Android数据存储（External Storage,外部存储）

External Storage

 

所有Android设备都支持可以保存文件的共享外部存储器，这个外部存储器可以是可移动存储器（如SD卡），也可以是内置在设备中的外部存储器（不可移动）。外部存储器上的文件时全部可读的，当设备通过USB连接电脑和电脑互传文件时，外部存储器上的文件不可修改。

当外部存储器被挂载到电脑上或被移除，文件对android设备就不可见了，且此时外部存储器上的文件是没有安全保障的。所有程序都可以读写外部存储器上的文件，用户也可以删除这些文件。

检查存储器的可用性

对存储器进行任何操作之前，为了防止存储设备因挂载到电脑、丢失、只可读或其他原因造成无法使用，必须调用getExternalStorageState() 方法来检查存储设备是否可用。下面的代码演示了如何检查其可用性（可读可写）：

booleanmExternalStorageAvailable = false;
booleanmExternalStorageWriteable = false;
Stringstate = Environment.getExternalStorageState();

if (Environment.MEDIA_MOUNTED.equals(state)) {
    // We can read and write the media
    mExternalStorageAvailable =mExternalStorageWriteable = true;
} else if (Environment.MEDIA_MOUNTED_READ_ONLY.equals(state)) {
    // We can only read the media
    mExternalStorageAvailable = true;
    mExternalStorageWriteable = false;
} else {
    // Something else is wrong. It may be one of many other states, but all we      //need
    //  to know is we can neither read nor write
    mExternalStorageAvailable =mExternalStorageWriteable = false;
}

如果你是用API Level 8或更高的版本，调用getExternalFilesDir()来打开外部存储器中你想用来保存文件的目录。这个方法需要一个指定子目录类型的参数，如DIRECTORY_MUSIC和DIRECTORY_RINGTONES.如果相应的目录不存在，该方法将在必要时创建所需的目录。通过指定目录的类型，可以保证android媒体扫描器可以方便的给系统中的文件归类（如，铃声文件归为铃声而不是音乐）。程序被卸载后，相应目录及里面的内容都会被删除。

如果是用API Level 7或者更老的版本，就需要调用getExternalStorageDirectory()来获取外部存储器的根目录。向外部存储器写入数据时的目录如下：

/Android/data/<package_name>/files/

如果希望外部存储器上的文件不是只对指定的程序有用，并且不会因为卸载程序而被删除，就应该把这些文件保存在外部存储器的公共目录下，这些目录在外部存储器的根目录下，如Music/, Pictures/, Ringtones/等。

API Level 8及更高版本中，调用getExternalStoragePublicDirectory(),如前面所述，传递一个目录的类型参数，该方法就会创建相应的目录。类型参数可以为：

DIRECTORY_MUSIC, DIRECTORY_PODCASTS,DIRECTORY_RINGTONES, DIRECTORY_ALARMS, DIRECTORY_NOTIFICATIONS,DIRECTORY_PICTURES,DIRECTORY_MOVIES,IRECTORY_DOWNLOADS, or DIRECTORY_DCIM.

API Level 7及较低版本中，调用getExternalStorageDirectory()来获取外部存储器的跟目录，然后就可以把文件保存在以下相应的文件夹中：

·        Music/ - Media scanner classifies all media found here as user music.

·        Podcasts/ - Media scanner classifies all media found here asa podcast.

·        Ringtones/ - Media scanner classifies all media found here as aringtone.

·        Alarms/ - Media scanner classifies all media found here asan alarm sound.

·        Notifications/ - Media scanner classifies all media found here asa notification sound.

·        Pictures/ - All photos (excluding those taken with the camera).

·        Movies/ - All movies (excluding those taken with thecamcorder).

·        Download/ - Miscellaneous downloads.

