Pre-populate Room Database with Volley

Question

Preface

So, I am pretty new to android development and at the moment I am developing an application that retrieves data from a REST server as a JSON response. The application needs to be functional offline, so in an effort to accomplish this, based on android recommendation, I decided to use Room Database and followed this tutorial: Room with View tutorial .

The tutorial itself was well done, and I was able to follow the basic set up and tailor it a bit to my needs.

Problem

I am however having a bit of trouble pre-populating the RoomDatabase with data from my server. The tutorial illustrated the pre-populating aspect by using static data, and this works fine. However, when I try to modify that to use the data fetched from the server, via the Volley request, is where I am running into problems.

Code

When I attempt to run the following

public abstract class AppDatabase extends RoomDatabase {

    public abstract MachineMovementDao machineMovementDao();

    private static volatile AppDatabase INSTANCE;
    private static Context ctx;

    public static AppDatabase getDatabase(final Context context) {
        ctx = context;
        if (INSTANCE == null) {
            synchronized (AppDatabase.class) {
                if (INSTANCE == null) {
                    INSTANCE = Room.databaseBuilder(context.getApplicationContext(),
                        AppDatabase.class, "local_database")
                        .addCallback(sRoomDatabaseCallback)
                        //.fallbackToDestructiveMigration()
                        .build();
                }
            }
        }
        return INSTANCE;
    }

    public static RoomDatabase.Callback sRoomDatabaseCallback =
        new RoomDatabase.Callback() {

        @Override
        public void onOpen(@NonNull SupportSQLiteDatabase db) {
            super.onOpen(db);
            new PopulateDbAsync(INSTANCE).execute();
        }
    };

    private static class PopulateDbAsync extends AsyncTask<Void, Void, Void> {

        private final MachineMovementDao machineMovementDao;

        PopulateDbAsync(AppDatabase db) {
            machineMovementDao = db.machineMovementDao();
        }

        @Override
        protected Void doInBackground(final Void... params) {
            machineMovementDao.deleteAll();

            String url = "http://192.168.10.124:8888/movement/machine/all";

            JsonArrayRequest jsonArrayRequest = new JsonArrayRequest
                (Request.Method.GET, url, null, new Response.Listener<JSONArray>() {
                    @Override
                    public void onResponse(JSONArray response) {
                        try {
                            for (int i = 0; i < response.length(); i++) {
                                JSONObject jsonObject = response.getJSONObject(i);
                                MachineMovement movement = new MachineMovement(jsonObject);
                                machineMovementDao.insert(movement);
                            }
                        } catch (JSONException e) {
                            System.out.println(e.toString());
                        }

                    }
                }, new Response.ErrorListener() {
                    @Override
                    public void onErrorResponse(VolleyError volleyError) {
                        System.out.println(">>>>>>>>>>>>>>>>>>> > Error:" + volleyError.toString());
                    }
            });

            DataVolleyRequest.getInstance(ctx.getApplicationContext())
                .addToRequestQueue(jsonArrayRequest);

            /*
            MachineMovement movement = new MachineMovement("AMUSEMENT_MACHINE", "00003",
                "SN969NI7VCSXN", 1);
            movement.setLocationFromID(1);
            movement.setLocationToID(2);
            machineMovementDao.insert(movement);
            movement = new MachineMovement("AMUSEMENT_MACHINE", "00003",
                "SN969NI7VCSXN", 1);
            movement.setLocationFromID(2);
            movement.setLocationToID(1);
            machineMovementDao.insert(movement);
            */
            return null;
        }
    }
}

Error

I get the following error

E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.sunnygroup.backoffice.tdcapp, PID: 18175
java.lang.IllegalStateException: Cannot access database on the main thread since it may potentially lock the UI for a long period of time.
    at android.arch.persistence.room.RoomDatabase.assertNotMainThread(RoomDatabase.java:204)
    at android.arch.persistence.room.RoomDatabase.beginTransaction(RoomDatabase.java:251)
    at com.sunnygroup.backoffice.tdcapp.daos.MachineMovementDao_Impl.insert(MachineMovementDao_Impl.java:115)
    at com.sunnygroup.backoffice.tdcapp.database.AppDatabase$PopulateDbAsync$1.onResponse(AppDatabase.java:78)
    at com.sunnygroup.backoffice.tdcapp.database.AppDatabase$PopulateDbAsync$1.onResponse(AppDatabase.java:71)
    at com.android.volley.toolbox.JsonRequest.deliverResponse(JsonRequest.java:90)
    at com.android.volley.ExecutorDelivery$ResponseDeliveryRunnable.run(ExecutorDelivery.java:102)
    at android.os.Handler.handleCallback(Handler.java:739)
    at android.os.Handler.dispatchMessage(Handler.java:95)
    at android.os.Looper.loop(Looper.java:135)
    at android.app.ActivityThread.main(ActivityThread.java:5593)
    at java.lang.reflect.Method.invoke(Native Method)
    at java.lang.reflect.Method.invoke(Method.java:372)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:960)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:755)

line 78 is the line in my code where you see

machineMovementDao.insert(movement);

Any assistance would be greatly appreciated

Answer 1

You can either allow room to run on the main thread ( https://developer.android.com/reference/androidx/room/RoomDatabase.Builder.html#allowMainThreadQueries() )

In your case:

AppDatabase.class, "local_database")
                        .addCallback(sRoomDatabaseCallback)
                        .allowMainThreadQueries()
                        .build();

Note that this is not recommended on production apps.

Or you can run that on a background thread. You can either run that instruction on an asyncTask or look into integrating RxJava into your Room implementation. Here is an example.