Android create file external storage

Saving Files

This lesson teaches you to

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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 File APIs.

A 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 java.io .

Choose Internal or External Storage

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.

• It’s always available.
• Files saved here are accessible by only your app by default.
• When the user uninstalls your app, the system removes all your app’s files from internal storage.

Internal storage is best when you want to be sure that neither the user nor other apps can access your files.

• 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.
• It’s world-readable, so files saved here may be read outside of your control.
• When the user uninstalls your app, the system removes your app’s files from here only if you save them in the directory from getExternalFilesDir() .

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 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.

Obtain Permissions for External Storage

To write to the external storage, you must request the WRITE_EXTERNAL_STORAGE permission in your manifest file:

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 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.

However, if your app uses the 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.

Save a File on Internal Storage

When saving a file to internal storage, you can acquire the appropriate directory as a File by calling one of two methods:

getFilesDir() Returns a File representing an internal directory for your app. getCacheDir() Returns a 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.

To create a new file in one of these directories, you can use the File() constructor, passing the File provided by one of the above methods that specifies your internal storage directory. For example:

Alternatively, you can call openFileOutput() to get a FileOutputStream that writes to a file in your internal directory. For example, here’s how to write some text to a file:

Or, if you need to cache some files, you should instead use createTempFile() . For example, the following method extracts the file name from a URL and creates a file with that name in your app’s internal cache directory:

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 MODE_PRIVATE for your files on the internal storage, they are never accessible to other apps.

Save a File on External Storage

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 getExternalStorageState() . If the returned state is equal to MEDIA_MOUNTED , then you can read and write your files. For example, the following methods are useful to determine the storage availability:

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

Public files 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.

For example, photos captured by your app or other downloaded files.

Private files 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.

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 getExternalStoragePublicDirectory() method to get a 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 DIRECTORY_MUSIC or DIRECTORY_PICTURES . For example:

If you want to save files that are private to your app, you can acquire the appropriate directory by calling 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:

If none of the pre-defined sub-directory names suit your files, you can instead call getExternalFilesDir() and pass null . This returns the root directory for your app’s private directory on the external storage.

Remember that 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 getExternalStoragePublicDirectory() .

Regardless of whether you use getExternalStoragePublicDirectory() for files that are shared or getExternalFilesDir() for files that are private to your app, it’s important that you use directory names provided by API constants like DIRECTORY_PICTURES . These directory names ensure that the files are treated properly by the system. For instance, files saved in DIRECTORY_RINGTONES are categorized by the system media scanner as ringtones instead of music.

Query Free Space

If you know ahead of time how much data you’re saving, you can find out whether sufficient space is available without causing an IOException by calling getFreeSpace() or 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 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 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.

Delete a File

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 delete() on itself.

If the file is saved on internal storage, you can also ask the Context to locate and delete a file by calling deleteFile() :

Note: When the user uninstalls your app, the Android system deletes the following:

• All files you saved on internal storage
• All files you saved on external storage using getExternalFilesDir() .

However, you should manually delete all cached files created with getCacheDir() on a regular basis and also regularly delete other files you no longer need.

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Data and file storage overview

Android uses a file system that’s similar to disk-based file systems on other platforms. The system provides several options for you to save your app data:

• App-specific storage: Store files that are meant for your app’s use only, either in dedicated directories within an internal storage volume or different dedicated directories within external storage. Use the directories within internal storage to save sensitive information that other apps shouldn’t access.
• Shared storage: Store files that your app intends to share with other apps, including media, documents, and other files.
• Preferences: Store private, primitive data in key-value pairs.
• Databases: Store structured data in a private database using the Room persistence library.

