android-4.4.4_r1/s

page.title=Saving Files

trainingnavtop=true

@jd:body


<div id="tb-wrapper">
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<h2>This lesson teaches you to</h2>
<ol>
  <li><a href="#InternalVsExternalStorage">Choose Internal or External Storage</a></li>
  <li><a href="#GetWritePermission">Obtain Permissions for External Storage</a></li>
  <li><a href="#WriteInternalStorage">Save a File on Internal Storage</a></li>
  <li><a href="#WriteExternalStorage">Save a File on External Storage</a></li>
  <li><a href="#GetFreeSpace">Query Free Space</a></li>
  <li><a href="#DeleteFile">Delete a File</a></li>
</ol>

<h2>You should also read</h2>
<ul>
  <li><a href="{@docRoot}guide/topics/data/data-storage.html#filesInternal">Using the Internal
Storage</a></li>
  <li><a href="{@docRoot}guide/topics/data/data-storage.html#filesExternal">Using the External
Storage</a></li>
</ul>

</div>
</div>

<p>Android uses a file system that's
similar to disk-based file systems on other platforms. This lesson describes
how to work with the Android file system to read and write files with the {@link java.io.File}
APIs.</p>

<p>A {@link java.io.File} object is suited to reading or writing large amounts of data in
start-to-finish order without skipping around. For example, it's good for image files or
anything exchanged over a network.</p>

<p>This lesson shows how to perform basic file-related tasks in your app.
The lesson assumes that you are familiar with the basics of the Linux file system and the
standard file input/output APIs in {@link java.io}.</p>


<h2 id="InternalVsExternalStorage">Choose Internal or External Storage</h2>

<p>All Android devices have two file storage areas: "internal" and "external" storage.  These names
come from the early days of Android, when most devices offered built-in non-volatile memory
(internal storage), plus a removable storage medium such as a micro SD card (external storage).
Some devices divide the permanent storage space into "internal" and "external" partitions, so even
without a removable storage medium, there are always two storage spaces and
the API behavior is the same whether the external storage is removable or not.
The following lists summarize the facts about each storage space.</p>

<div class="col-5" style="margin-left:0">
<p><b>Internal storage:</b></p>
<ul>
<li>It's always available.</li>
<li>Files saved here are accessible by only your app by default.</li>
<li>When the user uninstalls your app, the system removes all your app's files from
internal storage.</li>
</ul>
<p>Internal storage is best when you want to be sure that neither the user nor other apps can
access your files.</p>
</div>

<div class="col-7" style="margin-right:0">
<p><b>External storage:</b></p>
<ul>
<li>It's not always available, because the user can mount the external storage as USB storage
and in some cases remove it from the device.</li>
<li>It's world-readable, so
files saved here may be read outside of your control.</li>
<li>When the user uninstalls your app, the system removes your app's files from here
only if you save them in the directory from {@link android.content.Context#getExternalFilesDir
getExternalFilesDir()}.</li>
</ul>
<p>External storage is the best
place for files that don't require access restrictions and for files that you want to share
with other apps or allow the user to access with a computer.</p>
</div>


<p class="note" style="clear:both">
<strong>Tip:</strong> Although apps are installed onto the internal storage by
default, you can specify the <a
href="{@docRoot}guide/topics/manifest/manifest-element.html#install">{@code
android:installLocation}</a> attribute in your manifest so your app may
be installed on external storage. Users appreciate this option when the APK size is very large and
they have an external storage space that's larger than the internal storage. For more
information, see <a
href="{@docRoot}guide/topics/data/install-location.html">App Install Location</a>.</p>


<h2 id="GetWritePermission">Obtain Permissions for External Storage</h2>

<p>To write to the external storage, you must request the
  {@link android.Manifest.permission#WRITE_EXTERNAL_STORAGE} permission in your <a
href="{@docRoot}guide/topics/manifest/manifest-intro.html">manifest file</a>:</p>

<pre>
&lt;manifest ...>
    &lt;uses-permission android:name=&quot;android.permission.WRITE_EXTERNAL_STORAGE&quot; /&gt;
    ...
&lt;/manifest>
</pre>

<div class="caution"><p><strong>Caution:</strong>
Currently, all apps have the ability to read the external storage
without a special permission. However, this will change in a future release. If your app needs
to read the external storage (but not write to it), then you will need to declare the {@link
android.Manifest.permission#READ_EXTERNAL_STORAGE} permission. To ensure that your app continues
to work as expected, you should declare this permission now, before the change takes effect.</p>
<pre>
&lt;manifest ...>
    &lt;uses-permission android:name=&quot;android.permission.READ_EXTERNAL_STORAGE&quot; /&gt;
    ...
&lt;/manifest>
</pre>
<p>However, if your app uses the {@link android.Manifest.permission#WRITE_EXTERNAL_STORAGE}
permission, then it implicitly has permission to read the external storage as well.</p>
</div>

