code: public static String outputStream2String(final OutputStream out, final String charsetName) {
          if (out == null || isSpace(charsetName)) return null;
          try {
              return new String(outputStream2Bytes(out), charsetName);
          } catch (UnsupportedEncodingException e) {
              e.printStackTrace();
              return null;
          }
      }
docs: outputStream转string按编码
  
  @param out         输出流
  @param charsetName 编码格式
  @return 字符串