I have the following code which is executed asynchronously. I would like to make it synchronous in order to follow some logical flow but I cannot work out how.
You will see that scanning is set to true to indicate that the method is still working, at the beginning - I then initiate a findPrinters(...) command - this contains a DiscoveryHandler which runs asynchronously - foundPrinter() is called each time an item is discovered. discoveryFinished() is when the discovery process is successfully completed, and discoveryError(...) is called whenever an error occurs.
I rely on something being set in my DiscoveryHandler before I would like to return from this method. Hence why I have while (scanning) underneath it. But this feels like a hack to me, and not the correct way of doing things. I cannot get wait() and notify() working. Can someone tell me what the correct way to do this is please?
private boolean findPrinter(final Context ctx) {
try {
scanning = true;
BluetoothDiscoverer.findPrinters(ctx, new DiscoveryHandler() {
public void foundPrinter(DiscoveredPrinter device) {
if (device instanceof DiscoveredPrinterBluetooth) {
DiscoveredPrinterBluetooth btDevice = (DiscoveredPrinterBluetooth) device;
if (btDevice.friendlyName.startsWith("XXXX")) {
try {
connection = new BluetoothConnection(btDevice.address);
connection.open();
if (connection.isConnected()) {
address = btDevice.address;
}
} catch (Exception ex) {
}
}
}
}
public void discoveryFinished() {
scanning = false;
}
public void discoveryError(String arg0) {
scanning = false;
}
});
} catch (Exception ex) {
}
while (scanning) {}
return false;
}
You could do this with CountDownLatch, which might be the lightest synchronization primitive in java.util.concurrent:
private boolean findPrinter(final Context ctx) {
final CountDownLatch latch = new CountDownLatch(1);
final boolean[] result = {false};
...
BluetoothDiscoverer.findPrinters(ctx, new DiscoveryHandler() {
...
public void discoveryFinished() {
result[0] = true;
latch.countDown();
}
public void discoveryError(String arg0) {
result[0] = false;
latch.countDown();
}
...
}
// before final return
// wait for 10 seconds for the response
latch.await(10, TimeUnit.SECONDS);
//return the result, it will return false when there is timeout
return result[0];
}
There are a bunch of ways you can do this and wait()/notify() is probably not the best since you probably want to return something from your async method. As such I suggest using something like a BlockingQueue. Here is a simplified example of how you can do this:
private boolean findPrinter(final Context ctx) {
final BlockingQueue<?> asyncResult = new SynchronousQueue<?>();
try {
BluetoothDiscoverer.findPrinters(ctx, new DiscoveryHandler() {
public void foundPrinter(DiscoveredPrinter device) {
if (device instanceof DiscoveredPrinterBluetooth) {
DiscoveredPrinterBluetooth btDevice = (DiscoveredPrinterBluetooth) device;
if (btDevice.friendlyName.startsWith("XXXX")) {
try {
connection = new BluetoothConnection(btDevice.address);
connection.open();
if (connection.isConnected()) {
address = btDevice.address;
}
} catch (Exception ex) {
}
}
}
}
public void discoveryFinished() {
asyncResult.put(true);
}
public void discoveryError(String arg0) {
asyncResult.put(arg0);
}
});
} catch (Exception ex) {
}
Object result = asyncResult.take();
if (result instanceof Boolean) {
return (Boolean) result;
} else if (result instanceof String) {
logError((String) result);
}
return false;
}
One problem with using SynchronousQueue here though is that if discoveryFinished()/discoveryError() is called more than once, then the thread executing the code asynchronously will block forever since the SynchronousQueue assumes there will be exactly one take() per every put() and will block if a put() is made without a corresponding take() or vice versa. So if in your case those methods can be called more than once you would probably use a different kind of BlockingQueue instead (see documentation).
Related
I am trying to call a method multiple times every 60 seconds until a success response from the method which actually calls a rest end point on a different service. As of now I am using do while loop and using
Thread.sleep(60000);
to make the main thread wait 60 seconds which I feel is not the ideal way due to concurrency issues.
I came across the CountDownLatch method using
CountDownLatch latch = new CountDownLatch(1);
boolean processingCompleteWithin60Second = latch.await(60, TimeUnit.SECONDS);
#Override
public void run(){
String processStat = null;
try {
status = getStat(processStatId);
if("SUCCEEDED".equals(processStat))
{
latch.countDown();
}
} catch (Exception e) {
e.printStackTrace();
}
}
I have the run method in a different class which implements runnable. Not able to get this working. Any idea what is wrong?
