servlet一调用dao就报错?什么毛病?

[java阿俊]

servlet一调用dao就报错?什么毛病?
jsp正常调用

public class test {
       //测试类,测试正常
	public static void main(String []args) {
		CostDao dao=new CostDao();
		List<Cost> list=dao.findAll();
		System.out.println(list);
	}
}

,

public class CostDao implements Serializable {

	public List<Cost> findAll() {
		try {
			Connection conn = 
				DBUtil.getConnection();
			String sql = 
				"select * from cost "
				+ "order by cost_id";
			Statement smt = 
				conn.createStatement();
			ResultSet rs = smt.executeQuery(sql);
			List<Cost> list = new ArrayList<Cost>();
			while(rs.next()) {
				Cost c = new Cost();
				c.setCostId(rs.getInt("cost_id"));
				c.setName(rs.getString("name"));
				c.setBaseDuration(rs.getInt("base_duration"));
				c.setBaseCost(rs.getDouble("base_cost"));
				c.setUnitCost(rs.getDouble("unit_cost"));
				c.setStatus(rs.getString("status"));
				c.setDescr(rs.getString("descr"));
				c.setCreatime(rs.getTimestamp("creatime"));
				c.setStartime(rs.getTimestamp("startime"));
				c.setCostType(rs.getString("cost_type"));
				list.add(c);
			}
			return list;
		} catch (ClassNotFoundException e) {
			e.printStackTrace();
			throw new RuntimeException(
				"找不到驱动类",e);
		} catch (SQLException e) {
			e.printStackTrace();
			throw new RuntimeException(
				"查询资费失败",e);
		} finally {
			DBUtil.closeConnection();
		}
	}
	
	
}

,

public class MainServlet extends HttpServlet {

	@Override
	protected void service(
		HttpServletRequest req, 
		HttpServletResponse res) 
		throws ServletException, IOException {
		//获取并判断访问路径
		String path = req.getServletPath();
		if("/findCost.do".equals(path)) {
			findCost(req,res);
		} else {
			throw new RuntimeException("查无此页");
		}
	}
	
	//查询资费
	protected void findCost(
		HttpServletRequest req, 
		HttpServletResponse res) 
		throws ServletException, IOException {
		//查询所有资费
		CostDao dao = new CostDao();
               
               //就是在这出的错,把findAll一注释就不报错了,jsp正常
		List<Cost> list = dao.findAll();
		//转发至jsp
		req.setAttribute("costs", list);
		//当前:/netctoss/findCost.do
		//目标:/netctoss/WEB-INF/cost/find.jsp
		req.getRequestDispatcher
			("WEB-INF/cost/find.jsp").forward(req, res);
	}

}