Performing Asynchronous operations on Vaadin UI - spring-boot

Question

I have a Spring Boot project with Vaadin integration (v14). I want my application to do some background operation and represent the results on the Vaadin-based frontend. For this I have a view which is a Polymer template generated with Vaadin Designer (.js) and connected to a Java companion class. To this view I've just simply added a button initialized with the following listener:
_btnMyTriggerButton.addClickListener(event -> {
CompletableFuture<Void> c = CompletableFuture.supplyAsync(() -> {
for (int i = 0; i < 5; i++)
{
try
{
System.out.println("Waiting");
Thread.sleep(1000);
UI.getCurrent().access(() -> {
Notification.show("Waiting");
});
}
catch (InterruptedException e)
{
}
}
return "Waiting over. Greet!";
}).thenAccept(s -> {
System.out.println(s);
UI.getCurrent().access(() -> {
Notification.show(s);
});
});
I'm trying to access the UI as the documentation says. However when this is being executed it only reaches the first "Waiting", then stops. If I remove the UI interaction (Notification.show()) the output is printed to the backend as desired, but not when attempting do any interaction on the UI..
The structure of my java companion class is the following:
#Tag("my-view")
#JsModule("./my-view.js")
#Route("")
#Push(PushMode.AUTOMATIC)
public class MyView extends PolymerTemplate<MyView.MyModel>
{
#Id("trigger-button")
private Button _btnMyTriggerButton;
MyView() {
// listener initialization code described above
}
public interface MyModel extends TemplateModel
{
}
}
Do I miss something to achieve asynchronous behavior on this webpage?
Any help is appreciated.

Answer 1

The problem in this case is that the first callback terminates with a NullPointerException because UI.getCurrent() returns null when it's run from a background thread. CompletableFuture will just ignore the exception unless you explicitly handle it (e.g. using handle instead of thenAccept) or block on the result.
You can fix this by adding UI ui = UI.getCurrent(); in the beginning of the click listener and then referencing ui in both the supplyAsync and thenAccept callbacks instead of using UI.getCurrent() there.