为什么在JavaFX 2.2服务中两次调用了我的计划方法

Question

See the following example:

public class Test extends Application {
  public static void main(final String[] args) {
    launch(args);
  }

  @Override
  public void start(final Stage primaryStage) {
    primaryStage.setTitle("Hi!");
    primaryStage.setScene(new Scene(new BorderPane()));
    primaryStage.show();

    Service<Void> myService = new Service<Void>() {
      @Override
      protected Task<Void> createTask() {
        return new Task<Void>() {
          @Override
          protected void cancelled() {
            super.cancelled();
            System.out.println(Thread.currentThread().getName() + " - cancelled() called");
          }

          @Override
          protected void failed() {
            super.failed();
            System.out.println(Thread.currentThread().getName() + " - failed() called");
          }

          @Override
          protected void running() {
            super.running();
            System.out.println(Thread.currentThread().getName() + " - running() called");
          }

          @Override
          protected void succeeded() {
            super.succeeded();
            System.out.println(Thread.currentThread().getName() + " - succeeded() called");
          }

          @Override
          protected void scheduled() {
            super.scheduled();
            System.out.println(Thread.currentThread().getName() + " - scheduled() called");
          }

          @Override
          protected Void call() throws Exception {
            System.out.println(Thread.currentThread().getName() + " - call() called");
            return null;
          }
        };
      }
    };
    myService.stateProperty().addListener(new ChangeListener<State>() {
      @Override
      public void changed(ObservableValue<? extends State> arg0, State arg1, State newValue) {
        System.out.println("State: " + newValue);                   
      }
    });
    myService.start();
  }
}

This gives the following output:

State: SCHEDULED
JavaFX Application Thread - scheduled() called
Thread-4 - call() called
JavaFX Application Thread - scheduled() called
State: RUNNING
JavaFX Application Thread - running() called
State: SUCCEEDED
JavaFX Application Thread - succeeded() called

Note that this line occurs twice:

JavaFX Application Thread - scheduled() called

This means the "scheduled()" method is called twice. I would have expected the method to be called only once. Am I doing something wrong, is this is bug, do I misinterpret the API?

Thanks!

Answer 1

The lifecycle of the Worker object is defined as follows. When created, the Worker object is in the READY state. Upon being scheduled for work, the Worker object transitions to the SCHEDULED state. After that, when the Worker object is performing the work, its state becomes RUNNING. Note that even when the Worker object is immediately started without being scheduled, it first transitions to the SCHEDULED state and then to the RUNNING state.

from this Concurency JavaFX