How to make spring controller json response wait until firebase query ends?

I'm using an ajax call to spring controller to start a firebase query, save the result to local database and return items saved in the local database (including the results saved from the current and previous firebase queries too). The problem is that since the firebase query runs async, the results from the local database query are returned before the firebase query finishes.
How can I make the returned local database query wait until the firebase query finished?

	@PostMapping("/firebase-download")
	@ResponseBody
	public List<FirebaseTransactionRecord> firebaseDownload() {
		
		firebaseService.downloadAndSaveFirebaseTransactions();
        // wait for the query to end
		return firebaseTransactionRecordRepository.findBySentFalse();
	}	

	@Override
	@Transactional
	public void downloadAndSaveFirebaseTransactions() {

			final List<FirebaseTransactionRecord> firebaseTransactions = new ArrayList<>();

			firebaseDb.child(StringValueConstant.DB_CHILD_TEMPORARY_TRANSACTIONS)
					.addValueEventListener(new ValueEventListener() {

						@Override
						public void onDataChange(DataSnapshot snapshot) {

							for (DataSnapshot userSnapshot : snapshot.getChildren()) {
								userSnapshot.getChildren().forEach(transactionSnapshot -> {
									FirebaseTransactionRecord record = firebaseTransactionMapper
											.toFirebaseTransactionRecord(transactionSnapshot);
									firebaseTransactions.add(record);
								});
							}

							saveFirebaseTransactionRecords(firebaseTransactions);
						}

						@Override
						public void onCancelled(DatabaseError error) {
							log.error("Error reading Firebase data:", error);
						}
					});
	}

	@Override
	@Transactional
	public void saveFirebaseTransactionRecords(List<FirebaseTransactionRecord> firebaseTransactions) {
		firebaseTransactions.forEach(firebaseTransaction -> {
			if (!firebaseTransactionRecordRepository.existsByReferralNumber(firebaseTransaction.getReferralNumber())) {
				firebaseTransactionRecordRepository.save(firebaseTransaction);
			}
		});		
	}	

Something with a CountdownLatch should do the trick.

Set the initial latch counter to 1 (for the main listener), then set it to the number of child nodes in onDataChange. Next pass the latch to saveFirebaseTransactionRecords and decrease it when each transaction completes. Once the latch reaches 0, you can exit out of downloadAndSaveFirebaseTransactions