XStream序列化时字段怎样添加type?

实体类如下

 public class Root {
    private String REQUESTID;       
    private String REQUESTTIME;     
    private String REQUESTSERVICE;  
    private String ACCNBR;          
    private String QRYBEGINDATE;    
    private String QRYENDDATE;  
    /*
        get set省略。。。
    */
    public String getXml(Root root){
        XStream xstream = new XStream();
        xstream.alias("ROOT", Root.class); 
        String xml = xstream.toXML(root);
        return "<?xml version=\"1.0\" encoding=\"utf-8\"?>"+xml;
    }
}

getXml序列化得到的xml是这样的

 <?xml version="1.0" encoding="utf-8"?><ROOT>
  <REQUESTID>xxx</REQUESTID>
  <REQUESTTIME>xxx</REQUESTTIME>
  <REQUESTSERVICE>xxx</REQUESTSERVICE>
  <ACCNBR>xxx</ACCNBR>
  <QRYBEGINDATE>xxx</QRYBEGINDATE>
  <QRYENDDATE>xxx</QRYENDDATE>
</ROOT>

但是想要的效果是这样的

 <?xml version="1.0" encoding="UTF-8"?>
<ROOT>
  <REQUESTID  type="string"> xxx</REQUESTID>    
  <REQUESTTIME type="string">xxx</REQUESTTIME>      
  <REQUESTSERVICE type="string">xxx</REQUESTSERVICE>
  <ACCNBR type="string">xxx</ACCNBR>                
  <QRYBEGINDATE type="string">xxx</QRYBEGINDATE>        
  <QRYENDDATE type="string">xxx</QRYENDDATE>                
</ROOT>

请问怎么在字段里添加type=“”？
还有就是这样的xml在反序列化时怎么再生成实体类？

http://www.blogjava.net/DLevin/archive/2012/11/30/392240.html