import javafx.application.Application;
import javafx.event.ActionEvent;
import javafx.event.EventHandler;
import javafx.scene.Scene;
import javafx.scene.control.Button;
import javafx.scene.layout.StackPane;
import javafx.scene.layout.VBox;
import javafx.stage.DirectoryChooser;
import javafx.stage.Modality;
import javafx.stage.Stage;

public class DirChooser extends Application {
    public static void main(String[] args) {
        launch(args);
    }
    
    @Override
    public void start(final Stage primaryStage) {
        primaryStage.setScene(new Scene(new StackPane(), 300, 250));
        primaryStage.show();

        showMessageBox(primaryStage);
    }

    private void showMessageBox(final Stage primaryStage) {
        final Stage msgBox = new Stage();
        msgBox.initModality(Modality.APPLICATION_MODAL);
        msgBox.setResizable(false);

        Button okButton = new Button("OK");
        okButton.setOnAction(new EventHandler<ActionEvent>() {
            @Override
            public void handle(ActionEvent event) {
                msgBox.close();

                DirectoryChooser dirChooser = new DirectoryChooser();
                System.out.println("selected file: " + dirChooser.showDialog(primaryStage));
                System.out.println(
                        "On Mac you can see this line while the chooser is still active! " +
                        "No Directoy has been selected yet and therefore the selected file is null. " +
                        "The actual selected Directory is lost somewhere in Nirvana...");
            }
        });

        VBox contentBox = new VBox();
        contentBox.getChildren().add(okButton);

        msgBox.setScene(new Scene(contentBox, 200, 100));
        msgBox.showAndWait();
    }
}