当使用Hibernate维护数据库是可能出现一下错误：
2014-10-16 20:05:03.352 ERROR - Communications link failure
The last packet successfully received from the server was 58,566 milliseconds ago. The last packet sent successfully to the server was 11 milliseconds ago.
Caused by: com.mysql.jdbc.exceptions.jdbc4.CommunicationsException: Communications link failure
The last packet successfully received from the server was 58,566 milliseconds ago. The last packet sent successfully to the server was 11 milliseconds ago.
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:57)
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
at java.lang.reflect.Constructor.newInstance(Constructor.java:526)
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:2562)
at com.mysql.jdbc.ConnectionImpl.setTransactionIsolation(ConnectionImpl.java:5252)
at org.hibernate.service.jdbc.connections.internal.DriverManagerConnectionProviderImpl.getConnection(DriverManagerConnectionProviderImpl.java:184)
at org.hibernate.internal.AbstractSessionImpl$NonContextualJdbcConnectionAccess.obtainConnection(AbstractSessionImpl.java:292)
at org.hibernate.engine.jdbc.internal.LogicalConnectionImpl.obtainConnection(LogicalConnectionImpl.java:214)
... 8 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)
... 17 more
以上错误讲直接导致无法打开一个Connection错误：
org.hibernate.exception.JDBCConnectionException: Could not open connection
at org.hibernate.exception.internal.SQLStateConversionDelegate.convert(SQLStateConversionDelegate.java:132)
at org.hibernate.exception.internal.StandardSQLExceptionConverter.convert(StandardSQLExceptionConverter.java:49)
at org.hibernate.engine.jdbc.spi.SqlExceptionHelper.convert(SqlExceptionHelper.java:125)
at org.hibernate.engine.jdbc.spi.SqlExceptionHelper.convert(SqlExceptionHelper.java:110)
at org.hibernate.engine.jdbc.internal.LogicalConnectionImpl.obtainConnection(LogicalConnectionImpl.java:221)
at org.hibernate.engine.jdbc.internal.LogicalConnectionImpl.getConnection(LogicalConnectionImpl.java:157)
at org.hibernate.engine.transaction.internal.jdbc.JdbcTransaction.doBegin(JdbcTransaction.java:67)
at org.hibernate.engine.transaction.spi.AbstractTransactionImpl.begin(AbstractTransactionImpl.java:160)
at model.dao.UrlDao.save(UrlDao.java:34)
at model.dao.UrlDao.saveAllUrls(UrlDao.java:49)
at task.BackUpTableTask.run(BackUpTableTask.java:32)
at java.util.TimerThread.mainLoop(Timer.java:555)
at java.util.TimerThread.run(Timer.java:505)
检查发现，该Connection已经失效了，所以问题一定出在mysql的过期设置上。
修改mysql的配置文件（/etc/mysql/my.cnf）的wait_timeout的设置，该设置在linux版本中的最大值为365D，最直观的方法就是增大该值。但只是指标不治本的。
最佳的方法是让连接池维护者能够对维护的连接自己判断，在c3p0中可以做如下添加，使得连接池在取得连接时自动校验：
<property name="hibernate.c3p0.validate">true</property>