Database global instance

So I want to have one database instance for all application activities. I found the following code:

public class MyApplication extends Application {

    private static SQLiteDatabase mDB = null;

    @Override
    public void onCreate() {
        super.onCreate();
    DataBaseOpenHelper m_OpenHelper = new DataBaseOpenHelper( this );
    mDB = m_OpenHelper.getWritableDatabase();
    }

    public static SQLiteDatabase getDB() {
       开发者_JS百科 return mDB;
    }
}

I don`t understand when I can close SQLiteDatabase instance.

This was an issue for me when I first started out with Android, as there aren't many tutorials on the web that describe how to correctly allow access to your database across the entire application (don't ask me why). Here's some sample code that exhibits three possible approaches.

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;

    /**
     * create the database helper when the application is launched 
     */
    @Override
    public void onCreate() {
        mDbHelper = new DataDbAdapter(this);
        mDbHelper.open();
    }

    /** 
     * close the database helper when the application terminates.
     */
    @Override
    public void onTerminate() {
        mDbHelper.close();
        mDbHelper = null;
    }

    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!

You dont really need to close it. It will be automatically closed when your process dies. Instead of using the Application object, you can just make your DB helper object a singleton for easier access. BTW, getWritableDatabase() and getReadableDatabase() are not that different. The only difference is that getReadableDatabase() might work if you are out of space, while the other will throw an exception.

I dont think this is recommended, if anything you could have a global instance of DataBaseOpenHelper, and call getWritableDatabase() at the points where you actually need the DB. And obviously, only get the writable instance when you need it, otherwise use getReadableDatabase().

I'm not sure exactly the reason behind this, but my guess is that this is an expensive object to get and/or hold on to, so you don't want to create it when the app loads and hold on to a global instance. Maybe someone else can clarify this point. I wish the Android docs would explain this better because I also had the impulse to do this.