LinearLayout、RelativeLayout、FrameLayout内容居中显示

<?xml version="1.0" encoding="utf-8"?> 
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" 
    android:orientation="vertical" 
    android:layout_width="fill_parent" 
    android:layout_height="fill_parent" 
    > 
    <LinearLayout  
        android:layout_width="fill_parent" 
        android:layout_height="wrap_content" 
        android:gravity="center_horizontal" 
        android:orientation="vertical"> 
        <ImageView  
            android:layout_width="wrap_content" 
            android:layout_height="wrap_content" 
            android:background="@drawable/icon" 
            /> 
    </LinearLayout> 
    <FrameLayout  
        android:layout_width="fill_parent" 
        android:layout_height="wrap_content"> 
        <ImageView  
            android:layout_width="wrap_content" 
            android:layout_height="wrap_content" 
            android:background="@drawable/icon" 
            android:layout_gravity="center" 
            /> 
    </FrameLayout> 
    <RelativeLayout  
        android:layout_width="fill_parent" 
        android:layout_height="wrap_content"> 
        <ImageView  
            android:layout_width="wrap_content" 
            android:layout_height="wrap_content" 
            android:background="@drawable/icon" 
            android:layout_centerInParent="true" 
            /> 
    </RelativeLayout> 
</LinearLayout>