code: public String listFolder(ListFolderRequest request) throws AbstractCosException {
  
  		request.check_param();
  		request.setCosPath(request.getCosPath() + request.getPrefix());
  		String url = buildUrl(request);
  		long signExpired = System.currentTimeMillis() / 1000 + this.config.getSignExpired();
  		String sign = Sign.getPeriodEffectiveSign(request.getBucketName(), request.getCosPath(), this.cred, signExpired);
  
  		HttpRequest httpRequest = new HttpRequest();
  		httpRequest.setUrl(url);
  		httpRequest.addHeader(RequestHeaderKey.Authorization, sign);
  		httpRequest.addHeader(RequestHeaderKey.USER_AGENT, this.config.getUserAgent());
  		httpRequest.addParam(RequestBodyKey.OP, RequestBodyValue.OP.LIST);
  		httpRequest.addParam(RequestBodyKey.NUM, String.valueOf(request.getNum()));
  		httpRequest.addParam(RequestBodyKey.CONTEXT, request.getContext());
  		httpRequest.addParam(RequestBodyKey.ORDER, String.valueOf(request.getOrder().ordinal()));
  		httpRequest.addParam(RequestBodyKey.PATTERN, request.getPattern().toString());
  		httpRequest.setMethod(HttpMethod.GET);
  
  		return httpClient.sendHttpRequest(httpRequest);
      }
docs: 获取目录列表请求
  
  @param request
  获取目录列表请求
  @return JSON格式的字符串, 格式为{"code":$code, "message":"$mess"}, code为0表示成功,
  其他为失败, message为success或者失败原因
  @throws AbstractCosException
  SDK定义的COS异常, 通常是输入参数有误或者环境问题(如网络不通)