查询sqlserver 出现Object has been closed问题？

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)

查询代码：
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()){}
出现这个问题的原因是什么，有什么好的解决方法。