You could use a CompletableFuture instead of CountDownLatch to return the result:
CompletableFuture<String> future = new CompletableFuture<>();
invokeYourLogicInAnotherThread(future);
String result = future.get(); // this blocks
And in another thread (possibly in a loop):
#Override
public void run() {
String processStat = null;
try {
status = getStat(processStatId);
if("SUCCEEDED".equals(processStat))
{
future.complete(processStat);
}
} catch (Exception e) {
future.completeExceptionally(e);
}
}
future.get() will block until something is submitted via complete() method and return the submitted value, or it will throw the exception supplied via completeExceptionally() wrapped in an ExecutionException.
There is also get() version with timeout limit:
String result = future.get(60, TimeUnit.SECONDS);
Finally got it to work using Executor Framework.
final int[] value = new int[1];
pollExecutor.scheduleWithFixedDelay(new Runnable() {
Map<String, String> statMap = null;
#Override
public void run() {
try {
statMap = coldService.doPoll(id);
} catch (Exception e) {
}
if (statMap != null) {
for (Map.Entry<String, String> entry : statMap
.entrySet()) {
if ("failed".equals(entry.getValue())) {
value[0] = 2;
pollExecutor.shutdown();
}
}
}
}
}, 0, 5, TimeUnit.MINUTES);
try {
pollExecutor.awaitTermination(40, TimeUnit.MINUTES);
} catch (InterruptedException e) {
}
I am writing a job queue using BlockingQueue and ExecutorService. It basically waiting new data in the queue, if there are any data put into the queue, executorService will fetch data from queue. But the problem is that i am using a loop that loops to wait the queue to have data and thus the cpu usage is super high.
I am new to use this api. Not sure how to improve this.
ExecutorService mExecutorService = Executors.newSingleThreadExecutor();
BlockingQueue<T> mBlockingQueue = new ArrayBlockingQueue();
public void handleRequests() {
Future<T> future = mExecutorService.submit(new WorkerHandler(mBlockingQueue, mQueueState));
try {
value = future.get();
} catch (InterruptedException | ExecutionException e) {
e.printStackTrace();
}
if (mListener != null && returnedValue != null) {
mListener.onNewItemDequeued(value);
}
}
}
private static class WorkerHandler<T> implements Callable<T> {
private final BlockingQueue<T> mBlockingQueue;
private PollingQueueState mQueueState;
PollingRequestHandler(BlockingQueue<T> blockingQueue, PollingQueueState state) {
mBlockingQueue = blockingQueue;
mQueueState = state;
}
#Override
public T call() throws Exception {
T value = null;
while (true) { // problem is here, this loop takes full cpu usage if queue is empty
if (mBlockingQueue.isEmpty()) {
mQueueState = PollingQueueState.WAITING;
} else {
mQueueState = PollingQueueState.FETCHING;
}
if (mQueueState == PollingQueueState.FETCHING) {
try {
value = mBlockingQueue.take();
break;
} catch (InterruptedException e) {
Log.e(TAG, e.getMessage(), e);
break;
}
}
}
Any suggestions on how to improve this would be much appreciated!
You don't need to test for the queue to be empty, you just take(), so the thread blocks until data is available.
When an element is put on the queue the thread awakens an value is set.
If you don't need to cancel the task you just need:
#Override
public T call() throws Exception {
T value = mBlockingQueue.take();
return value;
}
If you want to be able to cancel the task :
#Override
public T call() throws Exception {
T value = null;
while (value==null) {
try {
value = mBlockingQueue.poll(50L,TimeUnit.MILLISECONDS);
break;
} catch (InterruptedException e) {
Log.e(TAG, e.getMessage(), e);
break;
}
}
return value;
}
if (mBlockingQueue.isEmpty()) {
mQueueState = PollingQueueState.WAITING;
} else {
mQueueState = PollingQueueState.FETCHING;
}
if (mQueueState == PollingQueueState.FETCHING)
Remove these lines, the break;, and the matching closing brace.
I have an executorService that does not wait for the executorService part to complete and it directly returns the return value without waiting
below is my code: Please see if I am implementing the executorService properly and help me correct it if required
public boolean validateForm() {
flag=true;
executorService = Executors.newSingleThreadExecutor();
Future f = executorService.submit(new Runnable() {
public void run() {
Log.e("FLAGssssss", "" + flag);
checkSourceCode(new BooleanCallBack() {
#Override
public void onSuccess(boolean result) {
Log.e("RESULT ISSSSS", "" + result);
validateCode = result;
Log.e("validateSourceCode ISSSSS", "" + validateSourceCode(result));
if (validateSourceCode(result) == false) {
flag = false;
}
Log.e("FLAG ISSSSS", "" + flag);
}
});
}
});
try {
if (f.get() != null) {
flag = true;
}
Log.e("FUTURE IS", "" + f.get());
} catch (InterruptedException e) {
e.printStackTrace();
} catch (ExecutionException e) {
e.printStackTrace();
}
if (!accept_checkbox.isChecked()) {
Toast.makeText(getActivity().getBaseContext(), "Accept The Terms & Conditions To Proceed", Toast.LENGTH_LONG).show();
// accept_checkbox.requestFocus();
flag = false;
}
if (!validateAddress()) {
flag = false;
}
if (!validatelandmark()) {
flag = false;
}
if (!validateDistrict()) {
flag = false;
}
if (!validatePincode()) {
flag = false;
}
if (!validateFullfillment()) {
flag = false;
}
if (flag) {
saveData();
}
executorService.shutdown();
return flag; //flag is returned even before the executorService above is executed first
}
It looks like there is some confusion on what constitutes a callback from being constructed vs. a callback executing.
