Busy Android Developer's GuideBusy Android Developer's Guideto Peristence

Ted NewardNeward & Associates

http://www.tedneward.com | ted@tedneward.com

CredentialsCredentials

Who is this guy?– Architectural Consultant, Neudesic Software– Principal, Architect, Consultant and Mentor

ask me how I can help your project or your team– Microsoft MVP (F#, C#, Architect)– JSR 175, 277 Expert Group Member– Author

Professional F# 2.0 (w/Erickson, et al; Wrox, 2010)Effective Enterprise Java (Addison-Wesley, 2004)C# In a Nutshell (w/Drayton, et all; OReilly, 2003)SSCLI Essentials (w/Stutz, et al; OReilly, 2003)Server-Based Java Programming (Manning, 2000)

– Blog: http://blogs.tedneward.com– Papers: http://www.tedneward.com/writings– Twitter: @tedneward

PreferencesPreferences

Android SharedPreferences– essentially, an XML file stored in your app's data area

•as such, it's probably the easiest to use ...•... but it has the usual limitations of XML

– usage is pretty straightforward•Activity.getPreferences() to return a SharedPreferences object•MODE_PRIVATE makes the prefs invisible to all but this app•MODE_WORLD_READABLE/WRITEABLE allows other apps access•to view existing data, call get*()•to change, call edit() first, then put*()s, then commit() to write

PreferencesPreferences

public class Calc extends Activity {• public static final String PREFS_NAME = "MyPrefsFile";•• @Override• protected void onCreate(Bundle state){• super.onCreate(state);• . . .•• // Restore preferences• SharedPreferences settings = getSharedPreferences(PREFS_NAME, 0);• boolean silent = settings.getBoolean("silentMode", false);• setSilent(silent);• }•• @Override• protected void onStop(){• super.onStop();•• SharedPreferences settings = getSharedPreferences(PREFS_NAME, 0);• settings.edit().putBoolean("silentMode", true).commit();• // ... or break it up into three steps if you prefer• }•}

File I/OFile I/O

Android exposes the filesystem to standard Java I/O– Two major categories of storage:

•internal storage•... and external storage (which may or may not be actually removable)

– Beyond where the files are stored, there's no real difference

except that external storage can disappear at any time, isn't secure, and can be read over the USB cable

– Any sort of file storage is possible from hereso anything that takes a File is a candidate for use

File I/OFile I/O

"Internal storage": storage permanent to the device– use openFileOutput() to return a java.io.FileOutputStream– use openFileInput() for a java.io.FileInputStream

both use MODE_PRIVATE, MODE_APPEND, MODE_WORLD_READABLE/WRITEABLE

– getFilesDir() returns File for app's internal storage directory

– deleteFile(), fileList()

File I/OFile I/O

"Cached" data is a little different– getCacheDir() returns a File to the "cache" directory– Android feels free to delete this data in low-storage

scenariosbut Android docs suggest not to rely on this; police yourself

File I/OFile I/O

"External storage": removable storage (SD cards)– API 7 and earlier

•getExternalStorageState() tells us SD card state•getExternalStorageDirectory() returns a java.io.File for the directory

– Files that should be shared go in a slightly different place•from getExternalStorageDirectory(), store to Music/, Podcasts/, Ringtones/, Alarms/, Notifications/, Pictures/, Movies/, or Downloads/

File I/OFile I/O

"External storage": removable storage (SD cards)– API 8 and later

•getExternalStorageState() tells us SD card state•getExternalFilesDir() returns a java.io.File for the directory

– Files that should be sharde go in a slightly different place•getExternalStoragePublicDirectory(), passing in DIRECTORY_MUSIC, DIRECTORY_RINGTONES, etc

SQLiteSQLite

SQLite is a small-scale footprint RDBMS– ".. a software library that implements a self-contained,

serverless, zero-configuration, transactional SQL database engine."

http://www.sqlite.org– Mostly SQL-92 compliant, but there are a few SQLisms

unsupported:•RIGHT and FULL OUTER JOIN•Complete ALTER TABLE support•Complete trigger support•Writing to VIEWs•GRANT and REVOKE

– Storage is single-file database, stored locally

SQLiteSQLite

Android supports SQLite (android.database.sqlite)– SQLiteDatabase: core class for all things SQLite

•openDatabase() or openOrCreateDatabase()•beginTransaction(), endTransaction()•insert(), delete(), update(), replace(), query()•execSQL()

– queries return either a "projection" (String[]) or a Cursor object

•either can generally be bound to a View Adapter•... or you can manually iterate/bind

SQLiteSQLite

Android convenience classes– SQLiteOpenHelper: extend this to receive "events" about

the database•onCreate()•onUpgrade()•onOpen()•provide a database name, and a version (to super())

– SQLiteStatement: precompiled SQL for fast reuseessentially, a JDBC PreparedStatement

SQLiteSQLite

Android SDK ships with sqlite3 tool

•SQL console to SQLite files on device

•generally you use adb to open a shell, then sqlite3:

$ adb -s emulator-5554 shell•# sqlite3 /data/data/com.example.google.rss.rssexample/databases/rssitems.db•SQLite version 3.3.12•Enter ".help" for instructions•.... enter commands, then quit...•sqlite> .exit

db4odb4o

db4o is a small-footprintobject database– http://www.db4o.com– object database means no mapping files

no ORM, no impedance mismatch, no external schema...– simply define POJOs (no annotations required, even)

db4odb4o

Opening the database

ObjectContainer db = null;•try {• db = Db4o.openFile("mydata.db4o");•• // . . .•}•catch (Exception ex) {• // Do more than just log the exception, please•}•finally { • if (db != null)• db.close();•}

db4odb4o

Storing an object

// Create the new Person•Person p = new Person("Matthew", "Neward", 13, new Address(...));••// Store•db.set(p);•db.commit();

db4odb4o

Fetching an object (prototype-based)

// Fetch all the Persons•ObjectSet persons = • // use empty/default object (all fields hold null/0 values)• db.get(new Person());• // or could use type parameter: • // db.get(Person.class);•while (persons.hasNext())• System.out.println(persons.next());••// Fetch all the Newards•ObjectSet newards = db.get(new Person(null, "Neward", 0, null);•while (newards.hasNext())• System.out.println(newards.next());

db4odb4o

Updating an object

// Fetch Matthew Neward (note the absence of error-checking)•Person matthew = (Person)• db.get(new Person("Matthew", "Neward", 0, null).next();••// Happy Birthday!•matthew.setAge(matthew.getAge() + 1);••// Store•db.set(matthew);•db.commit();

db4odb4o

db4o provides more functionality than just what's shown here– Native queries– Automatic reference retrieval ("fetch depth")– Native refactoring support– Store to memory or to disk (or across the network)– Transactions model– ... and more

Wrapping upWrapping up

"What do you know?"

SummarySummary

Android provides a local-storage option... that offers the traditional storage options... that can be customized in many ways... that ultimately will only get more powerful

ResourcesResources

Busy Coder's Guide to Android (and more)Mark Murphy, http://www.commonsware.com

Android websitehttp://developer.android.com

Presentations by this guy– Busy Android Dev's Guide to UI– Busy Android Dev's Guide to Communication– ... and more