Android : Focus on Spinner not Work

I have a spinner in my activity and i want that it open the select menu as soon as the activity starts

This is my layout excerpt that the Spinner appears

<LinearLayout 
    android:layout_width="fill_parent"
    android:layout_height="0dp"
    android:background="#fff"
    android:layout_weight="5">

    <Spinner
        android:id="@+id/spinner"
        android:layout_width="fill_parent"
        android:layout_height="fill_parent"
        android:prompt="@string/app_name"
        android:gravity="center_vertical"/>

</LinearLayout>

And this is my class that should open the Menu

Spinner spn = (Spinner) findViewById(R.id.spinner);

spn.setFocusable(true);
spn.setFocusableInTouchMode(true);

spn.requestFocus();