有关getExternalStoragePublicDirectory(String type)和getExternalStorageDirectory()的使用，下面给出官方文档上的例子。

publicstatic File getExternalStoragePublicDirectory (String type)

void createExternalStoragePublicPicture() {

   // Create a path where we will place our picture in the user's

   // public pictures directory. Note that you should be careful about

   // what you place here, since the user often manages these files.  For

   // pictures and other media owned by the application, consider

   // Context.getExternalMediaDir().

    File path =Environment.getExternalStoragePublicDirectory(

           Environment.DIRECTORY_PICTURES);

   File file = new File(path, "DemoPicture.jpg");

   try {

       // Make sure the Pictures directory exists.

       path.mkdirs();

       // Very simple code to copy a picture from the application's

       // resource into the external file. Note that this code does

       // no error checking, and assumes the picture is small (does not

       // try to copy it in chunks). Note that if external storage is

       // not currently mounted this will silently fail.

       InputStream is = getResources().openRawResource(R.drawable.balloons);

       OutputStream os = new FileOutputStream(file);

       byte[] data = new byte[is.available()];

        is.read(data);

       os.write(data);

       is.close();

       os.close();

       // Tell the media scanner about the new file so that it is

       // immediately available to the user.

       MediaScannerConnection.scanFile(this,

                new String[] { file.toString() }, null,

                newMediaScannerConnection.OnScanCompletedListener() {

           public void onScanCompleted(String path, Uri uri) {

               Log.i("ExternalStorage", "Scanned " + path +":");

               Log.i("ExternalStorage", "-> uri=" + uri);

           }

       });

    }catch (IOException e) {

       // Unable to create file, likely because external storage is

       // not currently mounted.

       Log.w("ExternalStorage", "Error writing " + file,e);

    }

}

 

void deleteExternalStoragePublicPicture() {

   // Create a path where we will place our picture in the user's

   // public pictures directory and delete the file.  If external

   // storage is not currently mounted this will fail.

   File path = Environment.getExternalStoragePublicDirectory(

           Environment.DIRECTORY_PICTURES);

   File file = new File(path, "DemoPicture.jpg");

   file.delete();

}

 

boolean hasExternalStoragePublicPicture() {

   // Create a path where we will place our picture in the user's

   // public pictures directory and check if the file exists.  If

   // external storage is not currently mounted this will think the

   // picture doesn't exist.

   File path = Environment.getExternalStoragePublicDirectory(

           Environment.DIRECTORY_PICTURES);

   File file = new File(path, "DemoPicture.jpg");

   return file.exists();

}     

publicstatic File getExternalStorageDirectory ()

BroadcastReceiver mExternalStorageReceiver;

boolean mExternalStorageAvailable = false;

boolean mExternalStorageWriteable = false;

 

void updateExternalStorageState() {

   String state = Environment.getExternalStorageState();

   if (Environment.MEDIA_MOUNTED.equals(state)) {

       mExternalStorageAvailable = mExternalStorageWriteable = true;

    }else if (Environment.MEDIA_MOUNTED_READ_ONLY.equals(state)) {

       mExternalStorageAvailable = true;

        mExternalStorageWriteable = false;

    }else {

       mExternalStorageAvailable = mExternalStorageWriteable = false;

    }

   handleExternalStorageState(mExternalStorageAvailable,

           mExternalStorageWriteable);

}

 

void startWatchingExternalStorage() {

   mExternalStorageReceiver = new BroadcastReceiver() {

       @Override

       public void onReceive(Context context, Intent intent) {

           Log.i("test", "Storage: " + intent.getData());

           updateExternalStorageState();

        }

   };

   IntentFilter filter = new IntentFilter();

   filter.addAction(Intent.ACTION_MEDIA_MOUNTED);

   filter.addAction(Intent.ACTION_MEDIA_REMOVED);

   registerReceiver(mExternalStorageReceiver, filter);

   updateExternalStorageState();

}

 

void stopWatchingExternalStorage() {

   unregisterReceiver(mExternalStorageReceiver);

}