Android地图没有显示？

Question

我尝试制作一个简单的地图视图，但它只显示了灰色网格。

activity_main.xml中

   <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:paddingBottom="@dimen/activity_vertical_margin"
       android:paddingLeft="@dimen/activity_horizontal_margin"
       android:paddingRight="@dimen/activity_horizontal_margin"
       android:paddingTop="@dimen/activity_vertical_margin"
       tools:context=".MainActivity" >  

       <com.google.android.maps.MapView
             android:id="@+id/mapview"
             android:layout_width="fill_parent"
             android:layout_height="fill_parent"
             android:enabled="true"
             android:clickable="true"
            android:apiKey="AIzaSyCc0MsNEdpsfiEQYx5DLol7AWKizrBIKB4"
       />

    </RelativeLayout>

MainActivity.java

    package com.example.mapapp;

    import com.google.android.maps.MapActivity;
    import com.google.android.maps.MapView;
    import android.os.Bundle;
    import android.view.Menu;

    public class MainActivity extends MapActivity {

      @Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.activity_main);
        }

        @Override
        public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
         getMenuInflater().inflate(R.menu.main, menu);
          return true;
       }

        @Override
        protected boolean isRouteDisplayed() {
          // TODO Auto-generated method stub
           return false;
        }

   }

AndroidManifest

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.mapapp"
    android:versionCode="1"
    android:versionName="1.0" >

    <uses-sdk
        android:minSdkVersion="7"
        android:targetSdkVersion="17" />

     <uses-permission android:name="android.permission.INTERNET" />
     <application
           android:allowBackup="true"
           android:icon="@drawable/ic_launcher"
           android:label="@string/app_name"
           android:theme="@style/AppTheme" >
          <uses-library android:name="com.google.android.maps" />
          <activity
               android:name="com.example.mapapp.MainActivity"
               android:label="@string/app_name" >
               <intent-filter>
                   <action android:name="android.intent.action.MAIN" />
                   <category android:name="android.intent.category.LAUNCHER" />
               </intent-filter>
          </activity>
      </application>

   </manifest>

它只显示灰色网格视图。 如果我错了，请给我解决方案？

输出： -