当手机开机、插拔USB连接时，Android系统会启动MediaScanner，扫描SD卡和内存里面的文件，扫描的结果保存在data/data/com.android.providers.media/databases/external.db下。通过DDMS可以找到，数据库中包含了图片、视频、音乐等文件的信息。开发者不用遍历文件，只要使用ContentProvider就可以获取SD卡中不同种类的文件列表。

数据库external.db：
sqlite3 external.db
.tables
所有的表：
album_art              audio_genres           images               
album_info             audio_genres_map       log                  
albums                 audio_genres_map_noid  search               
android_metadata       audio_meta             searchhelpertitle    
artist_info            audio_playlists        semc_metadata        
artists                audio_playlists_map    semc_metadata_version
artists_albums_map     files                  thumbnails           
audio                  files_ext              video                
audio_ext              files_with_ext         videothumbnails

选择images并查询；

.mode column         (.mode column 将设置为列显示模式，.header 将显示列名)

.header ON
select * from images;

结果如下：
_id|_data|_size|_display_name|mime_type|title|date_added|date_modified|description|picasa_id|isprivate|latitude|longitude|datetaken|orientation|mini_thumb_magic|bucket_id|bucket_display_name|width|height
86|/mnt/sdcard/DCIM/100ANDRO/DSC_0001.jpg|607597|DSC_0001.jpg|image/jpeg|DSC_0001|1371782044|1361346484||||||1361346484000|90|7241443144086764191|1278012876|100ANDRO|2048|1536

示例代码：

列表界面：

public class PlayerActivity extends Activity {
private ListView listView;
private CursorAdapter adapter;
private Cursor cursor;
private String path;
@SuppressWarnings("deprecation")
@Override
protected void onCreate(Bundle savedInstanceState) {
// TODO Auto-generated method stub
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
listView = (ListView)findViewById(R.id.listView);
ContentResolver contentResolver = getContentResolver();
cursor = contentResolver.query(MediaStore.Images.Media.EXTERNAL_CONTENT_URI,
null, null, null, null);
adapter = new SimpleCursorAdapter(this, android.R.layout.simple_list_item_2, cursor,
new String[] {
MediaStore.Images.ImageColumns.TITLE, MediaStore.Images.ImageColumns.SIZE
}, new int[] {
android.R.id.text1, android.R.id.text2
});
listView.setAdapter(adapter);
listView.setOnItemClickListener(new OnItemClickListener() {
public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
// TODO Auto-generated method stub
try {
if (cursor.moveToPosition(position)) {
path = cursor.getString(cursor.getColumnIndex("_data"));
}
} catch (Exception e) {
// TODO: handle exception
e.printStackTrace();
}
Intent intent = new Intent(PlayerActivity.this, ShowActivity.class);
intent.putExtra("path", path);
startActivity(intent);
}
});
}
@Override
protected void onDestroy() {
// TODO Auto-generated method stub
super.onDestroy();
cursor.close();
}
}

显示界面：

public class ShowActivity extends Activity {
private ImageView imageView;
@Override
protected void onCreate(Bundle savedInstanceState) {
// TODO Auto-generated method stub
super.onCreate(savedInstanceState);
this.requestWindowFeature(Window.FEATURE_NO_TITLE);
setContentView(R.layout.show);
imageView = (ImageView)findViewById(R.id.show);
Intent intent = getIntent();
String path = intent.getStringExtra("path");
BitmapDrawable drawable = new BitmapDrawable(getResources(), path);
imageView.setImageDrawable(drawable);
}
}

MediaStore：基于SQLite的多媒体数据库，包含了视频、音频、图片等所有的多媒体文件的信息。
MediaStore.Audio：音频信息
MediaStore.Files：非媒体信息
MediaStore.Images：图片信息
MediaStore.Video：视频信息
MediaStore.MediaColumns：部分MediaProvider表的常用列