Synchronize the asynchronous callback method call

We are working on a solution which is like this;

Request: (We receive the request via API call and send to third-party via a library we use)
OUR-Client --> OUR-API --> THIRD-PARTY

Response: (This response we receive from third-party asynchronously through a callback method given in the library we are using)
THIRD-PARTY --> OUR-CODE --> OUR-Client

Here is the below code and want to get rid of Thread.sleep() call and make use of the callback to provide response.

----- API Method -------------

    @GetMapping
    	public ResponseEntity<String> getData(@RequestBody String requestId) throws SessionNotFound, InterruptedException {
    		dataService.get(requestId);
    		String msg;
    		long start = System.currentTimeMillis();
    		do {
                // We want to get rid of this sleep() statement and some way to callback here as soon there is message.
    			Thread.sleep(30);
    			msg = clientApp.getRespnse(requestId);
    		} while(msg == null);
    		return ResponseEntity.ok(msg);
    	}


------- Service Class and Methods ---------------

@Service
public class DataService {
	
	
	@Autowired
	private ClientApp clientApp;
	
	public void get(String requestId) throws SessionNotFound {
// This method is from the library we use. This only submits the request, response is received on different method.
        send(requestId);        
	}


------- Component Class and Methods ---------------

@Component
public class ClientFixApp {

    private Map<String, String> responseMap = new HashMap<>();

// This method is callback from the third party library, whenever there is response this method will get invoked and this message we need to send as response of the API call.
    @Override
	public void onResponse(String requestId)
			throws FieldNotFound, IncorrectDataFormat, IncorrectTagValue, UnsupportedMessageType {
		responseMap.put(msgId, jsonMsg);
	}

  public String getRespnse(String requestId) {
		return responseMap.get(requestId);
	}
}

DataService and ClientFixApp are flawed by design (the very fact it is 2 different classes while there must be one, speaks a lot). Truly asynchronous programs must allow to register user procedure as a callack, called when the I/O operation finished (successfully or not). ClientFixApp silently writes the result in a table, leaving for client no other option except polling.

You can use any existing asynchronous http library. For example, below is the code for the library java.net.http included in Java 11 and later. Other libraries have similar functionality.

public static CompletableFuture<HttpResponse<String>> doGet(String uri) {
    HttpClient client = HttpClient.newHttpClient();
    HttpRequest request = HttpRequest.newBuilder()
            .uri(URI.create(uri))
            .build();
    return client.sendAsync(request, HttpResponse.BodyHandlers.ofString());
}

public static void main(String[] args) {
    CompletableFuture<HttpResponse<String>> future = doGet("https://postman-echo.com/get");
    future.thenApply(HttpResponse::body)
            .thenAccept(System.out::println)
            .join();
}