Android AsyncTask and SQLite DB instance

问题

I have a problem and I am not sure how to approach it. An activity in my app has multiple AsyncTasks which access single SQLiteOpenHelper. I initialize and open the helper in onCreate() and I am closing it in onStop(). I also check if it has been initialized in onResume().

Since I have published my app I received number of errors with Null Exception in doInBackground where I try to access the DB helper. I know that this is happens because the DB is closed ( onStop() ) just before the doInBackground is called, fair enough.

My question is, where should I close the DB connection? Is it right to use a single instance of the DB helper in the Activity and access it from multiple threads(AsyncTasks)? Or I should use separate DB helper instance for each AsyncTask?

This is a simplified skeleton of my activity:

public class MyActivity extends Activity{
    private DbHelper mDbHelper;
    private ArrayList<ExampleObject> objects;

    @Override
    public void onStop(){
        super.onStop();
        if(mDbHelper != null){
            mDbHelper.close();
            mDbHelper = null;
        }
    }

    @Override
    public void onResume(){
        super.onResume();
        if(mDbHelper == null){
            mDbHelper = new DbHelper(this);
            mDbHelper.open();
        }
    }

    @Override 
    public void onCreate(Bundle icicle) { 
        super.onCreate(icicle); 
        DbHelper mDbHelper = new DbHelper(this);
        mDbHelper.open();
    }

    private class DoSomething extends AsyncTask<String, Void, Void> {

        @Override
        protected Void doInBackground(String... arg0) {
            objects = mDbHelper.getMyExampleObjects();
            return null;
        }

        @Override
        protected void onPostExecute(final Void unused){
            //update UI with my objects
        }
    }

    private class DoSomethingElse extends AsyncTask<String, Void, Void> {

        @Override
        protected Void doInBackground(String... arg0) {
            objects = mDbHelper.getSortedObjects();
            return null;
        }

        @Override
        protected void onPostExecute(final Void unused){
            //update UI with my objects
        }
    }
}

回答1:

You mentioned that you cancel the AsyncTask before closing the DB. But you should keep in mind that cancelling the AsyncTask just signals the task to be cancelled and you need to check isCancelled() in doInBackground() and do the needful to stop DB operations.

Before closing the DB, you then need to check getStatus() to be sure that the AsyncTask has stopped.

回答2:

You don't need to manage db connection for each activity. You can do it in an instance of android.app.Application and access db using this instance. Something like this:

public class MyApplication extends Application {

    // Synchronized because it's possible to get a race condition here
    // if db is accessed from different threads. This synchronization can be optimized
    // thought I wander if it's necessary
    public synchronized static SQLiteDatabase db() {
        if(self().mDbOpenHelper == null) {
            self().mDbOpenHelper = new MyDbOpenHelper();
        }
        return self().mDbOpenHelper.getWritableDatabase();
    }

    public static Context context() {
        return self();
    }

    @Override
    public void onCreate() {
        super.onCreate();
        mSelf = this;
    }

    private static MyApplication self() {
        if (self == null) throw new IllegalStateException();
        return mSelf;
    }

    private MyDbOpenHelper mDbOpenHelper;

    private static MyApplication mSelf;
}

This way you can be sure you Db is always accessible.

And yes, having one instance of Db helper is a good practice. Thread syncronisation is made for you by default.

回答3:

It is good to use a single DB Helper. The problem is that when the user leaves the Activity the DB is closed but the AsyncTask may still run. So you should check that the DB is not null when you are trying to access it, and if it is null this may mean that your Activity was destroyed and cancel that task.

来源：https://stackoverflow.com/questions/7514021/android-asynctask-and-sqlite-db-instance