Android中关于外部存储的一些重要函数

桃扇骨 2022-03-26 08:26 248阅读 0赞

**一、简介**

关于Android的数据外部存储，在API Level 8之前，所有的文件都是建议放在 Environment . [getExternalStorageState()][getExternalStorageState] 目录下的；从API Level 8开始，对于应用程序的私有文件应该放在 [Context.getExternalFilesDir][] 目录下，非私有的（shared）的文件应该放在目录下 Environment. [getExternalStoragePublicDirectory(String)][getExternalStoragePublicDirectory_String] 所指定的目录下。对于缓存文件应该放在 Context. getExternalCacheDir() 目录下。另外在准备把数据保存外部存储之前应该先通过 Environment . [getExternalStorageState()][getExternalStorageState] 获取其状态，再根据其状态确定其是否可用，如果不可用，可以考虑将数据保存在内部存储中。

关于Android的内部存储可以参照《 [Android中关于内部存储的一些重要函数][Android] 》

**二、** Context 中关于外部存储的 重要函数

#### public abstract [File][] getExternalCacheDir () ####

Since:  [API Level 8][]

Returns the absolute path to the directory on the external filesystem (that is somewhere on `Environment.getExternalStorageDirectory()` where the application can place cache files it owns.

This is like `getCacheDir()` in that these files will be deleted when the application is uninstalled, however there are some important differences:

该函数像`getCacheDir()` 一样，保存在该目录下的文件会在卸载该应用程序时被删除。然而又有以下几点不同：

*  The platform does not monitor the space available in external storage, and thus will not automatically delete these files. Note that you should be managing the maximum space you will use for these anyway, just like with `getCacheDir()`. 系统并不会监视外部存储的可用容量。因此程序员应该自己管理其占用的最大空间，自己做文件的清理机制
 *  External files are not always available: they will disappear if the user mounts the external storage on a computer or removes it. See the APIs on`Environment` for information in the storage state.
 *  There is no security enforced with these files. All applications can read and write files placed here. 系统对该目录下的文件并没做安全访问机制，所有的应用程序都可以读写该目录下的文件。

##### Returns #####

*  Returns the path of the directory holding application cache files on external storage. Returns null if external storage is not currently mounted so it could not ensure the path exists; you will need to call this method again when it is available.

##### See Also #####

*  `getCacheDir()`

#### public abstract [File][] getExternalFilesDir ([String][] type) ####

Since:  [API Level 8][]

Returns the absolute path to the directory on the external filesystem (that is somewhere on `Environment.getExternalStorageDirectory()`) where the application can place persistent files it owns. These files are private to the applications, and not typically visible to the user as media.

该目录下的文件对其应用程序是私有的。其目录下的文件也不会被当做多媒体文件而为用户所见

This is like `getFilesDir()` in that these files will be deleted when the application is uninstalled, however there are some important differences:

该函数像 [getFilesDir()][getFilesDir]  一样，保存在该目录下的文件会在卸载该应用程序时被删除。然而又有以下几点不同：

*  External files are not always available: they will disappear if the user mounts the external storage on a computer or removes it. See the APIs on`Environment` for information in the storage state.
 *  There is no security enforced with these files. All applications can read and write files placed here. 系统对该目录下的文件并没做安全访问机制，所有的应用程序都可以读写该目录下的文件。

Here is an example of typical code to manipulate a file in an application's private storage:

void  createExternalStoragePrivateFile ()  \{

/\* Create a path where we will place our private file on external

storage.

\*/

File  file  =  new  File ( getExternalFilesDir ( null ),  "DemoFile.jpg" );

try  \{

/\* 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 ();

\}  catch  ( IOException  e )  \{

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

not currently mounted.

\*/

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

void  deleteExternalStoragePrivateFile ()  \{

/\* Get path for the file on external storage.  If external

storage is not currently mounted this will fail.

\*/

File  file  =  new  File ( getExternalFilesDir ( null ),  "DemoFile.jpg" );

if  ( file  !=  null )  \{

file . delete ();

boolean  hasExternalStoragePrivateFile ()  \{

/\* Get path for the file on external storage.  If external

storage is not currently mounted this will fail.

\*/

File  file  =  new  File ( getExternalFilesDir ( null ),  "DemoFile.jpg" );

if  ( file  !=  null )  \{

return  file . exists ();

return  false ;

If you supply a non-null type to this function, the returned file will be a path to a sub-directory of the given type. Though these files are not automatically scanned by the media scanner, you can explicitly add them to the media database with `MediaScannerConnection.scanFile`. Note that this is not the same as`Environment.getExternalStoragePublicDirectory()`, which provides directories of media shared by all applications. The directories returned here are owned by the application, and their contents will be removed when the application is uninstalled. Unlike`Environment.getExternalStoragePublicDirectory()`, the directory returned here will be automatically created for you.

