LinearLayout、RelativeLayout、FrameLayout居中显示

LinearLayout、RelativeLayout、FrameLayout居中显示需要设置的属性不一样

布局：

<?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>