Cache vs Data storage for downloaded content

Question

I'm downloading some text data by issuing a HTTP GET request to server. I want to save downloaded text file to re-use it on request if it has already been downloaded. But I want to keep my data private, so that no other apps could access it. On the other hand, it would be OK if Android removed that files if there's not enough disk space.

So, my question is - should I store downloaded content in App Data folder or in cache folder? Is there any difference between two?

First, I used to save files in App Data folder, using a method like

public void save(String fileName, String data) {
    FileOutputStream fos;
    try {
        fos = mContext.openFileOutput(fileName, Context.MODE_PRIVATE);
        fos.write(data.getBytes());
        fos.close();
    } catch (Exception e) {
        e.printStackTrace();
    }
}

Using this method I can set Private mode for my files so that no other apps could access them. But then I thought about moving files to cache directory, I need to do something like

private void save(String filename, String data) {
    File cacheDir = new File(mContext.getCacheDir(), "app_directory");
    cacheDir.mkdir();
    try {
        FileOutputStream fos = new FileOutputStream(new File(cacheDir, filename));
        fos.write(data.getBytes());
        fos.close();
    } catch (Exception e) {
        e.printStackTrace();
    }
}

I cannot set Private attribute to my files anymore, so, as I understand, any application will be able to get access to my data. Am I right?

Maybe there's a way to make files in cache directory private? Or it doesn't really matter where to save files to?

Answer 1

Both the CacheDir and the FilesDir are app specific, and can not be accessed by any other app.

Both of these however can be accessed if the user has rooted their device.

The CacheDir is for temp files, that may be deleted if so required to free up space by Android OS. The Files dir will not be cleared unless explicitly done so by the app, the user, or if the app is uninstalled.

Answer 2

This is covered in the docs:

http://developer.android.com/guide/topics/data/data-storage.html

Saving cache files

If you'd like to cache some data, rather than store it persistently, you should use getCacheDir() to open a File that represents the internal directory where your application should save temporary cache files.

When the device is low on internal storage space, Android may delete these cache files to recover space. However, you should not rely on the system to clean up these files for you. You should always maintain the cache files yourself and stay within a reasonable limit of space consumed, such as 1MB. When the user uninstalls your application, these files are removed.

Data in the cache can only be accessed your app (if its not rooted, but thats a user choice to minimze security)