Persisting Data to the Device

Overview

The Android framework offers several options and strategies for persistence:

• Shared Preferences - Easily save basic data as key-value pairs in a private persisted dictionary.
• Local Files - Save arbitrary files to internal or external device storage.
• SQLite Database - Persist data in tables within an application specific database.
• ORM - Describe and persist model objects using a higher level query/update syntax.

Shared Preferences

Settings can be persisted for your application by using SharedPreferences to persist key-value pairs:

SharedPreferences pref =   
    PreferenceManager.getDefaultSharedPreferences(this);
String username = pref.getString("username", "n/a"); 

SharedPreferences can be edited by getting access to the Editor instance:

SharedPreferences pref =   
    PreferenceManager.getDefaultSharedPreferences(this);
Editor edit = pref.edit();
edit.putString("username", "billy");
edit.putString("user_id", "65");
edit.commit(); 

Local Files

Android can read/write files to internal as well as external storage. Applications have access to an application-specific directory where preferences and sqlite databases are also stored. Every Activity has helpers to get the writeable directory. File I/O API is a subset of the normal Java File API.

Writing files is as simple as getting the stream using openFileOutput method] and writing to it using a BufferedWriter:

// Use Activity method to create a file in the writeable directory
FileOutputStream fos = openFileOutput("filename", MODE_WORLD_WRITEABLE);
// Create buffered writer
BufferedWriter writer = new BufferedWriter(new OutputStreamWriter(fos));
writer.write("Hi, I'm writing stuff");
writer.close();

Reading the file back is then just using a BufferedReader and then building the text into a StringBuffer:

BufferedReader input = null;
input = new BufferedReader(
new InputStreamReader(openFileInput("myfile")));
String line;
StringBuffer buffer = new StringBuffer();
while ((line = input.readLine()) != null) {
  buffer.append(line + "\n");
}
String text = buffer.toString();

You can also inspect and transfer files to emulators or devices using the DDMS File Explorer perspective which allows you to access to filesystem on the device.

SQLite

For maximum control, developers can use SQLite directly by leveraging the SQLiteOpenHelper for executing SQL requests and managing a local database. You can read more in the following tutorials:

• Vogella SQLite Tutorial
• AndroidHive Tutorial
• Techotopia Tutorial

A SQLite tutorial is covered in the book Programming Android, published by O'Reilly. The authors maintain complete working examples from the book on Github. The "MicroJobs" example shows how to use the database helper class in an activity.

• Programming Android SQLite Tutorial

In many cases, rather than interacting with SQL directly, Android apps can leverage one of the many available higher-level ORMs (object relational mappers) to persist Java models to a database table.

Object Relational Mappers

Instead of accessing the SQLite database directly, there is no shortage of higher-level wrappers for managing SQL persistence. There are many popular ORMs for Android, but one of the easiest to use is ActiveAndroid (cliffnotes). Here's a few alternatives as well:

• SugarORM - Very easy syntax, uses reflection to infer data (cliffnotes)
• Siminov - Another viable alternative syntax
• greenDAO - Slightly different take (DAO vs ORM)
• ORMLite - Lightweight and speed is prioritized
• androrm - Newer but promising take on a simple ORM

For this class, we selected ActiveAndroid. With ActiveAndroid, building models that are SQLite backed is easy and explicit using annotations. Instead of manually creating and updating tables and managing SQL queries, simply annotate your model classes to associate fields with database columns:

@Table(name = "Users")
public class User extends Model {
  @Column(name = "Name")
  public String name;

  @Column(name = "Age")
  public int age;
  
  // Make sure to define this constructor (with no arguments)
  // If you don't querying will fail to return results!
  public User() {
    super();
  }
  
  // Be sure to call super() on additional constructors as well
  public User(String name, int age){
    super();
    this.name = name;
    this.age = age;
  }
}

Inserting or updating objects no longer requires manually constructing SQL statements, just use the model class as an ORM:

User user = new User();
user.name = "Jack";
user.age = 25;
user.save();
// or delete easily too
user.delete();

ActiveAndroid queries map to SQL queries and are built by chaining methods.

List<User> users = new Select()
    .from(User.class).where("age > ?", 25)
    .limit(25).offset(0)
    .orderBy("age ASC").execute();

This will automatically query the database and return the results as a List for use. For more information, check out our ActiveAndroid Guide for links to more resources and answers to common questions.

References

• http://developer.android.com/guide/topics/data/data-storage.html
• http://developer.android.com/training/basics/data-storage/index.html
• http://mobile.tutsplus.com/tutorials/android/data-management-options-for-android-applications/
• http://developer.android.com/reference/android/content/SharedPreferences.html