Problem in save signature given from user

Question

I have a problem with save signature given from user in internal memory. Signature is created and I want to save as a png file in sinuture folder in internal storage. But my codes dont work and thats not be saved. this is my codes:

 String DIRECTORY = Environment.getRootDirectory().getPath() + "/Signature/";
String pic_name = new SimpleDateFormat("yyyyMMdd_HHmmss", Locale.getDefault()).format(new Date());
String StoredPath = DIRECTORY + pic_name + ".png";

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    canvasLL = (LinearLayout) findViewById(R.id.canvasLL);
    mSignature = new signature(getApplicationContext(), null);
    mSignature.setBackgroundColor(Color.WHITE);
    // Dynamically generating Layout through java code
    canvasLL.addView(mSignature, ViewGroup.LayoutParams.MATCH_PARENT, ViewGroup.LayoutParams.MATCH_PARENT);
    btnClear = (Button) findViewById(R.id.btnclear);
    btnSave = (Button) findViewById(R.id.btnsave);
    view = canvasLL;

    btnClear.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            mSignature.clear();
        }
    });

    btnSave.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {

            view.setDrawingCacheEnabled(true);
            mSignature.save(view,StoredPath);
            Toast.makeText(getApplicationContext(), "Successfully Saved", Toast.LENGTH_SHORT).show();

        }
    });

    // Method to create Directory, if the Directory doesn't exists
    file = new File(DIRECTORY);
    if (!file.exists()) {
        file.mkdir();
    }

}

public class signature extends View {

    private static final float STROKE_WIDTH = 5f;
    private static final float HALF_STROKE_WIDTH = STROKE_WIDTH / 2;
    private final Paint paint = new Paint();
    private final Path path = new Path();
    private float lastTouchX;
    private float lastTouchY;
    private final RectF dirtyRect = new RectF();

    public signature(Context context, AttributeSet attrs) {
        super(context, attrs);
        paint.setAntiAlias(true);
        paint.setColor(Color.BLACK);
        paint.setStyle(Paint.Style.STROKE);
        paint.setStrokeJoin(Paint.Join.ROUND);
        paint.setStrokeWidth(STROKE_WIDTH);
    }

    public void save(View v, String StoredPath) {
        Log.v("log_tag", "Width: " + v.getWidth());
        Log.v("log_tag", "Height: " + v.getHeight());
        if (bitmap == null) {
            bitmap = Bitmap.createBitmap(canvasLL.getWidth(), canvasLL.getHeight(), Bitmap.Config.RGB_565);
        }
        Canvas canvas = new Canvas(bitmap);
        try {
            // Output the file
            FileOutputStream mFileOutStream = new FileOutputStream(StoredPath);
            v.draw(canvas);

            // Convert the output file to Image such as .png
            bitmap.compress(Bitmap.CompressFormat.PNG, 90, mFileOutStream);

            mFileOutStream.flush();
            mFileOutStream.close();

        } catch (Exception e) {
            Log.v("log_tag", e.toString());
        }

    }

    public void clear() {
        path.reset();
        invalidate();

    }

    @Override
    protected void onDraw(Canvas canvas) {
        canvas.drawPath(path, paint);
    }

    @SuppressLint("ClickableViewAccessibility")
    @Override
    public boolean onTouchEvent(MotionEvent event) {
        float eventX = event.getX();
        float eventY = event.getY();

        switch (event.getAction()) {
            case MotionEvent.ACTION_DOWN:
                path.moveTo(eventX, eventY);
                lastTouchX = eventX;
                lastTouchY = eventY;
                return true;

            case MotionEvent.ACTION_MOVE:

            case MotionEvent.ACTION_UP:

                resetDirtyRect(eventX, eventY);
                int historySize = event.getHistorySize();
                for (int i = 0; i < historySize; i++) {
                    float historicalX = event.getHistoricalX(i);
                    float historicalY = event.getHistoricalY(i);
                    expandDirtyRect(historicalX, historicalY);
                    path.lineTo(historicalX, historicalY);
                }
                path.lineTo(eventX, eventY);
                break;

            default:
                debug("Ignored touch event: " + event.toString());
                return false;
        }

        invalidate((int) (dirtyRect.left - HALF_STROKE_WIDTH),
                (int) (dirtyRect.top - HALF_STROKE_WIDTH),
                (int) (dirtyRect.right + HALF_STROKE_WIDTH),
                (int) (dirtyRect.bottom + HALF_STROKE_WIDTH));

        lastTouchX = eventX;
        lastTouchY = eventY;

        return true;
    }

    private void debug(String string) {

        Log.v("log_tag", string);

    }

    private void expandDirtyRect(float historicalX, float historicalY) {
        if (historicalX < dirtyRect.left) {
            dirtyRect.left = historicalX;
        } else if (historicalX > dirtyRect.right) {
            dirtyRect.right = historicalX;
        }

        if (historicalY < dirtyRect.top) {
            dirtyRect.top = historicalY;
        } else if (historicalY > dirtyRect.bottom) {
            dirtyRect.bottom = historicalY;
        }
    }

    private void resetDirtyRect(float eventX, float eventY) {
        dirtyRect.left = Math.min(lastTouchX, eventX);
        dirtyRect.right = Math.max(lastTouchX, eventX);
        dirtyRect.top = Math.min(lastTouchY, eventY);
        dirtyRect.bottom = Math.max(lastTouchY, eventY);
    }
}

whould it be possible any body help me to save this sinature as a png file