Android Development:File System Approaches

Richard S. Stansbury2015

File System-based Data Storage

• Packaged Files– Raw Resources– XML Resources– Assets

• Internal and External Storage– Options

• Application Specific Internal• Application Specific External• Public Shared Storage

– Permission required for external write and/or read

Packaged Files: Raw Resources

• What are raw resources?– Data files of arbitrary format

• How are they included?– Stored in the res/raw folder– Example res/raw/blah.txt– Resources r = getResources();– InputStream i = r.openRawResource(R.raw.blah);

• What can be done to/with them? – Read only– Support’s reference of static application data

Packaged Files: XML Resources

• Why?– If your static application data is XML, then better to include

here than under raw.• How?

– Data stored in “res/xml/”– e.g. res/xml/blah.xml

• Resources r = getResources();• XMLResourceParser xml = r.getXml(R.xml.blah);

• What do I do with this?– Return’s an XML Pull Parser– http://developer.android.com/reference/org/xmlpull/v1/X

mlPullParser.html

Packaged Files: Assets

• How? – Create a folder in your root folder called “assets/”– AssetManager am = getAssets();– InputStream is = am.open(<path>);

• e.g. “content/blah.html” would access the path and folder specified in the “assets/” folder

• Or,– For WebView

• w.loadURL(file://android_asset/<path>); • Loads the content as HTML into the webview

Example: Assets

public void loadDatabase() throws IOException{

AssetManager am = getAssets();InputStream inputStream = am.open("movies.csv");BufferedReader input = new BufferedReader(new

InputStreamReader(inputStream));

String movieStr;while ((movieStr = input.readLine()) != null) {

_mdb.addMovie(Movie.parseMovie(movieStr));}

}

File-system Storage

• Access files for read/write at runtime– Persistent data

• Data that will remain for the life of your application (or longer)

– Cached data• Data that is eligible for purge periodically by other applications /

system.

• You can open instances java.io classes such as File, InputStream, OutputWriter, etc.– Some issues must be considered on how to specify the path

• Internal vs. External options

File-System Storage: Internal

• Onboard and always available• Typically the built-in flash– Note: some devices have internal flash for both

the “internal storage” and “external storage”• e.g. Nexus 7

• Data only available to the application– Not accessible by other applications or external

means (Note: exceptions for rooted devices)

File-System Storage: Internal

• No Path: Pass file names to:– FileInputStream fis = openFileInput(<filename>)– FileOutputStream fos =

openFileOutput(<filename>, int mode)• Mode options

– http://developer.android.com/reference/android/content/Context.html#openFileOutput(java.lang.String, int)

• For paths:– Create file reference to root folder– Create file reference for sub-folder and filename

File-System Storage: Internal

• Within context– File root = getActivity().getFilesDir();• Or getActivity().getCacheDir();

– File file = new File(root, <FILENAME>);• Now, you can create input stream or output

writer (or the like) using the file File object.

File-System Storage: External

• Application specific storage folder– Accessible by all– File root = getActivity().getExternalFilesDir();

• Application specific cache folder– Accessible by all, but eligible for random sweep– File root = getActivity().getExternalCacheDir();

File-System Storage: External

• External storage in root folder– File root =

Environment.getExternalStorageDirectory();

File-System Storage: External

• Public directory (for things to share)– File root =

Environment.getExternalPublicDirectory(<type>)• <type>: Environment.

– DIRECTORY_MUSIC– DIRECTORY_PODCASTS– DIRECTORY_RINGTONES – DIRECTORY_ALARMS– DIRECTORY_NOTIFICATIONS– DIRECTORY_PICTURES– DIRECTORY_MOVIES– DIRECTORY_DOWNLOADS– DIRECTORY_DCIM

File-System Storage: Permissions

• Manifest.xml– <uses-permission

android:name=“android.permission.WRITE_EXTERNAL_STORAGE” />– <uses-permission

android:name=“android.permission.READ_EXTERNAL_STORAGE” />

• If you do have write permissions, you do not need to include read permissions (but not vice-versa)

File-System Storage: Multiple Users

• 4.2 and later now allows multiple users– getFilesDir() and getFilesExternalDir() provides

separate folders for each user

References

• Textbook• Resources:– http://developer.android.com/reference/android/content/

res/Resources.html• XML Pull Parser:– http://developer.android.com/reference/org/xmlpull/v1/

XmlPullParser.html• Asset Manager– http

://developer.android.com/reference/android/content/res/AssetManager.html