static void deleteContents(File dir) throws IOException {
        File[] files = dir.listFiles();
        if (files == null) {
            throw new IOException("not a readable directory: " + dir);
        }
        for (File file : files) {
            if (file.isDirectory()) {
                deleteContents(file);
            }
            if (!file.delete()) {
                throw new IOException("failed to delete file: " + file);
            }
        }
    }
Deletes the contents of {@code dir}. Throws an IOException if any file
could not be deleted, or if {@code dir} is not a readable directory.
VideoSqlLite/src/main/java/com/yc/videosqllite/disk/DiskUtils.java