The characteristics of these options are summarized in the following table:

Type of content	Access method	Permissions needed	Can other apps access?	Files removed on app uninstall?
App-specific files	Files meant for your app’s use only	From internal storage, getFilesDir() or getCacheDir() From external storage, getExternalFilesDir() or getExternalCacheDir()	Never needed for internal storage Not needed for external storage when your app is used on devices that run Android 4.4 (API level 19) or higher	No	Yes
Media	Shareable media files (images, audio files, videos)	MediaStore API	READ_EXTERNAL_STORAGE when accessing other apps’ files on Android 11 (API level 30) or higher READ_EXTERNAL_STORAGE or WRITE_EXTERNAL_STORAGE when accessing other apps’ files on Android 10 (API level 29) Permissions are required for all files on Android 9 (API level 28) or lower	Yes, though the other app needs the READ_EXTERNAL_STORAGE permission	No
Documents and other files	Other types of shareable content, including downloaded files	Storage Access Framework	None	Yes, through the system file picker	No
App preferences	Key-value pairs	Jetpack Preferences library	None	No	Yes
Database	Structured data	Room persistence library	None	No	Yes

The solution you choose depends on your specific needs:

How much space does your data require? Internal storage has limited space for app-specific data. Use other types of storage if you need to save a substantial amount of data. How reliable does data access need to be? If your app’s basic functionality requires certain data, such as when your app is starting up, place the data within internal storage directory or a database. App-specific files that are stored in external storage aren’t always accessible because some devices allow users to remove a physical device that corresponds to external storage. What kind of data do you need to store? If you have data that’s only meaningful for your app, use app-specific storage. For shareable media content, use shared storage so that other apps can access the content. For structured data, use either preferences (for key-value data) or a database (for data that contains more than 2 columns). Should the data be private to your app? When storing sensitive data—data that shouldn’t be accessible from any other app—use internal storage, preferences, or a database. Internal storage has the added benefit of the data being hidden from users.

Categories of storage locations

Android provides two types of physical storage locations: internal storage and external storage. On most devices, internal storage is smaller than external storage. However, internal storage is always available on all devices, making it a more reliable place to put data on which your app depends.

Removable volumes, such as an SD card, appear in the file system as part of external storage. Android represents these devices using a path, such as /sdcard .

Apps themselves are stored within internal storage by default. If your APK size is very large, however, you can indicate a preference within your app’s manifest file to install your app on external storage instead:

Permissions and access to external storage

On earlier versions of Android, apps needed to declare the READ_EXTERNAL_STORAGE permission to access any file outside the app-specific directories on external storage. Also, apps needed to declare the WRITE_EXTERNAL_STORAGE permission to write to any file outside the app-specific directory.

More recent versions of Android rely more on a file’s purpose than its location for determining an app’s ability to access, and write to, a given file. In particular, if your app targets Android 11 (API level 30) or higher, the WRITE_EXTERNAL_STORAGE permission doesn’t have any effect on your app’s access to storage. This purpose-based storage model improves user privacy because apps are given access only to the areas of the device’s file system that they actually use.

Android 11 introduces the MANAGE_EXTERNAL_STORAGE permission, which provides write access to files outside the app-specific directory and MediaStore . To learn more about this permission, and why most apps don’t need to declare it to fulfill their use cases, see the guide on how to manage all files on a storage device.

Scoped storage

To give users more control over their files and to limit file clutter, apps that target Android 10 (API level 29) and higher are given scoped access into external storage, or scoped storage, by default. Such apps have access only to the app-specific directory on external storage, as well as specific types of media that the app has created.

Use scoped storage unless your app needs access to a file that’s stored outside of an app-specific directory and outside of a directory that the MediaStore APIs can access. If you store app-specific files on external storage, you can make it easier to adopt scoped storage by placing these files in an app-specific directory on external storage. That way, your app maintains access to these files when scoped storage is enabled.

To prepare your app for scoped storage, view the storage use cases and best practices guide. If your app has another use case that isn’t covered by scoped storage, file a feature request. You can temporarily opt-out of using scoped storage.

View files on a device

To view the files stored on a device, use Android Studio’s Device File Explorer.

Additional resources

For more information about data storage, consult the following resources.

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