java - Using Singleton design pattern for SQLiteDatabase

Question

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 1

Click here to see my blog post on this subject.

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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!