在android中获取权限和文件夹中的文件后，仍未获取文件夹中的文件

<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE"/>

if (ContextCompat.checkSelfPermission(this, Manifest.permission.READ_EXTERNAL_STORAGE) != PackageManager.PERMISSION_GRANTED) {
    ActivityCompat.requestPermissions(this, new String[]{Manifest.permission.READ_EXTERNAL_STORAGE}, PERMISSION_REQUEST_CODE);
}

String folderPath = Environment.getExternalStorageDirectory().getPath() + "/文件夹名称/";
String filePath = folderPath + "文件名.txt";
File file = new File(filePath);

File folder = new File(folderPath);
if (!folder.exists()) {
    folder.mkdirs();
}

File file = new File(filePath);
file.setReadable(true);