JavaFX - 如何使用执行程序一次运行一个任务？

Question

我正在尝试创建一个从URL获取屏幕截图的Javafx应用程序。我遇到问题的地方是线程。在我的主要方法中，我正在尝试运行两个屏幕截图，但在加载第一页后它会卡住。我试图在提交给执行者的任务（下面的代码）中包装monitorPageStatus（）（因为它调用实际的saveToPng（）函数）方法。如何正确地将任务提交给执行者，以便截取两个截图？

public class InsightScreenshot {
   {
         // Clever way to init JavaFX once
         JFXPanel fxPanel = new JFXPanel();
   }
   private Browser browser;
   public Stage stage;
   private Timer timer = new java.util.Timer();

   private ExecutorService exec = Executors.newSingleThreadExecutor(r -> {
    Thread t = new Thread(r);
    t.setDaemon(true); // allows app to exit if tasks are running
    return t ;
});


   @SuppressWarnings("restriction")
   /**
   * 
    * @param url
   * @param imageName add png extension
   */
   public void showWindow(String url, String imagePath) {
         // JavaFX stuff needs to be done on JavaFX thread
         Platform.runLater(new Runnable() {
                private Stage window;

                @SuppressWarnings("restriction")
                @Override
                public void run() {
                      Stage window = new Stage();
                      window.setTitle(url);

                      browser = new Browser(url);
                      monitorPageStatus(imagePath, window);

                      VBox layout = new VBox();
                      layout.getChildren().addAll(browser);
                      Scene scene = new Scene(layout);
                      window.setScene(scene);
                      //window.setOnCloseRequest(we -> System.exit(0));
                      window.show();
                }
         });
   }


   private void monitorPageStatus(String imageName, Stage window) {


            timer.schedule(new TimerTask() {
        @SuppressWarnings("restriction")
        public void run() {
            Platform.runLater(() -> {

                if (browser.isPageLoaded()) {

                    System.out.println("Page now loaded, taking screenshot...");
                    saveAsPng(imageName);
                    window.close();
                    cancel();
                } else
                    System.out.println("Loading page...");

            });
        }
    }, 1000, 1000);

   }

   private void saveAsPng(String imageName) {
         WritableImage image = browser.snapshot(new SnapshotParameters(), null);
         //TODO change file path?
         File file = new File(imageName);
         try {
                ImageIO.write(SwingFXUtils.fromFXImage(image, null), "png", file);
                System.out.println("Screenshot saved as " + imageName);
         } catch (IOException e) {
                e.printStackTrace();
         }
   }

   public static void main(String[] args) {


         InsightScreenshot pic = new InsightScreenshot();
         pic.showWindow("ttps://www.google.com", "/images/google.png");

         InsightScreenshot pic2 = new InsightScreenshot();
         pic2.showWindow("https://www.facebook.com", "/images/fb.png");


   }
}