Android之背景图片设置为重复而不是默认的拉伸

<?xml version="1.0" encoding="utf-8"?>
<bitmap xmlns:android="http://schemas.android.com/apk/res/android"
android:src="@drawable/ic_launcher"
android:tileMode="repeat" >
</bitmap>

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:background="@drawable/bitmap_bg">
</RelativeLayout>