Javafx deadlock when using setVIsible method with DragEvent

Question

I have a pretty simple implementation of a Drag and Drop that gets a file location and then hides some elements on the stage to display information about the file. However my program gets completely deadlocked whenever I try use the setVisible() or any other command that interacts with the stage. If i just have a println after the drag with the file it works fine, but the moment I have any logic that manipulates the UI it just stops.

This is what the deadlock looks like, after a file is dragged in

"main@1" prio=5 tid=0x1 nid=NA waiting
  java.lang.Thread.State: WAITING
      at sun.misc.Unsafe.park(Unsafe.java:-1)
      at java.util.concurrent.locks.LockSupport.park(LockSupport.java:175)
      at java.util.concurrent.locks.AbstractQueuedSynchronizer.parkAndCheckInterrupt(AbstractQueuedSynchronizer.java:836)
      at java.util.concurrent.locks.AbstractQueuedSynchronizer.doAcquireSharedInterruptibly(AbstractQueuedSynchronizer.java:997)
      at java.util.concurrent.locks.AbstractQueuedSynchronizer.acquireSharedInterruptibly(AbstractQueuedSynchronizer.java:1304)
      at java.util.concurrent.CountDownLatch.await(CountDownLatch.java:231)

Here is the Minimised Main method that gets the drag event.

package sample;

import javafx.application.Application;
import javafx.event.EventHandler;
import javafx.fxml.FXMLLoader;
import javafx.scene.Parent;
import javafx.scene.Scene;
import javafx.scene.input.DragEvent;
import javafx.scene.input.Dragboard;
import javafx.scene.input.TransferMode;
import javafx.stage.Stage;

import java.io.File;

public class Main extends Application {

    @Override
    public void start(Stage primaryStage) throws Exception{
        Parent root = FXMLLoader.load(getClass().getResource("sample.fxml"));
        Scene scene = new Scene(root, 300, 275);
        scene.setOnDragOver(new EventHandler<DragEvent>() {
            @Override
            public void handle(DragEvent event) {
                Dragboard db = event.getDragboard();
                if (db.hasFiles()) {
                    event.acceptTransferModes(TransferMode.COPY);
                } else {
                    event.consume();
                }
            }
        });
        scene.setOnDragDropped(new EventHandler<DragEvent>() {
            @Override
            public void handle(DragEvent event) {

                Dragboard db = event.getDragboard();
                boolean success = false;
                if (db.hasFiles()) {
                    success = true;
                    String filePath = null;
                    for (File file:db.getFiles()) {
                        filePath = file.getAbsolutePath();
                        System.out.println(filePath);
                        Controller controller = new Controller();
                        //Part where it crashes,
                        controller.buttonTest.setVisible(false);
                    }
                }
                event.setDropCompleted(success);
                event.consume();
            }
        });
        primaryStage.setTitle("Hello World");
        primaryStage.setScene(scene);
        primaryStage.show();
    }


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

The Minimised Controller

package sample;
import javafx.scene.control.Button;

public class Controller {
    public Button buttonTest;
}

The Minimised fxml

<?xml version="1.0" encoding="UTF-8"?>
<?import javafx.scene.control.*?>
<?import javafx.scene.layout.*?>
<GridPane alignment="center" hgap="10" vgap="10" xmlns:fx="http://javafx.com/fxml/1" xmlns="http://javafx.com/javafx/8" fx:controller="sample.Controller">
   <columnConstraints>
      <ColumnConstraints />
   </columnConstraints>
   <rowConstraints>
      <RowConstraints />
   </rowConstraints>
   <children>
      <Button fx:id="buttonTest" mnemonicParsing="false" text="Test" />
   </children>
</GridPane>

I thought I might be able to handle this by splitting the dragEvent into it's own thread but as per the concurrency rules in javafx, it would not be able to manipulate the stage anyway since all graphics must be on the main javafxGUI thread.

Hoping someone can point me in the right direction, thanks!

Answer 1

Your code generates a null pointer exception when you drop something onto the window, because button is never initialized in the instance of Controller that you create by hand with Controller controller = new Controller();.

Elements defined in the fxml file are initialized in the controller instance associated with the FXMLLoader when you call load(). So I think what you are trying to do here is access that controller object. (I don't know why you create a new one.) You can do this by creating an FXMLLoader instance and calling getController() on it, instead of using the static load method:

@Override
public void start(Stage primaryStage) throws Exception{
    FXMLLoader loader = new FXMLLoader(getClass().getResource("sample.fxml"));
    Parent root = loader.load();
    Controller controller = loader.getController();

    // ...

    scene.setOnDragDropped(new EventHandler<DragEvent>() {
        @Override
        public void handle(DragEvent event) {

            Dragboard db = event.getDragboard();
            boolean success = false;
            if (db.hasFiles()) {
                success = true;
                String filePath = null;
                for (File file:db.getFiles()) {
                    filePath = file.getAbsolutePath();
                    System.out.println(filePath);
                    // Controller controller = new Controller();
                    //Part where it crashes,
                    controller.buttonTest.setVisible(false);
                }
            }
            event.setDropCompleted(success);
            event.consume();
        }
    });

    // ...

}