How to wait cancellation of task after Service#cancel? - javafx

Question

Look at this example:
package sample;
import javafx.application.Application;
import javafx.application.Platform;
import javafx.concurrent.Service;
import javafx.concurrent.Task;
import javafx.stage.Stage;
public class Main extends Application {
//NOTICE: This is class in **other file** (here is just for example)
private static class MyService extends Service {
#Override
protected Task createTask() {
return new Task() {
#Override
protected Object call() throws Exception {
System.out.println("Service: START");
while(true) {
System.out.println("Service: ITERATION");
// Thread.sleep(3000); // This raise InterruptedException after cancel, but how about such code (it won't raise exception):
for(long i = 0; i < 1_000_000_000; i++) {
}
if (isCancelled())
break;
}
System.out.println("Service: END");
return null;
}
};
}
}
#Override
public void start(Stage primaryStage) throws Exception {
MyService myService = new MyService();
myService.start();
Thread.sleep(5000);
myService.cancel();
System.out.println(myService.getState()); // Here is `CANCELLED` already but task isn't finished yet.
// <--- How to wait cancellation of Task here?
System.out.println("This command must be called after `Service: END`");
Platform.exit();
}
public static void main(String[] args) {
launch(args);
}
}
As you known call of Service#cancel doesn't wait cancellation of Task. So, I want to block main thread and await cancellation of Task. How can I do it?
P.S.
Looks like Service doesn't provide any callback/event handler to check real cancellation of Task. Is it right?

Answer 1

By default, Service.cancel() interrupts the Task. So an InterruptedException must be raised and your task will be terminated (forcefully).
One thing you could do is to store the created task in a global variable in your MyService class and override the cancel method like this:
class MyService extends Service {
private Task t;
#Override
public boolean cancel() {
if (t != null) {
return t.cancel(false);
} else {
return false;
}
}
#Override
protected Task createTask() {
t = new Task() { /* ... */ };
return t;
}
}
The rest will be easy. Add a change listener to the service state property (or use setOnCanceled() method) and do whatever you want to do after the state change, in the callback.

Answer 2

Never block the FX Application Thread.
The Service class does indeed define a setOnCancelled(...) method, which you use to register a callback:
myService.setOnCancelled(event -> {
System.out.println("Service was cancelled");
});
Note that when you cancel a Service, it will interrupt the thread if it is blocked. So if you don't catch the InterruptedException it will not exit the call method normally. This is why you don't see the "END" message.
Full example code:
import javafx.animation.PauseTransition;
import javafx.application.Application;
import javafx.application.Platform;
import javafx.concurrent.Service;
import javafx.concurrent.Task;
import javafx.stage.Stage;
import javafx.util.Duration;
public class ServiceCancellationTest extends Application {
//NOTICE: This is class in **other file** (here is just for example)
private static class MyService extends Service<Void> {
#Override
protected Task<Void> createTask() {
return new Task<Void>() {
#Override
protected Void call() throws Exception {
System.out.println("Service: START");
while(! isCancelled()) {
System.out.println("Service: ITERATION");
try {
Thread.sleep(3000);
} catch (InterruptedException interrupted) {
System.out.println("Task interrupted");
}
if (isCancelled())
break;
}
System.out.println("Service: END");
return null;
}
};
}
}
#Override
public void start(Stage primaryStage) throws Exception {
MyService myService = new MyService();
myService.start();
myService.setOnCancelled(event -> {
System.out.println("In cancelled callback: "+myService.getState()); // Here is `CANCELLED` already but task isn't finished yet.
});
// You should never block the FX Application Thread. To effect a pause,
// use a pause transition and execute the code you want in its
// onFinished handler:
PauseTransition pause = new PauseTransition(Duration.seconds(5));
pause.setOnFinished(event -> {
myService.cancel();
System.out.println("After calling cancel: "+myService.getState());
System.out.println("This command must be called after `Service: END`");
Platform.exit();
});
pause.play();
}
public static void main(String[] args) {
launch(args);
}
}