请教一个基础的JDBC有关问题

String sql = "SELECT id, name, orders, link, color, parent_id FROM productnav WHERE parent_id=? order by orders";List l = null;try {    PreparedStatement ps = conn.prepareStatement(sql);    ps.setString(1, parentId);    ResultSet rs = ps.executeQuery(sql);    l = new ArrayList();    while(rs.next()){        ProductNav nav = new ProductNav();        nav.setId(rs.getString(1));        nav.setName(rs.getString(2));        nav.setOrders(rs.getInt(3));        nav.setLink(rs.getString(4));        nav.setColor(rs.getString(5));        nav.setParentId(rs.getString(6));        l.add(nav);    }}catch(SQLException e){    e.printStackTrace();    log.error(e.getMessage());}

com.mysql.jdbc.exceptions.MySQLSyntaxErrorException: You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near '? order by orders' at line 1    at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:936)    at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:2934)    at com.mysql.jdbc.MysqlIO.sendCommand(MysqlIO.java:1616)    at com.mysql.jdbc.MysqlIO.sqlQueryDirect(MysqlIO.java:1708)    at com.mysql.jdbc.Connection.execSQL(Connection.java:3249)    at com.mysql.jdbc.Connection.execSQL(Connection.java:3178)    at com.mysql.jdbc.Statement.executeQuery(Statement.java:1203)    at org.apache.commons.dbcp.DelegatingStatement.executeQuery(DelegatingStatement.java:205)    at com.wyx.common.database.impl.MysqlOperation.listProductChildNav(MysqlOperation.java:109)    at com.wyx.actions.admin.ProductNavAction.listChilds(ProductNavAction.java:134)    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.*/    public Object[] readNode(String nodeName) {        Object nodeValue[]=null;        NodeList nodelist = doc.getElementsByTagName(nodeName);        nodeValue=new Object[nodelist.getLength()];        //System.out.println("getLength:" + nodelist.getLength());        //System.out.println("getNodeName"+"\t"+"getNodeValue"+"\t"+"getAttribute");        for (int i = 0; i < nodelist.getLength(); i++) {            Element element =(Element)nodelist.item(i);            //System.out.println(element.getNodeName()+"\t"+element.getNodeValue()+"\t"+element.getAttribute("num"));            nodeValue[i]=element.getNodeValue();        }        return nodeValue;    }        /**     * 读取XML文件节点属性内容     * @param nodeName 节点名称     * @param Attribute 节点属性名称     * @return     */    public Object[] readNode(String nodeName,String Attribute) {        Object AttrValue[]=null;        NodeList nodelist = doc.getElementsByTagName(nodeName);        AttrValue=new Object[nodelist.getLength()];        for (int i = 0; i < nodelist.getLength(); i++) {            Element element =(Element)nodelist.item(i);            //System.out.println(element.getNodeName()+"\t"+element.getNodeValue()+"\t"+element.getAttribute("num"));            AttrValue[i]=element.getAttribute(Attribute);        }        return AttrValue;    }        public uDom() {        // TODO Auto-generated constructor stub        try {            this.readxml();        } catch (ParserConfigurationException e) {            e.printStackTrace();        } catch (SAXException e) {            e.printStackTrace();        } catch (IOException e) {            e.printStackTrace();        }    }    public void readxml() throws ParserConfigurationException, SAXException,            IOException {                String nodename = null;        short nodetype = 0;        String nodevalue = null;                DocumentBuilder dbf = DocumentBuilderFactory.newInstance().newDocumentBuilder();        System.out.println("Get DocumentBuilder Successful...");        Document doc = dbf.parse("d:/TestFile/links.xml");        System.out.println("Get Document Successful...");        NodeList nl1 = doc.getElementsByTagName("message");        System.out.println("Get NodeList(links) Successful...");        System.out.println("getLength:" + nl1.getLength());        Node node2 = doc.getFirstChild().getChildNodes().item(1);        // Node node3 = doc.getFirstChild();        Node node1 = nl1.item(4);        // Node node2 = doc.getFirstChild();        // Node node3 = node2.getFirstChild();        // Node node4 = node3.getFirstChild();        for (int i = 0; i < 2; i++) {            System.out.println(node2.getChildNodes().item(i).getNodeName());            System.out.println(node2.getChildNodes().item(i).getNodeValue());        }        // System.out.println(node2.getFirstChild().getNextSibling().getNodeName());        System.exit(0);        nodename = node1.getFirstChild().getNodeName();        nodetype = node1.getFirstChild().getNodeType();        nodevalue = node1.getFirstChild().getNodeValue();        System.out.println("getNodeName:" + nodename);        System.out.println("getNodeType:" + nodetype);        System.out.println("getNodeValue:" + nodevalue);    }    /**     * @param args     */    public static void main(String[] args) {        // TODO Auto-generated method stub        uDom udom = new uDom("xml/test.XML");        System.out.println("读取XML文件成功~~");        Object[] temp=udom.readNode("root","num");        System.out.println("获得节点长度为:"+temp.length);        for(int i=0;i<temp.length;i++) {            //temp[i]可能会得到空指针异常            System.out.println(i+" Value: "+temp[i].toString());        }    }}