Develop Reference

How do I get results from my work queue task?

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.

Realm Async Task

I'm trying to use AsyncTransaction for inserting some object, but for the moment it is a failed...
I tried to debug, but without success either ...
See my code :
realm.executeTransactionAsync(
new Realm.Transaction() {
#Override
public void execute(#NonNull Realm realm) {
Log.i("Insert", "Insert start");
realm.insert(character);
}
}, new Realm.Transaction.OnSuccess() {
#Override
public void onSuccess() {
Log.i("Insert", "Insert complete");
finish();
}
}, new Realm.Transaction.OnError() {
#Override
public void onError(Throwable error) {
Log.i("Insert","Error " + error.getMessage());
}
});
When i debug, i see that I didn't go on any callback of the async-transaction, there is no log, there is nothing that can help me.
Thank in advance,
EDIT :
public RealmAsyncTask executeTransactionAsync(final Transaction transaction,
#Nullable final Realm.Transaction.OnSuccess onSuccess,
#Nullable final Realm.Transaction.OnError onError) {
checkIfValid();
//noinspection ConstantConditions
if (transaction == null) {
throw new IllegalArgumentException("Transaction should not be null");
}
// Avoid to call canDeliverNotification() in bg thread.
final boolean canDeliverNotification = sharedRealm.capabilities.canDeliverNotification();
// If the user provided a Callback then we have to make sure the current Realm has an events looper to deliver
// the results.
if ((onSuccess != null || onError != null)) {
sharedRealm.capabilities.checkCanDeliverNotification("Callback cannot be delivered on current thread.");
}
// We need to use the same configuration to open a background SharedRealm (i.e Realm)
// to perform the transaction
final RealmConfiguration realmConfiguration = getConfiguration();
// We need to deliver the callback even if the Realm is closed. So acquire a reference to the notifier here.
final RealmNotifier realmNotifier = sharedRealm.realmNotifier;
final Future<?> pendingTransaction = asyncTaskExecutor.submitTransaction(new Runnable() {
#Override
public void run() {
if (Thread.currentThread().isInterrupted()) {
return;
}
SharedRealm.VersionID versionID = null;
Throwable exception = null;
final Realm bgRealm = Realm.getInstance(realmConfiguration);
bgRealm.beginTransaction();
// NOTHING IS DONE AFTER IS POINT .....
try {
transaction.execute(bgRealm);
if (Thread.currentThread().isInterrupted()) {
return;
}
bgRealm.commitTransaction();
// The bgRealm needs to be closed before post event to caller's handler to avoid concurrency
// problem. This is currently guaranteed by posting callbacks later below.
versionID = bgRealm.sharedRealm.getVersionID();
} catch (final Throwable e) {
exception = e;
} finally {
try {
if (bgRealm.isInTransaction()) {
bgRealm.cancelTransaction();
}
} finally {
bgRealm.close();
}
}
final Throwable backgroundException = exception;
final SharedRealm.VersionID backgroundVersionID = versionID;
// Cannot be interrupted anymore.
if (canDeliverNotification) {
if (backgroundVersionID != null && onSuccess != null) {
realmNotifier.post(new Runnable() {
#Override
public void run() {
if (isClosed()) {
// The caller Realm is closed. Just call the onSuccess. Since the new created Realm
// cannot be behind the background one.
onSuccess.onSuccess();
return;
}
if (sharedRealm.getVersionID().compareTo(backgroundVersionID) < 0) {
sharedRealm.realmNotifier.addTransactionCallback(new Runnable() {
#Override
public void run() {
onSuccess.onSuccess();
}
});
} else {
onSuccess.onSuccess();
}
}
});
} else if (backgroundException != null) {
realmNotifier.post(new Runnable() {
#Override
public void run() {
if (onError != null) {
onError.onError(backgroundException);
} else {
throw new RealmException("Async transaction failed", backgroundException);
}
}
});
}
} else {
if (backgroundException != null) {
// FIXME: ThreadPoolExecutor will never throw the exception in the background.
// We need a redesign of the async transaction API.
// Throw in the worker thread since the caller thread cannot get notifications.
throw new RealmException("Async transaction failed", backgroundException);
}
}
}
});
return new RealmAsyncTaskImpl(pendingTransaction, asyncTaskExecutor);
}

I found the trick.
In my constructor, i add some RealmObject to a another, that create a error
(Can't not write on a non write transaction)
The second point, was i used beginTransaction() on the parent, but it block on the other part for asynctransaction
I change my code for using the RealmRecyclerView on the firstPart and i didn't have the problem anymore
Thanks

