Unable to get firebase id token synchronously on worker thread

Question

I have a class extending FirebaseMessagingService where in onMessageRecieve() method I schedule a work using worker manager. In the work class, I want to POST Firebase Id token to my server( for which I am using synchronized volley request ) but for the Firebase IdToken i am using Tasks.await() inside my work class but the logcat shows the following error:

E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.calendarapp, PID: 29746
java.lang.IllegalStateException: Must not be called on the main application thread
    at com.google.android.gms.common.internal.Preconditions.checkNotMainThread(com.google.android.gms:play-services-basement@@17.1.1:51)
    at com.google.android.gms.common.internal.Preconditions.checkNotMainThread(com.google.android.gms:play-services-basement@@17.1.1:48)
    at com.google.android.gms.tasks.Tasks.await(Unknown Source:16)
    at com.example.calendarapp.MyWorker$1.onComplete(MyWorker.java:130)
    at com.google.android.gms.tasks.zzj.run(Unknown Source:4)
    at android.os.Handler.handleCallback(Handler.java:883)
    at android.os.Handler.dispatchMessage(Handler.java:100)
    at android.os.Looper.loop(Looper.java:214)
    at android.app.ActivityThread.main(ActivityThread.java:7397)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:935)

FirebaseMessagingService

public class MyFirebaseMessagingService extends FirebaseMessagingService {
    private static final String TAG = "MyFirebaseMsgService";
    FirebaseAuth mAuth;
    @Override
    public void onMessageReceived(@NonNull RemoteMessage remoteMessage) {
        super.onMessageReceived(remoteMessage);
        sendNotification(remoteMessage);
        scheduleJob();
    }
    private void scheduleJob() {
        // [START dispatch_job]
        Constraints constraints = new Constraints.Builder()
                .setRequiredNetworkType(NetworkType.CONNECTED)
                .build();
        OneTimeWorkRequest work = new OneTimeWorkRequest.Builder(MyWorker.class)
                .setConstraints(constraints)
                .build();
        WorkManager.getInstance().beginWith(work).enqueue();
        // [END dispatch_job]
    }
    public void sendNotification(){
    ....
    }
}

MyWorker class

public class MyWorker extends Worker {
Context context_i;
private static String url="https://my_url";
public MyWorker(
        @NonNull Context context,
        @NonNull WorkerParameters params) {
    super(context, params);
    context_i=context;
}

@Override
public Result doWork() {
    final DatabaseHandler databaseHandler=new DatabaseHandler(context_i);
    HashMap<String,String> map_post=getPostObject();

    RequestFuture<JSONObject> future = RequestFuture.newFuture();

    JsonObjectRequest request = new JsonObjectRequest(Request.Method.POST, url, new JSONObject(map_post), future, future);

    VolleySingleton.getmInstance(getApplicationContext()).addToRequestQueue(request);
    try {
        try {
            databaseHandler.deleteDatabase();
        } catch (JSONException e) {
            e.printStackTrace();
        }

        JSONObject response = future.get(60, TimeUnit.SECONDS);
        Iterator<String> keys = response.keys();
        while(keys.hasNext()){
            String eventId= keys.next();
            try {
                JSONObject jsonObject = response.getJSONObject(eventId);
                String marker="none";
                if(jsonObject.has("markedAs")){
                    marker=jsonObject.getString("markedAs");
                }
                ArrayList<String> attachmentsList= new ArrayList<>();
                ArrayList<String> attachmentNameList=new ArrayList<>();
                if(jsonObject.has("attachments")) {
                    JSONObject attachmentJson = jsonObject.getJSONObject("attachments");
                    Iterator iterator = attachmentJson.keys();
                    while(iterator.hasNext()){
                        String name_of_attachment= (String) iterator.next();
                        attachmentsList.add(attachmentJson.getString(name_of_attachment));
                        attachmentNameList.add(name_of_attachment);
                    }
                }
                ListItems item =new ListItems(
                        jsonObject.getString("name"),
                        jsonObject.getString("desc"),
                        jsonObject.getString("byName"),
                        jsonObject.getString("date"),
                        "Time: "+jsonObject.getString("time"),
                        "Venue: "+jsonObject.getString("venue"),marker,eventId,attachmentsList
                );
                item.setNameList(attachmentNameList);
                item.setPhotoUrl(jsonObject.getString("photoURL"));
                databaseHandler.addEvent(item);
            } catch (JSONException e) {
                e.printStackTrace();
            }
        }
        databaseHandler.close();
        Log.d("worker_tag", response.toString());
        return Result.success();
    } catch (InterruptedException e) {
        e.printStackTrace();
        // exception handling
        return Result.failure();
    } catch (ExecutionException e) {
        e.printStackTrace();
        return Result.failure();
        // exception handling
    } catch (TimeoutException e) {
        e.printStackTrace();
        return Result.failure();
    }
}
public HashMap<String,String> getPostObject() {
    final HashMap<String,String> mapToken=new HashMap<String, String>();
    FirebaseAuth mAuth = FirebaseAuth.getInstance();
    final FirebaseUser user = mAuth.getCurrentUser();
    if (user != null) {
        user.getIdToken(true).addOnCompleteListener(new OnCompleteListener<GetTokenResult>() {
            @RequiresApi(api = Build.VERSION_CODES.KITKAT)
            @Override
            public void onComplete(@NonNull Task<GetTokenResult> task) {
                try {
                    Tasks.await(task);
                    if(task.isSuccessful()){
                        mapToken.put("token", Objects.requireNonNull(task.getResult()).getToken());
                        Log.d("PostToken", Objects.requireNonNull(task.getResult().getToken()));
                        Log.d("PostObject", String.valueOf(new JSONObject(mapToken)));

                    }
                } catch (ExecutionException e) {
                    e.printStackTrace();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        });
    }
    return mapToken;
}

Isnt Work class a background thread? If it is then why error show "must not apply to main application thread" ?

Thanks in advance.

Answer 1

user.getIdToken(true) returns a Task<GetTokenResult>. Since you're already on a background thread, you shouldn't use addOnCompleteListener() at all - just call Tasks.await() immediately to synchronously get the result.

if (user != null) {
    Task<GetTokenResult> task = user.getIdToken(true)
    try {
        GetTokenResult result = Tasks.await(task);
        mapToken.put("token", Objects.requireNonNull(result).getToken());
        Log.d("PostToken", Objects.requireNonNull(result).getToken()));
        Log.d("PostObject", String.valueOf(new JSONObject(mapToken)));
    } catch (ExecutionException e) {
        e.printStackTrace();
    } catch (InterruptedException e) {
        e.printStackTrace();
    }
}