Gravity is not set on Relative Layout

I am tackling relativeLayout problem in my application. I take one image and button. I want to make button is bottom of the image but is not set.

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent">
<ImageButton android:src="@drawable/layout_logo"
        android:layout_width="fill_parent"
        android:layout_height="fill_parent"
        android:id="@+id/image" />

<Button android:id="@+id/button3"
        android:layout_height="wr开发者_开发技巧ap_content"
        android:layout_width="fill_parent"
        android:drawableBottom="@id/image"
        android:gravity="bottom" />
</RelativeLayout>

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RelativeLayout uses android:layout_above and android:layout_below to position elements with regard to others. Changing your android:gravity="bottom" to android:layout_below="@id/image" should fix this.