Dan Dan - 1 month ago 16
Java Question

Using Singleton design pattern for SQLiteDatabase

I'm rather newbie on Android, and I'm working on a simple application to get some basic experience. My app is pretty simple and consists among other things of a broadcast receiver and some activities. Both components make use of a single database, so in theory it could happen that both try to access the db concurrently.

Currently I'm simply instantiating the db object (which is-a SQLite db helper class) each time I need it, and performing the needed operations: query, insert, etc.

From what I've been reading here and in some other documents, this has the problem of getting a "db locked" exception in case the db is accessed concurrently, so a better approach would be having a single instance of this db object so all components use the same db connection at all times.

Is the above reasoning correct? Would a singleton then be a good-enough solution for this? I know some purists may argue against it, but please note that this is a rather simple application so I can afford doing things I wouldn't in other cases.

Otherwise, what would a better option be? I've read about using content provider but it would be too much for this, besides that I'm not interested to share the data with other activities. I have indeed read this post and found it rather helpful.

Answer

Click here to see my blog post on this subject.


Here is some sample code that illustrates three possible approaches. These will allow access to the database throughout the application.

Approach #1: 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://android-developers.blogspot.nl/2009/01/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 #2: abstract the SQLite database with a `ContentProvider`

This is the approach I would suggest. For one, the new CursorLoader class requires ContentProviders, so if you want an Activity or Fragment to implement LoaderManager.LoaderCallbacks<Cursor> with a CursorLoader (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 this helps!

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