multithreadingjavafxtaskcancel-button

JavaFX - Cancel Task doesn't work

In a JavaFX application, I have a method which takes a long time on large input. I'm opening a dialog when it is loading and I'd like the user to be able to cancel/close out the dialog and the task will quit. I created a task and added its cancellation in the cancel button handling. But the cancellation doesn't happen, the task doesn't stop executing.

Task<Void> task = new Task<Void>() {
    @Override
    public Void call() throws Exception {
        // calling a function that does heavy calculations in another class
    };
    task.setOnSucceeded(e -> {
        startButton.setDisable(false);
    });
}
new Thread(task).start();

cancelButton.setOnAction(new EventHandler<ActionEvent>() {
@Override
public void handle(ActionEvent e) {
    System.out.println("Button handled");
    task.cancel();
}
);

Why isn't the task getting canceled when the button clicked?

Solution

  • You have to check on the cancel state (see Task's Javadoc). Have a look at this MCVE:

    public class Example extends Application {

    @Override
    public void start(Stage primaryStage) throws Exception {
        Task<Void> task = new Task<Void>() {
            @Override
            protected Void call() throws Exception {
                new AnotherClass().doHeavyCalculations(this);
                return null;
            }
        };

        Button start = new Button("Start");
        start.setOnMouseClicked(event -> new Thread(task).start());

        Button cancel = new Button("Cancel");
        cancel.setOnMouseClicked(event -> task.cancel());

        primaryStage.setScene(new Scene(new HBox(start, cancel)));
        primaryStage.show();
    }

    private class AnotherClass {

        public void doHeavyCalculations(Task<Void> task) {
            while (true) {
                if (task.isCancelled()) {
                    System.out.println("Canceling...");
                    break;
                } else {
                    System.out.println("Working...");
                }
            }
        }

    }

}

    Note that…