The following code is creating and submitting a new Runnable task, but the Runnable is only printing a message and then constructing a BooleanCallBack. I'm not sure exactly what BooleanCallBack does, but assuming it really is a callback, the code in the callback will not execute right away.
For simplicity, lets think of the Runnable like this:
Future future = executorService.submit(new Runnable() {
public void run() {
sysou("runnable is running");
checkSourceCode(new BooleanCallBack() {
#Override
public void onSuccess(boolean result) {
sysou("callback from the runnable is running");
}
});
}
});
When you invoke future.get(), it waits for the run() method to complete execution, which involves constructing the BooleanCallBack but not actually running it. Similar to how constructing a new Runnable object does not mean the code in the run() method is executed.
So if your code calls future.get(), the only thing you can be sure of is that the run() method has completed (i.e. you have gotten the "runnable is running" message). NOT that BooleanCallBack.onSuccess() has executed.
If you actually want to wait for the "callback from the runnable is running" bit, then you need to establish a reference to the BooleanCallBack so you can check its status.
I work on a wizard for creation of a java project and get a invalid thread access exception if I run it in the empty workspace for the first time. I try to implement my wizard similar to JavaProjectWizard, but I don't need the second page, so I try to perform finish from the first page and to initialize the second page in advance:
import org.eclipse.jdt.ui.wizards.NewJavaProjectWizardPageTwo;
import org.eclipse.jface.wizard.Wizard;
import org.eclipse.ui.INewWizard;
public class SomeNewWizard
extends Wizard
implements INewWizard {
private SomeWizardPageTwo javaWizardPageTwo;
#Override
public void addPages() {
if (javaWizardPageTwo == null)
someWizardPageTwo = new SomeWizardPageTwo(newSeeAppWizardPageOne);
}
#Override
public boolean performFinish() {
/*line 109*/someWizardPageTwo .createProvisonalProject();
final IWorkspaceRunnable op = new IWorkspaceRunnable() {
public void run(IProgressMonitor monitor)
throws CoreException, OperationCanceledException {
try {
someWizardPageTwo.performFinish(new SubProgressMonitor(monitor, 1));
}
catch (InterruptedException e) {
throw new OperationCanceledException(e.getMessage());
}
finally {
monitor.done();
}
}
};
try {
rule = null;
Job job = Job.getJobManager().currentJob();
if (job != null)
rule = job.getRule();
IRunnableWithProgress runnable = new IRunnableWithProgress() {
#Override
public void run(IProgressMonitor monitor)
throws InvocationTargetException, InterruptedException {
try {
JavaCore.run(op, rule, monitor);
}
catch (OperationCanceledException e) {
throw new InterruptedException(e.getMessage());
}
catch (CoreException e) {
throw new InvocationTargetException(e);
}
}
};
getContainer().run(true, true, runnable);
}
catch (InvocationTargetException e) {
handleFinishException(getShell(), e);
return false;
}
catch (InterruptedException e) {
return false;
}
return true;
}
public class SomeWizardPageTwo
extends NewJavaProjectWizardPageTwo
{
private NewJavaProjectWizardPageOne mainPage;
public SomeWizardPageTwo(NewJavaProjectWizardPageOne mainPage)
{
super(mainPage);
this.mainPage = mainPage;
}
#Override
public IProject createProvisonalProject()
{
return super.createProvisonalProject();
}
#Override
protected IWizardContainer getContainer()
{
if (mainPage == null)
return null;
return mainPage.getWizard().getContainer();
}
}
The stacktrace can be found here.
The root of the issue seems that ImageDescriptorRegistry is created from the wrong thread because the corresponding IRunnableWithProgress runs forked. But I wonder why does it work for the normal JavaProjectWizard then? And the main question is: how to make it work for my wizard?
getContainer().run(true, true, runnable); causes the runnable to be executed in a separate thread. The first parameter fork is responsible therefore.
The call to javaWizardPageTwo.performFinish() atempts to access the UI thread and causes the invalid thread access exception.
If you set the fork parameter to false, the code will be executed on the current thread.
Why don't you call javaWizardPageTwo.performFinish() directly?
There is not a bug in Display.checkDevice.
Your call to NewJavaProjectWizardPageTwo.performFinish is running in a background thread, but the code is using ImageDescriptorRegistry which needs to be initialized on the User Interface thread.
