Back in 2014, I wrote a series of blog posts to try to clear up confusion
around where you can read and write files in Android. This post is an updated
edition of the original post on removable storage.
There is a lot of confusion regarding Android’s storage model. That confusion
got a lot worse with Android 4.4’s changes to that storage model, and it has
not really improved much since. There
are countless Stack Overflow questions and the like where they clearly do not
quite grok the various pieces of Android’s storage model.
This is the third post in a three-part series covering this storage model, to help
clarify what is going on. Monday, we looked at
Yesterday, we looked at
Today, we will look at removable storage, the source
of an unfortunate amount of angst.
What Your Users Think “Removable Storage” Means
Many of your users will have a device that has some sort of removable media.
Often times this is a micro SD card. Some tablets or docks have a full SD card slot.
Plus, USB mass storage is possible via USB On-The-Go and USB Type C connectors (not to mention
devices or docks with a full USB host port).
Your users will think that they can work with removable storage much like they
can with a desktop or notebook.
Unfortunately, your users are largely mistaken, and are even more mistaken
with Android 4.4+. That’s because Google’s approach towards removable storage is…
What Google Thinks “Removable Storage” Means
In the beginning, as was noted yesterday, external storage was often in the form
of a removable micro SD card. At that time, many developers got in the habit of
thinking that external storage == removable storage.
However, as Android 3.0 and higher started rolling out en masse, developers soon
started to realize two things:
External storage != removable storage on most of those devices
There’s nothing in the Android SDK for removable storage
That’s right: until Android 4.4, there was no official support for removable media
in Android. Quoting Dianne Hackborn:
…keep in mind: until Android 4.4, the official Android platform has not supported SD cards at all except for two special cases: the old school storage layout where external storage is an SD card (which is still supported by the platform today), and a small feature added to Android 3.0 where it would scan additional SD cards and add them to the media provider and give apps read-only access to their files (which is also still supported in the platform today).
Android 4.4 is the first release of the platform that has actually allowed applications to use SD cards for storage. Any access to them prior to that was through private, unsupported APIs. We now have a quite rich API in the platform that allows applications to make use of SD cards in a supported way, in better ways than they have been able to before: they can make free use of their app-specific storage area without requiring any permissions in the app, and can access any other files on the SD card as long as they go through the file picker, again without needing any special permissions.
But… But… But… What About All These Apps That Use Removable Media?
They fall into three buckets:
Some are just relying on
MediaStoreindexing. So, for example, a video player can
find out about videos on all available media by querying the
MediaStore, and if
the device manufacturer hasn’t broken the
MediaStoreindexing of removable media,
the player will be able to play back videos on removable media.
Some are apps that ship with the hardware. The hardware manufacturer knows the
device and what the rules of the game are for that device. The hardware manufacturer
is also far less concerned about cross-device compatibility, as their apps aren’t
(usually) shipping on the Play Store. Hence, a hardware manufacturer has carte blanche
to work with removable media.
Some are apps written by developers who decided to go past the boundaries of
the Android SDK. There are various recipes online for examining various Linux
system files (and file-like substances) to determine what “mount points” exist,
and from there apply some heuristics to determine what represents removable media.
While reliability across devices could easily be an issue, beyond that, these
techniques worked… until Android 4.4, when everything changed.
What Happened in Android 4.4: The Good News
Two things happened in the timeframe of Android 4.4 that affect removable media.
On the plus side, we gained some measure of official Android SDK support for removable media.
getExternalCacheDirs() (note the plural form)
will not only return directories that we can use on “real” external storage,
but also will return directories that we can use on any available and supported
removable media. Our apps do not need any specific permissions to use any of those
However, apps can create/write files outside of their package-specific directories on secondary external storage devices by using the new CREATE_DOCUMENT intent, which involves the user in the file creation process.
What Happened in Android 4.4: The Angst-Riddled News
Since Android 4.2, there has been a request from Google for device manufacturers to
lock down removable media. Generally, this was ignored.
