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.
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.
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}.
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.
Internal storage:
Internal storage is best when you want to be sure that neither the user nor other apps can access your files.
External storage:
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.
Tip: Although apps are installed onto the internal storage by default, you can specify the {@code android:installLocation} 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 App Install Location.
To write to the external storage, you must request the {@link android.Manifest.permission#WRITE_EXTERNAL_STORAGE} permission in your manifest file:
<manifest ...>
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />
...
</manifest>
Caution: 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.
<manifest ...>
<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />
...
</manifest>
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.
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.
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:
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:
File file = new File(context.getFilesDir(), filename);
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:
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();
}
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:
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;
}
Note: 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.
Because the external storage may be unavailable—such as when the user has mounted the storage to a PC or has removed the SD card that provides the external storage—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:
/* 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;
}
Although the external storage is modifiable by the user and other apps, there are two categories of files you might save here:
For example, photos captured by your app or other downloaded files.
For example, additional resources downloaded by your app or temporary media files.
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:
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;
}
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.
For example, here's a method you can use to create a directory for an individual photo album:
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;
}
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.
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—such as when your app is a camera and the user will want to keep the photos—you should instead use {@link android.os.Environment#getExternalStoragePublicDirectory getExternalStoragePublicDirectory()}.
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.
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.
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.
Note: 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.
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.
myFile.delete();
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()}:
myContext.deleteFile(fileName);
Note: When the user uninstalls your app, the Android system deletes the following:
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.