<p>You don’t need any permissions to save files on the internal
storage. Your application always has permission to read and
write files in its internal storage directory.</p>


<h2 id="WriteInternalStorage">Save a File on Internal Storage</h2>

<p>When saving a file to internal storage, you can acquire the appropriate directory as a
{@link java.io.File} by calling one of two methods:</p>

<dl>
  <dt>{@link android.content.Context#getFilesDir}</dt>
  <dd>Returns a {@link java.io.File} representing an internal directory for your app.</dd>
  <dt>{@link android.content.Context#getCacheDir}</dt>
  <dd>Returns a {@link java.io.File} representing an internal directory for your app's temporary
cache files. Be sure to delete each file once it is no
longer needed and implement a reasonable size limit for the amount of memory you use at any given
time, such as 1MB. If the system begins running low on storage, it may delete your cache files
without warning.</dd>
</dl>

<p>To create a new file in one of these directories, you can use the {@link
java.io.File#File(File,String) File()} constructor, passing the {@link java.io.File} provided by one
of the above methods that specifies your internal storage directory. For example:</p>

<pre>
File file = new File(context.getFilesDir(), filename);
</pre>

<p>Alternatively, you can call {@link
android.content.Context#openFileOutput openFileOutput()} to get a {@link java.io.FileOutputStream}
that writes to a file in your internal directory. For example, here's
how to write some text to a file:</p>

<pre>
String filename = "myfile";
String string = "Hello world!";
FileOutputStream outputStream;

try {
  outputStream = openFileOutput(filename, Context.MODE_PRIVATE);
  outputStream.write(string.getBytes());
  outputStream.close();
} catch (Exception e) {
  e.printStackTrace();
}
</pre>

<p>Or, if you need to cache some files, you should instead use {@link
java.io.File#createTempFile createTempFile()}. For example, the following method extracts the
file name from a {@link java.net.URL} and creates a file with that name
in your app's internal cache directory:</p>

<pre>
public File getTempFile(Context context, String url) {
    File file;
    try {
        String fileName = Uri.parse(url).getLastPathSegment();
        file = File.createTempFile(fileName, null, context.getCacheDir());
    catch (IOException e) {
        // Error while creating file
    }
    return file;
}
</pre>

<p class="note"><strong>Note:</strong>
Your app's internal storage directory is specified
by your app's package name in a special location of the Android file system.
Technically, another app can read your internal files if you set
the file mode to be readable. However, the other app would also need to know your app package
name and file names. Other apps cannot browse your internal directories and do not have
read or write access unless you explicitly set the files to be readable or writable. So as long
as you use {@link android.content.Context#MODE_PRIVATE} for your files on the internal storage,
they are never accessible to other apps.</p>


<h2 id="WriteExternalStorage">Save a File on External Storage</h2>

<p>Because the external storage may be unavailable&mdash;such as when the user has mounted the
storage to a PC or has removed the SD card that provides the external storage&mdash;you
should always verify that the volume is available before accessing it. You can query the external
storage state by calling {@link android.os.Environment#getExternalStorageState}. If the returned
state is equal to {@link android.os.Environment#MEDIA_MOUNTED}, then you can read and
write your files. For example, the following methods are useful to determine the storage
availability:</p>

<pre>
/* Checks if external storage is available for read and write */
public boolean isExternalStorageWritable() {
    String state = Environment.getExternalStorageState();
    if (Environment.MEDIA_MOUNTED.equals(state)) {
        return true;
    }
    return false;
}

/* Checks if external storage is available to at least read */
public boolean isExternalStorageReadable() {
    String state = Environment.getExternalStorageState();
    if (Environment.MEDIA_MOUNTED.equals(state) ||
        Environment.MEDIA_MOUNTED_READ_ONLY.equals(state)) {
        return true;
    }
    return false;
}
</pre>

<p>Although the external storage is modifiable by the user and other apps, there are two
categories of files you might save here:</p>

<dl>
  <dt>Public files</dt>
  <dd>Files that
should be freely available to other apps and to the user. When the user uninstalls your app,
these files should remain available to the user.
  <p>For example, photos captured by your app or other downloaded files.</p>
  </dd>
  <dt>Private files</dt>
  <dd>Files that rightfully belong to your app and should be deleted when the user uninstalls
  your app. Although these files are technically accessible by the user and other apps because they
  are on the external storage, they are files that realistically don't provide value to the user
  outside your app. When the user uninstalls your app, the system deletes
  all files in your app's external private  directory.
  <p>For example, additional resources downloaded by your app or temporary media files.</p>
  </dd>
</dl>

