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 internal 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 first post in a three-part series covering this storage model, to help
clarify what is going on. Today, we will look at “internal storage”… including
its various definitions.
What Your Users Think “Internal Storage” Means
Your users will eventually wander into Settings > Storage on their device —
or the equivalent location, depending on their device model — and
may see a screen that describes “Internal Storage”:
And that will be that. The user thinks that all of the on-board flash
is “internal storage”.
Fortunately, Google has started to change that with Android 8.0, going with a
general “storage” term instead of “internal storage”.
However, users may still see “internal storage” in places like an Explorer
window in Windows, when their device is connected via USB.
What Google Thinks “Internal Storage” Means
Alas, that is not what the Android SDK thinks “internal storage” means, which is
where some of the confusion lies.
If you read the Android documentation on internal storage,
the description is… hand-wavy:
You can save files directly on the device’s internal storage. By default, files saved to the internal storage are private to your application and other applications cannot access them (nor can the user). When the user uninstalls your application, these files are removed.
In truth, the Android SDK’s definition of “internal storage” is a specific
directory, unique to your app, where your app can place files. As suggested in the
docs, those files by default are read-write for your app and deny-all for any
(exception: users running file managers with superuser privileges on rooted devices
can access anything)
There are a handful of base methods on
Context that give you access to
internal storage, including:
Other methods will rely upon these, such as
classes also will rely upon these, such as
Where Internal Storage Is Stored… Sometimes
If you look around various blog posts, StackOverflow answers, and books that
came out in 2012 or earlier, you will be told that your app’s “internal storage”
your.application.package.name is replaced by your application ID, as is
declared in the
package attribute in the manifest or modified via Gradle)
Inside of there will be some directories automatically created by Android as
you use some of those
Context methods. For example,
getFilesDir() returns a
File object that points to a
files/ directory inside of your app’s
Where Internal Storage Is Stored… The Rest of the Time
However, this is not always where your app’s internal storage resides. If there
is one rule for developers that you should take away from this blog post series,
NEVER HARDCODE PATHS
Every now and then, I will see developers do something like this:
File f=new File("/data/data/their.app.package.name/files/foo.txt");
This is really really dumb.
First, it is more typing than using
File f=new File(getFilesDir(), "foo.txt");
More importantly, internal storage is not always at the same place. Notably,
we have the notion of separate user profiles, starting in
Android 4.2 for
tablets and Android 5.0 for phones.
Each user gets his or her own “internal storage” area. While the aforementioned
directory is still used for the primary user, that is not guaranteed, and it will
not be used for secondary accounts.
Poking Around Internal Storage
The Device File Explorer tool in Android Studio 3.0+ can browse all of internal
storage on an emulator, plus internal storage of debuggable apps on production
devices. This should handle most of your scenarios:
From the command line, you can use the
run-as option with
adb at the command line. For example, to download a
database from the primary user’s internal storage to your development machine, you
adb shell 'run-as your.application.package.name cp /data/data/your.application.package.name/databases/dbname.db /sdcard'
You will need to change the destination to wherever on your device external
storage is mapped to (shown here as
/sdcard/, which will not work on all devices)
You may need to use
cpon older devices
Once the file is on external storage, you can use
adb pull to download
it to your development machine, or access it by other conventional means
(e.g., via mounting the device as a drive on your development machine).
Limitations of Internal Storage
On ancient Android 1.x and 2.x devices, internal storage was usually on a dedicated
filesystem partition, and that partition was usually rather tiny. The HTC Dream
(a.k.a., T-Mobile G1), the original Android device, had a whopping 70MB of
internal storage for use by all apps.
(And, no, that’s not a typo. We measured storage in megabytes back then.)
By the time the 2.3 devices were rolling out, internal storage might be as
big as 1GB.
Android 3.0 changed the storage model around, in such a fashion as internal
storage got more space. Devices that advertise as having 4GB, 8GB, 16GB, etc.
of storage space usually had all of that (less existing contents) available
for internal storage. We will get into what changed in Android 3.0 and its impacts
on the storage model as part of tomorrow’s blog post on external storage.
For Android 1.x and 2.x, internal storage was only really for small files, and you
needed to use external storage for everything else. Android 3.0+ means that
for most devices and most users, internal storage is fine for files that are not
meant to be used routinely by other apps or accessible by the user independently
from your app. Some power users, though, will find that even on-board flash is
insufficient for what they want to store, so they turn to removable storage…
which is a whole ‘nuther can of worms that we will get into later in the week.
Here are some FAQs about what the SDK refers to as internal storage:
Should I Make Files on Internal Storage World-Readable or World-Writeable?
$DEITY, no. Use
FileProvider and serve that content via that
ContentProvider implementation. Then, you at least have the option of using
Android’s permission system to manage access to those files, versus having
any app on the system be able to monkey with those files.
Well, How About android:sharedUserId?
I also counsel against this.
android:sharedUserId is an attribute you can place in your manifest that indicates
a logical user ID to be used for your app. Any other app that is installed that
is signed by the same signing key and requests the same
will use the same Linux user from a security standpoint. The net effect is that
those two apps will be able to work with each other’s files with impunity, as those
files are all owned by the same Linux user.
This attribute is really designed for pre-installed apps, such as some software
suite pre-loaded by the device manufacturer, carrier, or ROM mod maintainer. In
particular, once you ship your app, you cannot reliably change your
value without locking your user out of any existing files… as Android does not
change the ownership of existing files when it changes the Linux user account that
your app runs as.
There are various risks in having multiple processes work simultaneously with
files. Some subsystems, like SQLite, have built-in logic to deal with this.
But if you are doing your own file access yourself (e.g., via
File and Java I/O), then
you have to somehow deal with simultaenous access, which can get tricky.
You also have to deal with what happens when one app is uninstalled, taking away
files that another app was using. In a hub-and-spoke model, like an app and a suite
of plugins, perhaps this is not that risky. In other models, where apps are closer
to being peers, you cannot afford to have your app’s data vanish because the user
elected to uninstall some separate app.
Finally, you do not know what the future might bring. Right now, you might view
your set of apps as a tightly-coupled suite. Somebody who acquires those apps,
or acquires your firm, might wish to go a different route. Using data sharing
options that are more loosely coupled, like a
ContentProvider, gives you greater
flexibility than does assuming that your apps should co-mingle their files.
In an ideal world, your app should treat other apps as a fairly-reliable but not
always avaialble resource, just like you treat your own Web service.
How Do I Prevent Users of Rooted Devices From Accessing My Files on Internal Storage?
Simple: don’t put files on internal storage. Users of rooted devices can access
whatever they want on the device, so the only way to prevent them from accessing your
data is to not have it on the device.
Some developers will attempt to encrypt their files, using a hard-coded password,
to prevent rooted device users from using those files. This does erect a “speed bump”,
but it is a small one. All it takes is one person with interest to reverse-engineer
your app, determine how to decrypt those files, then write up a blog post or
discussion board entry on how to do it.
On the whole, there are relatively few people with rooted devices – I estimate it
at well under 1%. IMHO, you will have better success by focusing your
engineering work on writing a better app, rather than spending that time trying
to block rooted device users.
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