应用遇到异常报警：

Caused by: com.mysql.jdbc.exceptions.jdbc4.CommunicationsException: Communications link failure

The 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 more
Caused 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)

服务器设置的时间为20s.从而说明上面超时的原因。

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)

设置完毕。

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)

是会话级别。有些人会把会话参数误认为全局参数。

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)