysql.jdbc.SQLError.createCommunicationsException(SQLError.java:1118) at com.mysql.jdbc.MysqlIO.reuseAndReadPacket(MysqlIO.java:3055) at com.mysql.jdbc.MysqlIO.reuseAndReadPacket(MysqlIO.java:2941) at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:3489) at com.mysql.jdbc.MysqlIO.sendCommand(MysqlIO.java:1959) at com.mysql.jdbc.MysqlIO.sqlQueryDirect(MysqlIO.java:2113) at com.mysql.jdbc.ConnectionImpl.execSQL(ConnectionImpl.java:2568) at com.mysql.jdbc.PreparedStatement.executeInternal(PreparedStatement.java:2113) at com.mysql.jdbc.PreparedStatement.execute(PreparedStatement.java:1364) at com.mchange.v2.c3p0.impl.NewProxyPreparedStatement.execute(NewProxyPreparedStatement.java:989) at com.ibatis.sqlmap.engine.execution.SqlExecutor.executeQuery(SqlExecutor.java:185) at com.ibatis.sqlmap.engine.mapping.statement.MappedStatement.sqlExecuteQuery(MappedStatement.java:221) at com.ibatis.sqlmap.engine.mapping.statement.MappedStatement.executeQueryWithCallback(MappedStatement.java:189) ... 36 moreCaused by: java.io.EOFException: Can not read response from server. Expected to read 4 bytes, read 0 bytes before connection was unexpectedly lost. at com.mysql.jdbc.MysqlIO.readFully(MysqlIO.java:2502) at com.mysql.jdbc.MysqlIO.reuseAndReadPacket(MysqlIO.java:2952) ... 47 more
问题比较明显，应用连接已经失效。
23.579s之前创建了一个连接，之后再次使用此数据库连接就超时了。
查看数据库配置：
mysql> show global variables like 'wait_timeout';+---------------+-------+| Variable_name | Value |+---------------+-------+| wait_timeout | 20 |+---------------+-------+1 row in set (0.00 sec)
解决这个方法：
一：从应用层解决问题：修改连接池参数 缩短连接池内的连接超时时间。
二：从DB层解决问题：增大MySQL的连接失效时间。
mysql> show global variables like 'wait_timeout';+---------------+-------+| Variable_name | Value |+---------------+-------+| wait_timeout | 20 |+---------------+-------+1 row in set (0.00 sec)
动态修改global参数，wait_timeout;
set global wait_timeout=30;
再次查看参数：
mysql> show global variables like 'wait_timeout';+---------------+-------+| Variable_name | Value |+---------------+-------+| wait_timeout | 30 |+---------------+-------+1 row in set (0.00 sec)
时间的大小设置不能随便，时间过长，导致过多的connection Sleep,占用较多系统资源。时间设置过小也不行。从业务出发，结合查询的耗时与业务吞吐量来进行设置比较合适吧。
常见误解：
网上有很多文章 没有清晰的认识 会话变量与全局变量的不同，查看了会话变量的超时时间，最终可能会导致将全局变量 wait_timeout设置成了31536000s！！
[mysqld]
wait_timeout=31536000
my.cnf里设置的参数是global级别。不是会话级别。
然而：
mysql> show variables like 'wait_timeout';+---------------+-------+| Variable_name | Value |+---------------+-------+| wait_timeout | 28800 |+---------------+-------+1 row in set (0.00 sec)
正确的参数初始化设置应该为：
[mysqld]
wait_timeout=30
建议大家今后查询变量的时候不要使用缺省。
查询会话变量：
mysql> show session variables like '%wait%'; +--------------------------+----------+| Variable_name | Value |+--------------------------+----------+| innodb_lock_wait_timeout | 50 || innodb_spin_wait_delay | 6 || lock_wait_timeout | 31536000 || wait_timeout | 28800 |+--------------------------+----------+4 rows in set (0.00 sec)
查询全局变量：
mysql> show global variables like '%wait%'; +--------------------------+----------+| Variable_name | Value |+--------------------------+----------+| innodb_lock_wait_timeout | 50 || innodb_spin_wait_delay | 6 || lock_wait_timeout | 31536000 || wait_timeout | 30 |+--------------------------+----------+4 rows in set (0.00 sec)
