如何将获取的JSON数据与Room Persistence库集成

I am developing an Android Application (Android Studio - Java) which includes a sign in and registration process. I took care of the sign in and registration processes by implementing a connection between PHP files and a MySQL database through http. In short, I just created an AsyncTask class in java --called from another class-- and used it to post data to a PHP file, from there I just used the appropriate SQL commands. This part works fine.

This first part with the login and registration is important because every user will see a slightly different layout once they login. The layout is a RecyclerView composed of detailed CardView elements. Each CardView has a few TextViews with some details. The details are held by an Object i created in a separate class. To fill in the CardView elements a fetched some JSON data using a separate PHP file (and one more http connection). Parsing the data from JSON into Strings and ints was a straightforward endeavor, as was adding them to the list of custom objects. There is some code below showing how I fetched the data and added it to the Object list.

This is the complete AsyncTask class:

public class ScheduleWorker extends AsyncTask<String, Void, String> {
Context context;
AlertDialog alertDialog;
ScheduleWorker (Context ctx) { context = ctx; }

@Override
protected  void onPreExecute() { super.onPreExecute(); }

@Override
protected void onPostExecute (String s) {
    super.onPostExecute(s);
    Toast.makeText(context, s, Toast.LENGTH_SHORT).show();
    try {
        loadJSON(s);
    } catch (JSONException e) {
        e.printStackTrace();
    }
}
@Override
protected String doInBackground(String... params) {
    final String fetch_url = "http://192.168.1.70/newfetcher.php";

    try {
        String ussr_name = params[0];
        URL url = new URL(fetch_url);
        HttpURLConnection con = (HttpURLConnection) url.openConnection();
        con.setRequestMethod("POST");
        con.setDoOutput(true);
        con.setDoInput(true);
        OutputStream outputStream = con.getOutputStream();
        BufferedWriter bufferedWriter = new BufferedWriter(new OutputStreamWriter(outputStream, "UTF-8"));
        String post_data = URLEncoder.encode("user_name", "UTF-8")+"="+URLEncoder.encode(ussr_name, "UTF-8");
        bufferedWriter.write(post_data);
        bufferedWriter.flush();
        bufferedWriter.close();
        outputStream.close();
        StringBuilder sb = new StringBuilder();
        BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(con.getInputStream()));
        String json;

        while ((json = bufferedReader.readLine()) != null) {
            sb.append(json + "
");
        }

        return sb.toString().trim();
    } catch (Exception e) {
        return null;
    }
}

private void loadJSON(String json) throws JSONException {
    JSONArray jsonArray = new JSONArray(json);
    Course course;
    List<Course> courses = new ArrayList<Course>();
    int len = jsonArray.length();

    String[] titles = new String[len];
    String[] types = new String[len];
    String[] teachers = new String[len];
    int[] pics = new int[len];

    for (int i = 0; i < len; i++) {
        JSONObject obj = jsonArray.getJSONObject(i);

        titles[i] = obj.getString("title");
        types[i] = obj.getString("type");
        teachers[i] = obj.getString("teacher");
        pics[i] = obj.getInt("pic");
        course = new Course(titles[i], types[i], teachers[i], pics[i]);
        courses.add(course);
    }
}
}

Now, where I encountered problems is when I tried saving the List of Objects (and their details) to a Room Persistent Database. I based my code for the Room Database off an example from Google Developer CodeLabs. I adapted the code for my particular needs but I kept the underlying class structure. The structure includes: an Entity, a DAO, a RoomDatabase, a Repository, a ViewModel, a ViewHolder, an Adapter for the RecyclerView, and a class to populate the database. Everything seems fine except for the part where I populate the database. The example populates the database by using a callback and an AsyncTask within the RoomDatabase class.

Here is populating AsyncTask:

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

    @Override
    public void onOpen(@NonNull SupportSQLiteDatabase db) {
        super.onOpen(db);
        // If you want to keep the data through app restarts,
        // comment out the following line.
        new PopulateDbAsync(INSTANCE).execute();
    }
};

/**
 * Populate the database in the background.
 * If you want to start with more words, just add them.
 */
private static class PopulateDbAsync extends AsyncTask<Void, Void, Void> {

    private final WordDao mDao;

    PopulateDbAsync(WordRoomDatabase db) {
        mDao = db.wordDao();
    }

    @Override
    protected Void doInBackground(final Void... params) {
        // Start the app with a clean database every time.
        // Not needed if you only populate on creation.

        Word word = new Word("Hello");
        mDao.insert(word);
        word = new Word("World");
        mDao.insert(word);
        return null;
    }
}

My question is how do I populate the database with the detail arrays and/or the Object list? The example executes the callback every time a adding activity is called. I just want to populate the database when the user registers.