public void getXmlNodeValue(){

    String xmlString="<xml><A><aa></aa></A>...</xml>";

    //1.将xml document对象。
    Document doc=string2XmlDocument(result);

    //2.获取指定的node
   String result=null;
    Node aaNode=doc.selectSingleNode("//A/aa");
    if(aaNode!=null){
        result=aaNode.getStringValue();
    }
    System.out.pringln("aa节点值："+result);

}

public static Document string2XmlDocument(String xmlString){
        if(StringUtils.isBlank(xmlString)){
            return null;
        }
        logger.info(xmlString);
        SAXReader saxReader = new SAXReader();
        Document document=null;
        try {
            document = saxReader.read(new ByteArrayInputStream(xmlString.getBytes(AlipayConfig.input_charset)));
        } catch (Exception e) {
            logger.error(" string 2 xml document error:"+e);
        }
        return document;
    }