code: public static List<String> scanFolder(File folder) {
          String parent = folder.getAbsolutePath() + ValueConsts.SEPARATOR;
          List<String> list = new ArrayList<>(16);
          if (folder.isDirectory()) {
              String[] children = folder.list();
              if (Checker.isNotNull(children)) {
                  for (String child : children) {
                      File file = new File(parent + child);
                      if (file.isDirectory()) {
                          list.addAll(scanFolder(file));
                      } else {
                          list.add(file.getAbsolutePath());
                      }
                  }
              }
          }
          return list;
      }
docs: 扫描文件夹下面所有文件
  
  @param folder 文件夹
  
  @return 文件路径列表