Here's some example code that won't work if the snapshot is taken while the screen is locked (and it was locked after application start) in both Java 7 and 8.
 
import javafx.application.Application;
import javafx.application.Platform;
import javafx.scene.Scene;
import javafx.scene.image.Image;
import javafx.scene.layout.BorderPane;
import javafx.scene.paint.Color;
import javafx.scene.shape.Rectangle;
import javafx.stage.Stage;

public class SnapshotTester extends Application {

    @Override
    public void start(Stage primaryStage) throws Exception {

        final BorderPane borderPane = new BorderPane();
        final Rectangle rectangle = new Rectangle(20, 20, Color.BLUE);
        borderPane.setCenter(rectangle);
        Scene scene = new Scene(borderPane);
        primaryStage.setScene(scene);
        primaryStage.show();

        new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    Thread.sleep(2000);
                    Platform.runLater(new Runnable() {
                        @Override
                        public void run() {
                            Image snapshot = rectangle.snapshot(null, null);
                            System.out.println("Snapshot taken: " + snapshot);
                        }
                    });
                } catch (InterruptedException ignored) {
                }
            }
        }).start();
    }

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

Steps to reproduce:

1) Start application

2) Lock screen as soon as window is shown

3) Unlock after two seconds


You'll get the following exception which is unfortunately not very expressive:


java.lang.IllegalArgumentException: Unrecognized image loader: null
at javafx.scene.image.WritableImage.loadTkImage(WritableImage.java:230)
at javafx.scene.image.WritableImage.access$000(WritableImage.java:44)
at javafx.scene.image.WritableImage$1.loadTkImage(WritableImage.java:49)
at javafx.scene.Scene.doSnapshot(Scene.java:1144)
at javafx.scene.Node.doSnapshot(Node.java:1632)
at javafx.scene.Node.snapshot(Node.java:1710)

Might be related to RT-30362