protected void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_home);


        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {
            requestStoragePermission();
        }

        dirPath = Environment.getExternalStorageDirectory().getPath() + "/GACI";

        File projDir = new File(dirPath);
        if (!projDir.exists())
            projDir.mkdirs();
...
     }

private void requestStoragePermission() {
    if (ContextCompat.checkSelfPermission(this, Manifest.permission.READ_EXTERNAL_STORAGE) == PackageManager.PERMISSION_GRANTED)
        return;

    if (ActivityCompat.shouldShowRequestPermissionRationale(this, Manifest.permission.READ_EXTERNAL_STORAGE)) {
        //If the user has denied the permission previously your code will come to this block
        //Here you can explain why you need this permission
        //Explain here why you need this permission
    }
    //And finally ask for the permission
    ActivityCompat.requestPermissions(this, new String[]{Manifest.permission.READ_EXTERNAL_STORAGE}, STORAGE_PERMISSION_CODE);
}

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