android studio connection to neo4j

Question

I have to connect neo4j to android studio. I have a recyclerview that must be populated by neo4j. I made the connection and it works for me but the recyclerview is not populated because in debug I saw that the connection driver variable remains null.

 public static class AsyncTask_Drink extends AsyncTask<String, String, String> {

    static AsyncTask_Drink instance = null;

    Connection conn = null;

    Driver driver;

    @Override
    protected String doInBackground(String... strings) {
        try {
            String uri = "bolt://localhost:7687";
            String username = "neo4j";
            String password = "neo4j";
            driver = GraphDatabase.driver(uri, AuthTokens.basic(username, password));
            Session session = driver.session();
            Log.e("Client", "Connessione effettuata");
       } catch (Exception e) {
            e.printStackTrace();
            Log.e("Client", "Connessione non effettuata");
        }
        return null;
    }

    public Driver getConn() {
        return this.driver;
    }

    public static AsyncTask_Drink getInstance() throws Neo4jException {

        if (instance == null) {

            instance = new AsyncTask_Drink();
        }
        return instance;
    }
}`

public class DaoDrinkDatabase implements DaoDrink {

    public ArrayList<DrinkDomain> getDrinkCocktail() throws Neo4jException, ClassNotFoundException {

        ArrayList<DrinkDomain> drinkDomains = new ArrayList<DrinkDomain>();

        Driver driver =  DrinksFragment.AsyncTask_Drink.getInstance().getConn();

        try(Session session = driver.session()) {

            Result result = session.run("MATCH (d:drink) -[rel:DI_TIPO]->(t:Tipologia {tipo:\"Cocktail\"}) RETURN d,rel,t");

            while (result.hasNext()) {

                Record record = result.next();

                String name = (record.get("name").asString());
                String descriptor = (record.get("descriptor").asString());
                Double fee = (record.get("fee").asDouble());
                String pic = (record.get("pic").asString());

                drinkDomains.add(new DrinkDomain(name, descriptor, fee, pic));
            }
        }
        return drinkDomains;
    }


  public View onCreateView(@NonNull LayoutInflater inflater,
                             ViewGroup container, Bundle savedInstanceState) {
        DrinksViewModel drinksViewModel =
                new ViewModelProvider(this).get(DrinksViewModel.class);

        binding = FragmentDrinksBinding.inflate(inflater, container, false);
        View root = binding.getRoot();

        text_head = root.findViewById(R.id.text_head);
        category = root.findViewById(R.id.fragment_drinks_category);
        cocktail = root.findViewById(R.id.fragment_drinks_Cocktail);
        shake = root.findViewById(R.id.fragment_drinks_Shake);
        recyclerViewcategory = root.findViewById(R.id.fragment_drinks_recyclerView_category);
        recyclerViewcocktail = root.findViewById(R.id.fragment_drinks_recyclerView_cocktail);
        recyclerViewshake =  root.findViewById(R.id.fragment_drinks_recyclerView_shake);

        FunctionThread f = new FunctionThread("drinks");
        Thread t = new Thread(f);
        t.start();
        try {
            t.join();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        if(UserSingleton.getLogged()){

            text_head.setVisibility(View.INVISIBLE);
            category.setVisibility(View.VISIBLE);
            cocktail.setVisibility(View.VISIBLE);
            shake.setVisibility(View.VISIBLE);
            recyclerViewcategory.setVisibility(View.VISIBLE);
            recyclerViewcocktail.setVisibility(View.VISIBLE);
            recyclerViewshake.setVisibility(View.VISIBLE);

        }else{

            text_head.setVisibility(View.VISIBLE);
            category.setVisibility(View.VISIBLE);
            shake.setVisibility(View.INVISIBLE);
            cocktail.setVisibility(View.INVISIBLE);
            recyclerViewcategory.setVisibility(View.VISIBLE);
            recyclerViewcocktail.setVisibility(View.INVISIBLE);
            recyclerViewshake.setVisibility(View.INVISIBLE);

        }

        recyclerViewCategoryList = root.findViewById(R.id.fragment_drinks_recyclerView_category);
        recyclerViewCocktailList = root.findViewById(R.id.fragment_drinks_recyclerView_cocktail);
        recyclerViewShakeList = root.findViewById(R.id.fragment_drinks_recyclerView_shake);

        recyclerViewCategory();

        AsyncTask_Drink asyncTask_drink = new AsyncTask_Drink();
        asyncTask_drink.execute();

        recyclerViewCocktail();

        recyclerViewShake();

        return root;
    }`

Connection made to pass data from neo4j to android with java using recyclerview.

Error:

java.lang.NullPointerException: Attempt to invoke interface method 'org.neo4j.driver.Session org.neo4j.driver.Driver.session()' on a null object reference