Mysql不能连接

com.mysql.jdbc.CommunicationsException: Communications link failure due to under
lying exception:

** BEGIN NESTED EXCEPTION **

java.io.EOFException

STACKTRACE:

java.io.EOFException
        at com.mysql.jdbc.MysqlIO.readFully(MysqlIO.java:1913)
        at com.mysql.jdbc.MysqlIO.reuseAndReadPacket(MysqlIO.java:2304)
        at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:2803)
        at com.mysql.jdbc.MysqlIO.sendCommand(MysqlIO.java:1573)
        at com.mysql.jdbc.MysqlIO.sqlQueryDirect(MysqlIO.java:1665)
        at com.mysql.jdbc.Connection.execSQL(Connection.java:3170)
        at com.mysql.jdbc.Connection.execSQL(Connection.java:3099)
        at com.mysql.jdbc.Statement.executeQuery(Statement.java:1169)
        at com.xuchenguang.Dbrw.EQ(Dbrw.java:46)
        at com.xuchenguang.SMRev.run(SmsModemRev.java:34)
        at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExec
utor.java:650)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor
.java:675)
        at java.lang.Thread.run(Thread.java:595)


** END NESTED EXCEPTION **



Last packet sent to the server was 0 ms ago.
第719行:java.lang.NullPointerException
SMRev线程发现异常，丢弃!
2010-01-16 09:05:12 -DB- > 删除一个无效链接.
2010-01-16 09:05:12 -DB- > 创建一个新的链接.



http://www.wangzhongyuan.com/archives/610.html



问题分析：这个错误是Java程序没有连接上Mysql，或者链接超时导致无法进行任何与数据库相关的操作。这个错误的本质是一个网络的问题。

问题解决：首先，需要分别排查以下情况：
(1)MySql服务器是否支持TCP/IP连接；
(2)服务器端以及运行Java程序的客户端是否启用了防火墙，端口是否被占用；
(3)JDBC的驱动是否正确合适；
(4)是否启用了IPv6？如果现在使用的IPv6，那么换成IPv4再试试看；

如果上述情况都排除之后，还是出现如上的异常错误，那么很有可能是你电脑所处的网络存在问题，你可以尝试一下换个地方上网运行这个包含JDBC的Java程序，或者换个时间再次运行。

参考如下：

http://andyao.iteye.com/blog/38506

查看了Mysql的文档，以及Connector/J的文档以及在线说明发现，出现这种异常的原因是：

Mysql服务器默认的“wait_timeout”是8小时，也就是说一个connection空闲超过8个小时，Mysql将自动断开该connection。这就是问题的所在，在C3P0 pools中的connections如果空闲超过8小时，Mysql将其断开，而C3P0并不知道该connection已经失效，如果这时有Client请求connection，C3P0将该失效的Connection提供给Client，将会造成上面的异常。

参考二：

http://forums.sun.com/thread.jspa?threadID=5263225



That error simply means that the database cannot be reached.

As mentioned before, the URL and/or port number can be wrong, there can be a firewall which blocks the port, or the database is simply down or does not accept TCP/IP connections.

Which operating system exactly are you using? Which database implementation and version exactly are you using? Which JDBC driver implementation and version exactly are you using?