Which storage directory should I use for storing on Android 6+

on android

Android File System Structure

Android uses the Linux file system structure which has a single root.
So, both internal storage and external storage are directories which are partition in Android file system.

Partitions

There's some storage related partitions: /data, /storage and /sdcard that worth mentioning.

/data

You can consider /data to be internal storage partition.
When you perform a data or factory reset, it is the partition that you are wiping.

/data contains two directories: app/, data/

  • app/
    • store all package's .apk file
  • data/
    • each package's Internal Storage
/storage

In contrast to /data, /storage can be considered external storage partition in general.

/storage contains two directories: emulated/, self/

  • emulated/
    • user's external storage
  • self/
    • current user(*) identifier

*user of multi-user on Android 4.2+

/sdcard

Actually /sdcard/ and mnt/sdcard is a symlink to /storage/self/primary on Android 6.0+.

Tracing these symlinks: 

/sdcard ---> /storage/self/primary
/mnt/sdcard ---> /storage/self/primary

/storage/self/primary ---> /mnt/user/0/primary

/mnt/user/0/primary ---> /storage/emulated/0

/sdcard eventually links to /storage/emulated/0.

All Android devices have two file storage areas: "internal" and "external" storage. Many devices now divide the permanent storage space into separate "internal" and "external" partitions. Choose internal or external storage

It's noteworthy that /sdcard isn't your physical SD card.
/sdcard is Primary External Storage which divided from permanent storage.

If you had a physical SD card inserted then you had Secondary External Storage, you can find it under /storage.

Internal and external storage

Users cannot access internal storage, only the apps can but limited to their own package's directory.
You must have an external storage even if you have no SD card inserted. (Some modern devices have no SD card slot actually)

Internal storage

/data/data/your.package/

The files in internal storage would be removed on app uninstall.
The files are private, only your app can access it.

Public external storage

/storage/emulated/0

The files in public external storage would not be removed on app uninstall.
The files are shared, means other apps can access.

Private external storage

/storage/emulated/0/Android/data/your.package/

The files in private external storage would be removed on app uninstall.
The files are private, only your app can access it.

Storage related APIs

Internal storage:

  • Context.getCacheDir() - /data/data/your.package/cache/
  • Context.getFilesDir() - /data/data/your.package/files/

Public external storage:

  • Environment.getExternalStorageDirectory() - /storage/emulated/0/
  • Environment.getExternalStoragePublicDirectory(DIRECTORY_DCIM ...) - /storage/emulated/0/DCIM ...

Private external storage:

  • context.getExternalCacheDir() - /storage/emulated/0/Android/data/your.package/cache/
  • context.getExternalFilesDir() - /storage/emulated/0/Android/data/your.package/files/

Notice that Context is app related storage, Environment is public storage.

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