For Android 4.4+, Google amended the Compatibility Test Suite (CTS) that device manufacturers
must comply with in order to ship a device containing Google’s proprietary apps
(e.g., Play Store, Maps, Gmail; otherwise known as “GMS”). Quoting
However, new tests were added in CTS for 4.4 that validate whether or not secondary storage has the proper read-only permissions in non app-specific directories, presumably because of the new APIs to finally expose those paths to application developers. As soon as CTS includes these rules, OEMs have to support them to keep shipping devices with GMS (Google Play, etc.) on-board.
As a result, apps can read files on removable media using the various undocumented
and unsupported tricks for finding removable media. However, apps cannot write to or
otherwise modify such removable storage. Note that device manufacturers themselves
may have ways of dealing with this, but ordinary app developers do not.
android-platform Google Group has been the home of
a rather epic discussion thread
regarding Google’s decision here, including Dianne Hackborn’s entry linked to earlier
in this post.
Despite the hand-wringing, I do not expect Google to backpedal on this issue.
For every user that complains about how Android 4.4+ makes their micro SD card
useless, there’s a pundit complaining about how Android lets any app run amok
and read stuff on storage, raising privacy and security issues. In fact,
I would not be the
least bit surprised if read access to removable media is blocked in the future,
beyond an app’s own directory on that media.
Where Can I Learn More?
Two of the better resources for this topic are ones I linked to earlier in this
OK, So What Are We Supposed to Do?
Ideally, use the Storage Access Framework:
ACTION_OPEN_DOCUMENT(akin to a legacy system file-open dialog)
ACTION_OPEN_DOCUMENT_TREE(akin to a legacy system directory-open dialog)
ACTION_CREATE_DOCUMENT(akin to a legacy system save-as dialog)
On Android 7.0+, you can also use
StorageVolume to request access to specific
directory trees on removable storage.
All of these give you a
Uri back, representing a document or a document tree.
You would use those with things like
work with the content (or tree of content).
The big advantage of this approach is that your code is now storage-agnostic.
The same code will work with:
- External storage
- Removable storage
- Google Drive
- Samba/CIFS network file servers (if the user has Google’s app for that installed)
- Anything else exposed by a third-party documents provider
While your code will be somewhat different than when using
not that different:
You still use
OutputStreamto read and write content,
but you get them from
You still can traverse document trees, but you do so using
The biggest difference is that you should no longer assume that there is
a meaningful filename, using
DocumentFile to get a “display name” to show
Certain types of apps, such as media players, might be better served using
MediaStore and ignoring the filesystem entirely. Not only can you
MediaStore to find out the available media of relevance, but you
can create a
Uri that has the
MediaStore stream the content
to you, even for content that you might not be able to access.
But, That Doesn’t Fit My UI
From what I’ve seen, the #1 source of complaints about the “lost” removable
storage access comes from developers of file managers. Their argument is that
they need to be able to list all sources of files and traverse them.
That’s just not how Android works. With some tweaks to the UX, you can get
something that is far more Android-friendly.
For example, rather than considering your app to be the “source of truth” for
document trees, let the user do that, in the form of a “bookmark” system. Your
app would come with a pre-configured bookmark for external storage. If the user
taps an action bar item (or whatever) to add a new bookmark, you use
ACTION_OPEN_DOCUMENT_TREE and let them choose a tree from whatever they want.
You take persistable permissions on the
Uri that you get back, then save
Uri as part of your app data (e.g., in a database). You then use
for traversing the document tree for presentation in your UI.
With this approach, you fit better with Android, by using the Storage
Access Framework. Along the way, you offer more functionality to the users, as now your “file
manager” can manage content from a variety of places, including things that
are not local files. Similar approaches can be used for other types of apps,
switching from a “file” metaphor to a “content” metaphor and leveraging
the Storage Access Framework.
Need a speaker at your Android development meetup? Mark Murphy is available, in person in the Boston/Pittsburgh/DC triangle, or by remote anywhere in the world! Contact Mark for details!