String xml 格式：

<?xml version = '1.0' encoding = 'gb2312'?>
<ROOT>

<ER>0</ER>

<MSG>null</MSG>
   <RESULT num="1">
      <C_ITEMNO>430300200400052</C_ITEMNO>
      <C_ITEMID>53</C_ITEMID>
      <C_ITEMNAME>12月19日测试</C_ITEMNAME>
      <C_INITPERMMITFILE NULL="TRUE"/>
      <C_YEARPLANNO1>123</C_YEARPLANNO1>
      <C_YEARPLANNO2>2004</C_YEARPLANNO2>
      <C_YEARPLANNO3>3</C_YEARPLANNO3>
      <C_CODE_ZIP>430300</C_CODE_ZIP>
      <C_ITEMADDR>中国</C_ITEMADDR>
      <C_CODE_ITEMPROP>1</C_CODE_ITEMPROP>
      <C_CODE_INVESTMENT>1</C_CODE_INVESTMENT>
      <C_CODE_FUNDSOURCE>1</C_CODE_FUNDSOURCE>
      <C_SHOPDRAWINGCENSORNO NULL="TRUE"/>
      <C_MAPNO NULL="TRUE"/>
      <C_WATERCITYINTERFACE NULL="TRUE"/>
      <C_GASCITYINTERFACE NULL="TRUE"/>
      <C_FRAMEACCOUNTEDITION NULL="TRUE"/>
      <C_LICENCENO NULL="TRUE"/>
      <D_INCEPTDATE>2001-01-01 00:00:00.0</D_INCEPTDATE>
      <C_MEMO NULL="TRUE"/>
      <D_OPERATE>2004-12-19 16:33:36.0</D_OPERATE>
      <C_OPERATER>111</C_OPERATER>
      <C_PUNISHNOTENO NULL="TRUE"/>
   </RESULT>
</ROOT>

异常信息为：XML file has no top-element.  All well-formed XML files have a single top-level element.

把这个XML拷贝出来放在一个xxx.xml文件中，再来用dom4j解析就完全没有问题。
    File f = new File("xxx.xml");
    SAXReader reader = new SAXReader();
    Document document = reader.read(f);

谢谢。