写入外部存储权限始终被拒绝

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

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

@Override
public void onRequestPermissionsResult(int requestCode, String[] permissions, int[] grantResults) {
    if (requestCode == REQUEST_CODE) {
        if (grantResults.length > 0 && grantResults[0] == PackageManager.PERMISSION_GRANTED) {
            // 权限已授予，可以进行写入外部存储的操作
        } else {
            // 权限被拒绝，无法进行写入外部存储的操作
        }
    }
}