mysql-java.net.UnknownHostException:hostname

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用jsp连接mysql数据库时出现了这样的错误，怎么解决？

com.mysql.jdbc.CommunicationsException: Communications link failure due to underlying exception:
** BEGIN NESTED EXCEPTION **
java.net.UnknownHostExceptionMESSAGE: hostname
STACKTRACE:
java.net.UnknownHostException: hostname