private static String fromCollection(Collection<?> collection) throws Exception {
        String json = "";
        log.debug(String.format("method start Collection[collection:%s]", collection));
        if (collection != null && collection.size() > 0) {
            StringBuilder sb = new StringBuilder(collection.size() * 20); //假设一个元素平均长度为20
            for (Object obj : collection) {
                sb.append('<').append(Constant.XML_LIST_FLAG).append('>')
                    .append(fromObject(obj))
                    .append("</").append(Constant.XML_LIST_FLAG).append('>');  
            }
            json = sb.toString();
        } 
        log.debug(String.format("method stop return:String[%s]", json));
        return json;
    }