Android初学习 - LinearLayout、RelativeLayout、FrameLayout内容居中显示

XML如下：

 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>