內蒙古java團隊

java.sql.SQLException: [Microsoft][SQLServer 2000 Driver for JDBC]Object has been closed.
?at com.microsoft.jdbc.base.BaseExceptions.createException(Unknown Source)
?at com.microsoft.jdbc.base.BaseExceptions.getException(Unknown Source)
?at com.microsoft.jdbc.base.BaseStatement.validateClosedState(Unknown Source)
?at com.microsoft.jdbc.base.BaseResultSet.validateClosedState(Unknown Source)
?at com.microsoft.jdbc.base.BaseResultSet.getString(Unknown Source)
?at com.microsoft.jdbc.base.BaseResultSet.getString(Unknown Source)

查詢代碼：
String sql="select * from ws_D_E_C ORDER BY DEP_ID";
??????? cnn=cn.getCon();
???smt=cnn.createStatement();
?????rs=smt.executeQuery(sql);
while(rst.next()){}
出現這個問題的原因是什么，有什么好的解決方法。