//Java对象转化json格式
    public static void toJsonByJettisonMappedXmlDriver(){
        try {
            User user = getUser();
            FileOutputStream fileOutputStream = new FileOutputStream(file);
            OutputStreamWriter ow = new OutputStreamWriter(fileOutputStream, Charset.forName("UTF-8"));
            xstream = new XStream(new JettisonMappedXmlDriver());
            xstream.setMode(XStream.NO_REFERENCES);
            xstream.alias("user", User.class);
            xstream.toXML(user,ow);
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        }
    }