java 在解析xml成对象的时候，有什么办法将自闭和元素设置为null，而不是new Object

在用jdk自带的JAXB和xstream解析xml的时候，如果遇到这样的元素。转为对象后，变成new Object();
能否通过什么方式，让他变为null.

 @XStreamAlias("root")
public class RootBean {
    @XStreamAlias("DocInfo")
    private DocInfoBean docInfo;
    @XStreamAlias("PAGES")
    private List<PageBean> pages;
    public DocInfoBean getDocInfo() {
        return docInfo;
    }
    public void setDocInfo(DocInfoBean docInfo) {
        this.docInfo = docInfo;
    }
    public List<PageBean> getPages() {
        return pages;
    }
    public void setPages(List<PageBean> pages) {
        this.pages = pages;
    }

    //getter和setter


}

 public class DocInfoBean {
    @XStreamAlias("APP_CODE")
    private String appCode;
    @XStreamAlias("APP_NAME")
    private String appName;
    @XStreamAlias("BUSI_NO")
    private String busiNo;
        //getter和setter
        ...
        }

 @XStreamAlias("PAGE")
public class PageBean {
    @XStreamAlias("DOC_TYPE")
    @XStreamAsAttribute
    private String docType;
    @XStreamAlias("DOC_NAME")
    @XStreamAsAttribute
    private String docName;
    @XStreamAlias("PAGE_URL")
    @XStreamAsAttribute
    private String pageUrl;
    @XStreamAlias("PAGE_NAME")
    @XStreamAsAttribute
    private String pageName;
    @XStreamAlias("UP_USER")
    @XStreamAsAttribute
    private String upUser;
    @XStreamAlias("UP_ORG")
    @XStreamAsAttribute
    private String upOrg;
    @XStreamAlias("UP_TIME")
    @XStreamAsAttribute
    private String upTime;
        //getter和setter
        }

解析方法：

 private static RootBean getRootBean(String xml)  {
        InputStreamReader reader = null;
        XStream xStream=new XStream(new DomDriver("UTF-8", new XmlFriendlyNameCoder("-_", "_")));
        xStream.alias("root", RootBean.class);
        xStream.autodetectAnnotations(true);
        return (RootBean) xStream.fromXML(xml);
    }

需要解析的xml为：

<root>
  <DocInfo/>
  <PAGES>
    <PAGE DOC_TYPE="010" DOC_NAME="身份证" PAGE_URL="1111.jpg" PAGE_NAME="0000.jpg" UP_USER="12433124" UP_ORG="惠普" UP_TIME="34543312321"/>
    <PAGE DOC_TYPE="010" DOC_NAME="身份证" PAGE_URL="1111.jpg" PAGE_NAME="0000.jpg" UP_USER="12433124" UP_ORG="惠普" UP_TIME="34543312321"/>
    <PAGE DOC_TYPE="010" DOC_NAME="身份证" PAGE_URL="1111.jpg" PAGE_NAME="0000.jpg" UP_USER="12433124" UP_ORG="惠普" UP_TIME="34543312321"/>
  </PAGES>
</root>

http://blog.csdn.net/hingwu/article/details/1497892