<p>If you want to save public files on the external storage, use the
{@link android.os.Environment#getExternalStoragePublicDirectory
getExternalStoragePublicDirectory()} method to get a {@link java.io.File} representing
the appropriate directory on the external storage. The method takes an argument specifying
the type of file you want to save so that they can be logically organized with other public
files, such as {@link android.os.Environment#DIRECTORY_MUSIC} or {@link
android.os.Environment#DIRECTORY_PICTURES}. For example:</p>

<pre>
public File getAlbumStorageDir(String albumName) {
    // Get the directory for the user's public pictures directory.
    File file = new File(Environment.getExternalStoragePublicDirectory(
            Environment.DIRECTORY_PICTURES), albumName);
    if (!file.mkdirs()) {
        Log.e(LOG_TAG, "Directory not created");
    }
    return file;
}
</pre>


<p>If you want to save files that are private to your app, you can acquire the
appropriate directory by calling {@link
android.content.Context#getExternalFilesDir getExternalFilesDir()} and passing it a name indicating
the type of directory you'd like. Each directory created this way is added to a parent
directory that encapsulates all your app's external storage files, which the system deletes when the
user uninstalls your app.</p>

<p>For example, here's a method you can use to create a directory for an individual photo album:</p>

<pre>
public File getAlbumStorageDir(Context context, String albumName) {
    // Get the directory for the app's private pictures directory.
    File file = new File(context.getExternalFilesDir(
            Environment.DIRECTORY_PICTURES), albumName);
    if (!file.mkdirs()) {
        Log.e(LOG_TAG, "Directory not created");
    }
    return file;
}
</pre>

<p>If none of the pre-defined sub-directory names suit your files, you can instead call {@link
android.content.Context#getExternalFilesDir getExternalFilesDir()} and pass {@code null}. This
returns the root directory for your app's private directory on the external storage.</p>

<p>Remember that {@link android.content.Context#getExternalFilesDir getExternalFilesDir()}
creates a directory inside a directory that is deleted when the user uninstalls your app.
If the files you're saving should remain available after the user uninstalls your
app&mdash;such as when your app is a camera and the user will want to keep the photos&mdash;you
should instead use {@link android.os.Environment#getExternalStoragePublicDirectory
getExternalStoragePublicDirectory()}.</p>


<p>Regardless of whether you use {@link
android.os.Environment#getExternalStoragePublicDirectory
getExternalStoragePublicDirectory()} for files that are shared or
{@link android.content.Context#getExternalFilesDir
getExternalFilesDir()} for files that are private to your app, it's important that you use
directory names provided by API constants like
{@link android.os.Environment#DIRECTORY_PICTURES}. These directory names ensure
that the files are treated properly by the system. For instance, files saved in {@link
android.os.Environment#DIRECTORY_RINGTONES} are categorized by the system media scanner as ringtones
instead of music.</p>


<h2 id="GetFreeSpace">Query Free Space</h2>

<p>If you know ahead of time how much data you're saving, you can find out
whether sufficient space is available without causing an {@link
java.io.IOException} by calling {@link java.io.File#getFreeSpace} or {@link
java.io.File#getTotalSpace}. These methods provide the current available space and the
total space in the storage volume, respectively. This information is also useful to avoid filling
the storage volume above a certain threshold.</p>

<p>However, the system does not guarantee that you can write as many bytes as are
indicated by {@link java.io.File#getFreeSpace}.  If the number returned is a
few MB more than the size of the data you want to save, or if the file system
is less than 90% full, then it's probably safe to proceed.
Otherwise, you probably shouldn't write to storage.</p>

<p class="note"><strong>Note:</strong> You aren't required to check the amount of available space
before you save your file. You can instead try writing the file right away, then
catch an {@link java.io.IOException} if one occurs. You may need to do
this if you don't know exactly how much space you need. For example, if you
change the file's encoding before you save it by converting a PNG image to
JPEG, you won't know the file's size beforehand.</p>


<h2 id="DeleteFile">Delete a File</h2>

<p>You should always delete files that you no longer need. The most straightforward way to delete a
file is to have the opened file reference call {@link java.io.File#delete} on itself.</p>

<pre>
myFile.delete();
</pre>

<p>If the file is saved on internal storage, you can also ask the {@link android.content.Context} to locate and
delete a file by calling {@link android.content.Context#deleteFile deleteFile()}:</p>

<pre>
myContext.deleteFile(fileName);
</pre>

<div class="note">
<p><strong>Note:</strong> When the user uninstalls your app, the Android system deletes
the following:</p>
<ul>
<li>All files you saved on internal storage</li>
<li>All files you saved on external storage using {@link
android.content.Context#getExternalFilesDir getExternalFilesDir()}.</li>
</ul>
<p>However, you should manually delete all cached files created with
{@link android.content.Context#getCacheDir()} on a regular basis and also regularly delete
other files you no longer need.</p>
</div>