How to wait for callback before leaving method (Java)

I have a method in which I call another method that has a callback. I want to receive this callback before leaving my method. I saw some other posts in which latches are used. My code looks like this:
public void requestSecurityToken(<some params>){
final CountDownLatch latch = new CountDownLatch(1);
MyFunction.execute(<someParams>, new RequestListener<Login>() {
#Override
public void onRequestFailure(SpiceException spiceException) {
//TODO
}
#Override
public void onRequestSuccess(Login login) {
//handle some other stuff
latch.countDown();
}
});
try {
latch.await();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
This doesn't work, the method is stuck in the await() function. What happens is that, the method immediately jumps to the await(), and doesn't go into the onRequestSuccess() or onRequestFailure() method again. I guess this is a concurency problem... Any ideas on how to fix this issue?
EDIT: Added the line of code where I create the latch.

When you are doing this
new RequestListener<Login>
You are passing an object to your function , which implements an interface.
That is why those methods are not getting called , those methods are called only when you get the request result (success or failure).
You can do this instead.
MyFunction.execute(<someParams>, new RequestListener<Login>() {
#Override
public void onRequestFailure(SpiceException spiceException) {
someFunction();
}
#Override
public void onRequestSuccess(Login login) {
//handle some other stuff
someFunction();
latch.countDown();
}
});
public void someFunction()[
try {
latch.await();
} catch (InterruptedException e) {
e.printStackTrace();
}
}

Terminating thread from inside it's run method in Eclipse-RCP

I have done something like this in my code
public void doWork()
{
Job job = new Job("Job")
{
#Override
protected IStatus run(IProgressMonitor monitor) {
while (rsMemName.next()) {
Display.getDefault().syncExec(new Runnable() {
#Override
public void run() {
try {
String memId = rsMemName.getString("id");
if (doMemberTasks(memId)==false)
{
cnn.rollback();
return;
}
} catch (Exception ex) {
ex.printStackTrace();
try {
cnn.rollback();
return;
} catch (SQLException e) {
e.printStackTrace();
}
}
});
}
}
}
job.schedule();
}
What i want to do is exit from the whole method if doMemberTasks(memId) returns false.
But it doesn't return from the method and keep looping on ResultSet. how can i terminate the thread from the run method?
Please give any suggestions how could i achieve that.....
Thanks in advance....

This is because return will return only from the thread run method. What you can do is set a variable(flag) probably static, and check its value after the run code to put another return statement.

Yeah your best bet would be to have a flag,
boolean doWork = true;
...
while( doWork && rsMemName.next(){
...
if (doMemberTasks(memId)==false)
{
cnn.rollback();
doWork = false;
return;
}

Thread not continue running

I have a following code as below:
new Thread(new Test1Runnable()).start(); // Line (a)
public class Test1Runnable implements Runnable {
public void run() {
Test2Runnable task1 = new Test2Runnable();
ExecutorService executor = Executors.newSingleThreadExecutor();
try {
executor.submit(task1);
while(true) {
if(task1.isDone()) {
break;
}
// Thread.sleep(2000); // Line (b)
}
if(!task1.hasError()) { // Line (c)
executor.submit(new Test3Runnable());
}
} catch(Exception ex) {
if(executor != null) {
executor.shutdown();
}
}
}
}
public class Test2Runnable implements Runnable {
private Exception error;
private boolean done;
public void run() {
reset();
doRun();
done = true;
}
protected void doRun() {
try{
// ...
// ....
} catch(Exception ex) {
}
}
private void reset() {
error = null;
done = false;
}
public boolean isDone() {
return done;
}
public boolean hasError() {
return getError() != null || getNonSuccess() > 0;
}
public Exception getError() {
return error;
}
}
I have an issue when I run Test1Runnable at line (a) and comment Line (b) then the thread hang and not run to Line (c). If I uncomment line (b) or I add breakpoint at line (c) and activate remote debug the thread continue to run to the end as normal. Could anyone can give me some advice about this? Why the Thread not continue running? All threads run without any exception.

Looks like you have a race conditioin here, so result of the execution depends on timings, debug enabled, etc. The code posted is more or less fine, the error is likely to be in Test2Runnable class. I suppose there are some flags (isDone, hasError) that have visibility issues. Try to declare them volatile.
Please add Test2Runnable code here and I'll be able to give more precise answer.