Is it OK to have one instance of SQLiteOpenHelper shared by all Activities in an Android application?

Question

Would it be OK to have a single instance of SQLiteOpenHelper as a member of a subclassed Application, and have all Activities that need an instance of SQLiteDatabase get it from the one helper?

Answer 1

Click here to see my blog post on this subject.

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CommonsWare is right on (as usual). Expanding on his post, here is some sample code that illustrates three possible approaches. These will allow access to the database throughout the application.

Approach #1: subclassing `Application`

If you know your application won't be very complicated (i.e. if you know you'll only end up having one subclass of Application), then you can create a subclass of Application and have your main Activity extend it. This ensures that one instance of the database is running throughout the Application's entire life cycle.

public class MainApplication extends Application {      /**      * see NotePad tutorial for an example implementation of DataDbAdapter      */     private static DataDbAdapter mDbHelper;      /**      * Called when the application is starting, before any other       * application objects have been created. Implementations       * should be as quick as possible...      */     @Override     public void onCreate() {         super.onCreate();         mDbHelper = new DataDbAdapter(this);         mDbHelper.open();     }      public static DataDbAdapter getDatabaseHelper() {         return mDbHelper;     } } 

Approach #2: have `SQLiteOpenHelper` be a static data member

This isn't the complete implementation, but it should give you a good idea on how to go about designing the DatabaseHelper class correctly. The static factory method ensures that there exists only one DatabaseHelper instance at any time.

/**  * create custom DatabaseHelper class that extends SQLiteOpenHelper  */ public class DatabaseHelper extends SQLiteOpenHelper {      private static DatabaseHelper mInstance = null;      private static final String DATABASE_NAME = "databaseName";     private static final String DATABASE_TABLE = "tableName";     private static final int DATABASE_VERSION = 1;      private Context mCxt;      public static DatabaseHelper getInstance(Context ctx) {         /**           * use the application context as suggested by CommonsWare.          * this will ensure that you dont accidentally leak an Activitys          * context (see this article for more information:           * http://developer.android.com/resources/articles/avoiding-memory-leaks.html)          */         if (mInstance == null) {             mInstance = new DatabaseHelper(ctx.getApplicationContext());         }         return mInstance;     }      /**      * constructor should be private to prevent direct instantiation.      * make call to static factory method "getInstance()" instead.      */     private DatabaseHelper(Context ctx) {         super(context, DATABASE_NAME, null, DATABASE_VERSION);         this.mCtx = ctx;     } } 

Approach #3: abstract the SQLite database with a `ContentProvider`

This is the approach I would suggest. For one, the new LoaderManager class relies heavily on ContentProviders, so if you want an Activity or Fragment to implement LoaderManager.LoaderCallbacks<Cursor> (which I suggest you take advantage of, it is magical!), you'll need to implement a ContentProvider for your application. Further, you don't need to worry about making a Singleton database helper with ContentProviders. Simply call getContentResolver() from the Activity and the system will take care of everything for you (in other words, there is no need for designing a Singleton pattern to prevent multiple instances from being created).

Hope that helps!

Answer 2

Having a single SQLiteOpenHelper instance can help in threading cases. Since all threads would share the common SQLiteDatabase, synchronization of operations is provided.

However, I wouldn't make a subclass of Application. Just have a static data member that is your SQLiteOpenHelper. Both approaches give you something accessible from anywhere. However, there is only one subclass of Application, making it more difficult for you to use other subclasses of Application (e.g., GreenDroid requires one IIRC). Using a static data member avoids that. However, do use the Application Context when instantiating this static SQLiteOpenHelper (constructor parameter), so you do not leak some other Context.

And, in cases where you aren't dealing with multiple threads, you can avoid any possible memory leak issues by just using one SQLiteOpenHelper instance per component. However, in practice, you should be dealing with multiple threads (e.g., a Loader), so this recommendation is only relevant for trivial applications, such as those found in some books... :-)