Working with databases in android?

I have a database in my app and when I need to insert 开发者_如何学Pythonor delete something I have to open the database,of course. I do this every activity, so that means that I open the database even if it is open. In DDMS I get :"Leak found". What should I do to open my database only once time? Should I use a singleton class?

Yes, Singleton is the best option. You can use a common instance to access the database. If you want to share DB with external activities, then go for content provider.

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!