阿里巴巴数据源Druid使用时报discard connection异常，为什么

2014-11-06 22:36:28,537 ERROR [com.alibaba.druid.pool.DruidDataSource] - discard connection com.mysql.jdbc.CommunicationsException: The last communications with the server was 28816 seconds ago, which is longer than the server configured value of 'w