Odtnhj

I've implemented a simply work queue that receives tasks from a number of different threads. I want these tasks to return a value to their source thread, but can't figure out how to do that.
I've considered using a future, but there's no way to explicitly set the future's value. I could use a property, but I don't believe those are thread safe.

Every task is an implementation of DBRequest. The actual content varies, but the result of all activities is a string.
An asynchronous thread creates a DBRequest and submits it to the queue. The queue runs the task, which produces a string. How do I get that string back to the thread that created the DBRequest, and how can I cause my creator thread to wait for the result?

public interface DBRequest 
 String execute(VdtsSysDB vdtsSysDB, BoardLoad currentLoad);


public class DBQueue implements Runnable 
 private static DBQueue dbQueue;
 private LinkedBlockingQueue<DBRequest> queue = new LinkedBlockingQueue<>();
 private VdtsSysDB vdtsSysDB = new VdtsSysDB();
 private ReentrantLock lock = new ReentrantLock();
 private static final Logger LOG = LoggerFactory.getLogger(DBQueue.class);
 private boolean kill = false;

 private BoardLoad currentLoad;
 private ProgressController progressController;

 public static DBQueue getInstance() 
 if (dbQueue == null) synchronized (DBQueue.class) 
 if (dbQueue == null)
 dbQueue = new DBQueue();
 
 return dbQueue;
 

 private DBQueue() 
 

 public ReentrantLock getLock() 
 return lock;
 

 @Override
 public void run() 
 LOG.info("Starting DBQueue loop. Kill .", kill);
 while (!kill) 
 DBRequest dbRequest = removeRequest();
 if (dbRequest != null) 
 lock.lock();
 String result = dbRequest.execute(vdtsSysDB, currentLoad);
 lock.unlock();
 if (progressController != null) Platform.runLater(() ->
 progressController.updateDisplay(currentLoad));
 
 
 vdtsSysDB.getEntityManager().close();
 


 public void addRequest(DBRequest dbRequest) 
 try 
 queue.add(dbRequest);
 LOG.info("Added request.");
 catch (Exception e) 
 LOG.error("Can't add element.", e);
 
 

 private DBRequest removeRequest() 
 DBRequest result = null;
 try 
 //result = queue.poll(10, TimeUnit.SECONDS);
 result = queue.take();
 catch (Exception e) 
 LOG.error("Exception.", e);
 
 return result;
 

 public void killDBQueue() 
 kill = true;
 LOG.info("Shutting down DBQueue.");
 

 public static void start() 
 Thread thread = new Thread(DBQueue.getInstance(), "DBQueue Thread");
 thread.start();
 LOG.info("Starting DBQueue.");
 

 public BoardLoad getCurrentLoad() 
 if (currentLoad == null)
 currentLoad = BoardLoad.getLastOpenLoad(vdtsSysDB);
 return currentLoad;
 

 public void setCurrentLoad(BoardLoad proposedLoad) 
 // We can only have one open load, and by definition, the current load is open. So close it.
 if (this.currentLoad != null && !this.currentLoad.equals(proposedLoad)) 
 currentLoad.close(vdtsSysDB);
 if (proposedLoad != null) 
 this.currentLoad = vdtsSysDB.getEntityManager().find(BoardLoad.class, proposedLoad.getId());
 else this.currentLoad = null;
 
 

 public ProgressController getProgressController() 
 return progressController;
 

 public void setProgressController(ProgressController progressController) 
 this.progressController = progressController;
 

EDIT: I'm using this queue to synchronize database access, reducing the need for locks and ensuring that requests are completed sequentially. I don't believe there is any other way to achieve this sort of asynchronous request -> synchronous request change.
But I'd love to have that belief changed.

You should add a reference to the submitting thread in your DBRequest interface and implement a setResult(String result) (or similar) method to receive the result.
You can implement a CountDownLatch waiting (or similar) on your submitting thread run() method to wait setting latch up when sending request to queue and down in setResult method.
If I'm not clear just let me know and I'll elaborate.