Here is an example of typical code to manipulate a picture in an application's private storage and add it to the media database:

void  createExternalStoragePrivatePicture ()  \{

/\* Create a path where we will place our picture in our own private

pictures directory.  Note that we don't really need to place a

picture in DIRECTORY\_PICTURES, since the media scanner will see

all media in these directories; this may be useful with other

media types such as DIRECTORY\_MUSIC however to help it classify

your media for display to the user.

\*/

File  path  =  getExternalFilesDir ( Environment . DIRECTORY\_PICTURES );

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

try  \{

/\* 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 ,

new  MediaScannerConnection . 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  deleteExternalStoragePrivatePicture ()  \{

/\* 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  =  getExternalFilesDir ( Environment . DIRECTORY\_PICTURES );

if  ( path  !=  null )  \{

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

file . delete ();

boolean  hasExternalStoragePrivatePicture ()  \{

/\* 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  =  getExternalFilesDir ( Environment . DIRECTORY\_PICTURES );

if  ( path  !=  null )  \{

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

return  file . exists ();

return  false ;

##### Parameters #####

<table> 
 <tbody> 
  <tr> 
   <th>type</th> 
   <td>The type of files directory to return. May be null for the root of the files directory or one of the following Environment constants for a subdirectory:<code><a href="http://developer.android.com/reference/android/os/Environment.html#DIRECTORY_MUSIC" rel="nofollow">DIRECTORY_MUSIC</a></code>,&nbsp;<code><a href="http://developer.android.com/reference/android/os/Environment.html#DIRECTORY_PODCASTS" rel="nofollow">DIRECTORY_PODCASTS</a></code>,&nbsp;<code><a href="http://developer.android.com/reference/android/os/Environment.html#DIRECTORY_RINGTONES" rel="nofollow">DIRECTORY_RINGTONES</a></code>,&nbsp;<code><a href="http://developer.android.com/reference/android/os/Environment.html#DIRECTORY_ALARMS" rel="nofollow">DIRECTORY_ALARMS</a></code>,&nbsp;<code><a href="http://developer.android.com/reference/android/os/Environment.html#DIRECTORY_NOTIFICATIONS" rel="nofollow">DIRECTORY_NOTIFICATIONS</a></code>,&nbsp;<code><a href="http://developer.android.com/reference/android/os/Environment.html#DIRECTORY_PICTURES" rel="nofollow">DIRECTORY_PICTURES</a></code>,&nbsp;or&nbsp;<code><a href="http://developer.android.com/reference/android/os/Environment.html#DIRECTORY_MOVIES" rel="nofollow">DIRECTORY_MOVIES</a></code>.</td> 
  </tr> 
 </tbody> 
</table>

##### Returns #####

*  Returns the path of the directory holding application files on external storage. Returns null if external storage is not currently mounted so it could not ensure the path exists; you will need to call this method again when it is available.

##### See Also #####

*  `getFilesDir()`
 *  `getExternalStoragePublicDirectory(String)`

该函数返回的地址将是如下形式：/mnt/sdcard/Android/data/com.lenovo.robin/files

其中为context所对应的应用程序的包名。

**三、** Environment 中 关于 外部存储的 重要函数

#### public static [File][] getExternalStorageDirectory () ####

Since:  [API Level 1][API Level 8]

Gets the Android external storage directory. This directory may not currently be accessible if it has been mounted by the user on their computer, has been removed from the device, or some other problem has happened. You can determine its current state with `getExternalStorageState()`.

*Note: don't be confused by the word "external" here. This directory can better be thought as media/shared storage. It is a filesystem that can hold a relatively large amount of data and that is shared across all applications (does not enforce permissions). Traditionally this is an SD card, but it may also be implemented as built-in storage in a device that is distinct from the protected internal storage and can be mounted as a filesystem on a computer.*

In devices with multiple "external" storage directories (such as both secure app storage and mountable shared storage), this directory represents the "primary" external storage that the user will interact with.

Applications should not directly use this top-level directory, in order to avoid polluting the user's root namespace. Any files that are private to the application should be placed in a directory returned by `Context.getExternalFilesDir`, which the system will take care of deleting if the application is uninstalled. Other shared files should be placed in one of the directories returned by `getExternalStoragePublicDirectory(String)`.

该函数现在已经不合时宜，在API Level 8后，对于应用程序的私有文件应该放在[Context.getExternalFilesDir][]目录下，非私有的（shared）的文件应该放在目录下Environment提供的函数[getExternalStoragePublicDirectory(String)][getExternalStoragePublicDirectory_String]所指定的目录下

Here is an example of typical code to monitor the state of external storage:

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 );

##### See Also #####

*  `getExternalStorageState()`
 *  `isExternalStorageRemovable()`

#### public static [File][] getExternalStoragePublicDirectory ([String][] type) ####

Get a top-level public external storage directory for placing files of a particular type. This is where the user will typically place and manage their own files, so you should be careful about what you put here to ensure you don't erase their files or get in the way of their own organization.

