java XStream 对象转xml字符串问题

public static String textMessageToXml(TextMessage textMessage){
XStream xStream = new XStream();
xStream.alias("xml",textMessage.getClass());
return xStream.toXML(textMessage);
}

    调用时报错

    public static void main(String args[]){

    TextMessage s=new TextMessage();
    s.setContent("woshineirong");
    s.setMsgType("text");
    s.setToUserName("xuzhongyao");
    s.setFromUserName("sdnaisdasd");
    s.setCreateTime(new Date().getTime());
    String tem = textMessageToXml(s);
    System.out.println(tem);
}
    main方法正常运行