How can I solve this exception

By using Sybase jConnect-6_0 , I make connection and didn't cut it off. However , when it is about 30 ~ 100 hours long , then it throw the Exception like below
 
java.sql.SQLException: JZ006: Caught IOException: com.sybase.jdbc3.jdbc.SybConnectionDeadException: JZ0C0: Connection is already closed.
 at com.sybase.jdbc3.jdbc.ErrorMessage.createIOEKilledConnEx(Unknown Source)
 at com.sybase.jdbc3.jdbc.ErrorMessage.raiseErrorCheckDead(Unknown Source)
 at com.sybase.jdbc3.tds.Tds.a(Unknown Source)
 at com.sybase.jdbc3.tds.Tds.a(Unknown Source)
 at com.sybase.jdbc3.tds.Tds.nextResult(Unknown Source)
 at com.sybase.jdbc3.jdbc.ResultGetter.nextResult(Unknown Source)
 at com.sybase.jdbc3.jdbc.SybStatement.nextResult(Unknown Source)
 at com.sybase.jdbc3.jdbc.SybStatement.nextResult(Unknown Source)
 at com.sybase.jdbc3.jdbc.SybStatement.queryLoop(Unknown Source)
 at com.sybase.jdbc3.jdbc.SybStatement.executeQuery(Unknown Source)
 at com.sybase.jdbc3.jdbc.SybPreparedStatement.executeQuery(Unknown Source)

What's the reason of it?