Here is an example of typical code to manipulate a picture on the public external storage:

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 ,

new  MediaScannerConnection . 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 ();

##### Parameters #####

<table> 
 <tbody> 
  <tr> 
   <th>type</th> 
   <td>The type of storage directory to return. Should be one of&nbsp;<code><a href="http://developer.android.com/reference/android/os/Environment.html#DIRECTORY_MUSIC" rel="nofollow">DIRECTORY_MUSIC</a></code>,&nbsp;<code><a href="http://developer.android.com/reference/android/os/Environment.html#DIRECTORY_PODCASTS" rel="nofollow">DIRECTORY_PODCASTS</a></code>,&nbsp;<code><a href="http://developer.android.com/reference/android/os/Environment.html#DIRECTORY_RINGTONES" rel="nofollow">DIRECTORY_RINGTONES</a></code>,&nbsp;<code><a href="http://developer.android.com/reference/android/os/Environment.html#DIRECTORY_ALARMS" rel="nofollow">DIRECTORY_ALARMS</a></code>,<code><a href="http://developer.android.com/reference/android/os/Environment.html#DIRECTORY_NOTIFICATIONS" rel="nofollow">DIRECTORY_NOTIFICATIONS</a></code>,&nbsp;<code><a href="http://developer.android.com/reference/android/os/Environment.html#DIRECTORY_PICTURES" rel="nofollow">DIRECTORY_PICTURES</a></code>,<br>&nbsp;<code><a href="http://developer.android.com/reference/android/os/Environment.html#DIRECTORY_MOVIES" rel="nofollow">DIRECTORY_MOVIES</a></code>,&nbsp;<code><a href="http://developer.android.com/reference/android/os/Environment.html#DIRECTORY_DOWNLOADS" rel="nofollow">DIRECTORY_DOWNLOADS</a></code>,&nbsp;or&nbsp;<code><a href="http://developer.android.com/reference/android/os/Environment.html#DIRECTORY_DCIM" rel="nofollow">DIRECTORY_DCIM</a></code>.&nbsp;May not be null.</td> 
  </tr> 
 </tbody> 
</table>

##### Returns #####

*  Returns the File path for the directory. Note that this directory may not yet exist, so you must make sure it exists before using it such as with `File.mkdirs()`.

对于 Environment.getExternalStoragePublicDirectory(Environment.DIRECTORY\_MOVIES).getAbsolutePath() 返回的将是 /mnt/sdcard/Movies

#### public static [String][] getExternalStorageState () ####

Since:  [API Level 1][API Level 8]

Gets the current state of the primary "external" storage device.

See `getExternalStorageDirectory()` for more information.

返回值可以为Environment的以下常量之一

<table> 
 <tbody> 
  <tr> 
   <th colspan="12">Constants</th> 
  </tr> 
  <tr> 
   <td><a href="http://developer.android.com/reference/java/lang/String.html" rel="nofollow">String</a></td> 
   <td><a href="http://developer.android.com/reference/android/os/Environment.html#MEDIA_BAD_REMOVAL" rel="nofollow">MEDIA_BAD_REMOVAL</a></td> 
   <td><code><a href="http://developer.android.com/reference/android/os/Environment.html#getExternalStorageState%28%29" rel="nofollow">getExternalStorageState()</a></code>&nbsp;returns&nbsp;MEDIA_<p>再分享一下我老师大神的人工智能教程吧。零基础！通俗易懂！风趣幽默！还带黄段子！希望你也加入到我们人工智能的队伍中来！<a href="https://blog.csdn.net/jiangjunshow/article/details/77338485" rel="nofollow">https://blog.csdn.net/jiangjunshow</a></p> </td> 
  </tr> 
 </tbody> 
</table>

[getExternalStorageState]: http://developer.android.com/reference/android/os/Environment.html#getExternalStorageState%28%29
[Context.getExternalFilesDir]: http://developer.android.com/reference/android/content/Context.html#getExternalFilesDir%28java.lang.String%29
[getExternalStoragePublicDirectory_String]: http://developer.android.com/reference/android/os/Environment.html#getExternalStoragePublicDirectory%28java.lang.String%29
[Android]: http://hubingforever.blog.163.com/blog/static/171040579201281895042802/
[File]: http://developer.android.com/reference/java/io/File.html
[API Level 8]: http://developer.android.com/guide/topics/manifest/uses-sdk-element.html#ApiLevels
[String]: http://developer.android.com/reference/java/lang/String.html
[getFilesDir]: http://developer.android.com/reference/android/content/Context.html#getFilesDir%28%29