This is intermittent because sometimes something else that you do will have already initialized the registry.
I have to send a set of files to several computers through a certain port. The fact is that, each time that the method that sends the files is called, the destination data (address and port) is calculated. Therefore, using a loop that creates a thread for each method call, and surround the method call with a try-catch statement for a BindException to process the situation of the program trying to use a port which is already in use (different destination addresses may receive the message through the same port) telling the thread to wait some seconds and then restart to retry, and keep trying until the exception is not thrown (the shipping is successfully performed).
I didn't know why (although I could guess it when I first saw it), Netbeans warned me about that sleeping a Thread object inside a loop is not the best choice. Then I googled a bit for further information and found this link to another stackoverflow post, which looked so interesting (I had never heard of the ThreadPoolExecutor class). I've been reading both that link and the API in order to try to improve my program, but I'm not yet pretty sure about how am I supposed to apply that in my program. Could anybody give a helping hand on this please?
EDIT: The important code:
for (Iterator<String> it = ConnectionsPanel.list.getSelectedValuesList().iterator(); it.hasNext();) {
final String x = it.next();
new Thread() {
#Override
public void run() {
ConnectionsPanel.singleAddVideos(x);
}
}.start();
}
private static void singleAddVideos(String connName) {
String newVideosInfo = "";
for (Iterator<Video> it = ConnectionsPanel.videosToSend.iterator(); it.hasNext();) {
newVideosInfo = newVideosInfo.concat(it.next().toString());
}
try {
MassiveDesktopClient.sendMessage("hi", connName);
if (MassiveDesktopClient.receiveMessage(connName).matches("hello")) {
MassiveDesktopClient.sendMessage(newVideosInfo, connName);
}
} catch (BindException ex) {
MassiveDesktopClient.println("Attempted to use a port which is already being used. Waiting and retrying...", new Exception().getStackTrace()[0].getLineNumber());
try {
Thread.sleep(MassiveDesktopClient.PORT_BUSY_DELAY_SECONDS * 1000);
} catch (InterruptedException ex1) {
JOptionPane.showMessageDialog(null, ex1.toString(), "Error", JOptionPane.ERROR_MESSAGE);
}
ConnectionsPanel.singleAddVideos(connName);
return;
}
for (Iterator<Video> it = ConnectionsPanel.videosToSend.iterator(); it.hasNext();) {
try {
MassiveDesktopClient.sendFile(it.next().getAttribute("name"), connName);
} catch (BindException ex) {
MassiveDesktopClient.println("Attempted to use a port which is already being used. Waiting and retrying...", new Exception().getStackTrace()[0].getLineNumber());
try {
Thread.sleep(MassiveDesktopClient.PORT_BUSY_DELAY_SECONDS * 1000);
} catch (InterruptedException ex1) {
JOptionPane.showMessageDialog(null, ex1.toString(), "Error", JOptionPane.ERROR_MESSAGE);
}
ConnectionsPanel.singleAddVideos(connName);
return;
}
}
}
Your question is not very clear - I understand that you want to rerun your task until it succeeds (no BindException). To do that, you could:
try to run your code without catching the exception
capture the exception from the future
reschedule the task a bit later if it fails
A simplified code would be as below - add error messages and refine as needed:
public static void main(String[] args) throws Exception {
ScheduledExecutorService scheduler = Executors.newScheduledThreadPool(corePoolSize);
final String x = "video";
Callable<Void> yourTask = new Callable<Void>() {
#Override
public Void call() throws BindException {
ConnectionsPanel.singleAddVideos(x);
return null;
}
};
Future f = scheduler.submit(yourTask);
boolean added = false; //it will retry until success
//you might use an int instead to retry
//n times only and avoid the risk of infinite loop
while (!added) {
try {
f.get();
added = true; //added set to true if no exception caught
} catch (ExecutionException e) {
if (e.getCause() instanceof BindException) {
scheduler.schedule(yourTask, 3, TimeUnit.SECONDS); //reschedule in 3 seconds
} else {
//another exception was thrown => handle it
}
}
}
}
public static class ConnectionsPanel {
private static void singleAddVideos(String connName) throws BindException {
String newVideosInfo = "";
for (Iterator<Video> it = ConnectionsPanel.videosToSend.iterator(); it.hasNext();) {
newVideosInfo = newVideosInfo.concat(it.next().toString());
}
MassiveDesktopClient.sendMessage("hi", connName);
if (MassiveDesktopClient.receiveMessage(connName).matches("hello")) {
MassiveDesktopClient.sendMessage(newVideosInfo, connName);
}
for (Iterator<Video> it = ConnectionsPanel.videosToSend.iterator(); it.hasNext();) {
MassiveDesktopClient.sendFile(it.next().getAttribute("name"), connName);
}
}
}