When cancelling a Task, the exception property has the value IllegalStateException, when I expect it to have a value of null:

java.lang.IllegalStateException: Task must only be used from the FX Application Thread
at javafx.concurrent.Task.checkThread(Task.java:1219)
at javafx.concurrent.Task.getValue(Task.java:962)
at javafx.concurrent.Task$TaskCallable.call(Task.java:1446)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at java.lang.Thread.run(Thread.java:745)

Actually, in an earlier version it seems like it behaved as expected: see the Fig. 6.8 on page 273, chapter "Collections and Concurrency" in Pro JavaFX 2 by James Weaver.

Below is a runnable example that demonstrates the issue, using one of the examples in Task's api doc.

/*
 * Created on 05.06.2013
 *
 */
package chapter4layoutandcontrols;

import javafx.application.Application;
import javafx.application.Platform;
import javafx.beans.binding.Bindings;
import javafx.beans.binding.StringBinding;
import javafx.beans.value.ChangeListener;
import javafx.beans.value.ObservableValue;
import javafx.concurrent.Task;
import javafx.concurrent.Worker;
import javafx.event.ActionEvent;
import javafx.event.EventHandler;
import javafx.geometry.Insets;
import javafx.geometry.Pos;
import javafx.scene.Parent;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.control.Label;
import javafx.scene.control.TextArea;
import javafx.scene.layout.BorderPane;
import javafx.scene.layout.BorderPaneBuilder;
import javafx.scene.layout.HBox;
import javafx.scene.layout.HBoxBuilder;
import javafx.scene.layout.Pane;
import javafx.scene.layout.VBox;
import javafx.stage.Stage;

/**
 * Issue: Task throws IllegalStateException on cancel
 *
 * To reproduce:
 * run, start, cancel - the listener to the exception property shows the message
 * "Task must only be used from the FX Application Thread" (plus
 * its stacktrace) though all access seems to be on the fx-thread.
 *
 * @author Jeanette Winzenburg, Berlin
 */
@SuppressWarnings({"unchecked", "rawtypes"})
public class WorkerIllegalStateException extends Application {

    private Task worker;

    private Button startButton;

    private Button cancelButton;

    public WorkerIllegalStateException() {
        worker = createTask();
    }

    // This is the example from java doc of Task
    private Task createTask() {
        Task<Integer> task = new Task<Integer>() {
            @Override
            protected Integer call() throws Exception {
                int iterations;
                for (iterations = 0; iterations < 1000; iterations++) {
                    if (isCancelled()) {
                        updateMessage("Cancelled at " + iterations);
                        break;
                    }
                    updateMessage("Iteration " + iterations);
                    updateProgress(iterations, 1000);
    
                    // Now block the thread for a short time, but be sure
                    // to check the interrupted exception for cancellation!
                    try {
                        Thread.sleep(100);
                    } catch (InterruptedException interrupted) {
                        if (isCancelled()) {
                            updateMessage("Cancelled at: " + iterations);
                            break;
                        }
                    }
                }
                return iterations;
            }
        };
        return task;
    }

    private void hookupEvents() {
        startButton.setOnAction(actionEvent -> new Thread(worker).start());
        cancelButton.setOnAction(actionEvent -> worker.cancel());
        startButton.disableProperty().bind(
                worker.stateProperty().isNotEqualTo(Worker.State.READY));
        cancelButton.disableProperty().bind(
                worker.stateProperty().isNotEqualTo(Worker.State.RUNNING));
        // listen to changes of the exception property
        ChangeListener l = (observable, oldValue, newValue) -> {
            if (!(newValue instanceof Throwable)) return;
            ((Throwable) newValue).printStackTrace();
        };
        worker.exceptionProperty().addListener(l);
    }

    private Parent createContent() {
        startButton = new Button("Start");
        cancelButton = new Button("Cancel");
        hookupEvents();
        
        Label message = new Label();
        message.textProperty().bind(worker.messageProperty());
        Label state = new Label();
        state.textProperty().bind(Bindings.format("%s", worker.stateProperty()));
        VBox topPane = new VBox(10);
        topPane.setPadding(new Insets(10, 10, 10, 10));
        topPane.setAlignment(Pos.CENTER);
        topPane.getChildren().addAll(message, state);

        HBox buttonPane = new HBox(10);
        buttonPane.setPadding(new Insets(10, 10, 10, 10));
        buttonPane.setAlignment(Pos.CENTER);
        buttonPane.getChildren().addAll(startButton, cancelButton);
        BorderPane pane = new BorderPane();
        pane.setCenter(topPane);
        pane.setBottom(buttonPane);
        return pane;
    }

    @Override
    public void start(Stage stage) throws Exception {
        Scene scene = new Scene(createContent());
        stage.setScene(scene);
        stage.show();
    }

    public static void main(String[] args) {
        Application.launch(args);
    }
}