UI.getCurrent() is null in ErrorHandler

Throwing an exception in the ui.access() block from bg thread will call ErrorHandler.error() with UI.getCurrent() set to null. That will fail to show any notification from the ErrorHandler with the following stacktrace:

[Thread-8] ERROR com.vaadin.flow.component.UI - UI instance is not available. It means that you are calling this method out of a normal workflow where it's always implicitly set. That may happen if you call the method from the custom thread without 'UI::access' or from tests without proper initialization.
java.lang.IllegalStateException: UI instance is not available. It means that you are calling this method out of a normal workflow where it's always implicitly set. That may happen if you call the method from the custom thread without 'UI::access' or from tests without proper initialization.
	at com.vaadin.flow.component.notification.Notification.setOpened(Notification.java:426)
	at com.vaadin.flow.component.notification.Notification.open(Notification.java:303)
	at com.vaadin.flow.component.notification.Notification.show(Notification.java:227)
	at com.vaadin.flow.component.notification.Notification.show(Notification.java:244)
	at org.vaadin.example.Bootstrap.error(Bootstrap.java:20)
	at com.vaadin.flow.component.UI$1.handleError(UI.java:520)
	at com.vaadin.flow.server.FutureAccess.handleError(FutureAccess.java:76)
	at com.vaadin.flow.server.VaadinService.runPendingAccessTasks(VaadinService.java:2045)
	at com.vaadin.flow.server.VaadinSession.unlock(VaadinSession.java:681)
	at com.vaadin.flow.server.VaadinService.ensureAccessQueuePurged(VaadinService.java:2005)
	at com.vaadin.flow.server.VaadinService.accessSession(VaadinService.java:1972)
	at com.vaadin.flow.server.VaadinSession.access(VaadinSession.java:969)
	at com.vaadin.flow.component.UI.access(UI.java:506)
	at com.vaadin.flow.component.UI.access(UI.java:489)
	at org.vaadin.example.MainView$1.run(MainView.java:52)

The ErrorHandler does not document whether it’s called in Vaadin UI thread or not. If it doesn’t, this needs to be clearly documented. If yes, then the Vaadin environment should be set properly - the UI.getCurrent() should not return null. In either case, this needs to be documented in the ErrorHandler javadoc.

Minimal reproducible example

Attaching the skeleton starter app which demoes the issue. In short, this view demoes the issue:

public class MainView extends VerticalLayout {

    public MainView() {
        // Use TextField for standard text input
        TextField textField = new TextField("Wait for it");
        add(textField);

        final UI ui = UI.getCurrent();
        new Thread() {
            @Override
            public void run() {
                try {
                    Thread.sleep(1000);
                    ui.access(() -> {
                       throw new RuntimeException("Simulated failure");
                    });
                } catch (Throwable t) {
                    t.printStackTrace();
                }
            }
        }.start();
    }
}

Make sure to have the ErrorHandler set; wait for 1 second then check out the console log.

skeleton-starter-flow.zip

Expected behavior

The UI.getCurrent() should not be null.

Actual behavior

The UI.getCurrent() is null.

Versions:

- Vaadin / Flow version: 14.5.1 / 2.5.1
- Java version: 11

[TatuLund]

This is not a bug, it just means that you need to setup your ErrorHandler to be thread aware, e.g. this worked for me

public class InitListener implements VaadinServiceInitListener {

    @Override
    public void serviceInit(ServiceInitEvent serviceInitEvent) {
        serviceInitEvent.getSource().addUIInitListener(event -> {
            UI ui = event.getUI();
            VaadinSession session = ui.getSession();
            setErrorHandler(session, ui);
        }
    }

    private void setErrorHandler(VaadinSession session, UI ui) {
        session.setErrorHandler(error -> {
            ui.access(() -> {
                ConfirmDialog confirmDialog = new ConfirmDialog("Error",
                        "Internal Error: " + error.getThrowable().getMessage(),
                        "Do Something", confirmFire -> {
                        });
                String trace = "";
                for (StackTraceElement s : error.getThrowable()
                        .getStackTrace()) {
                    trace = trace + "  at " + s.getClassName() + ".java line "
                            + s.getLineNumber() + " " + s.getMethodName()
                            + "\n";
                }
                confirmDialog.add(new Pre(trace));
                confirmDialog.setWidth("500px");
                confirmDialog.setHeight("500px");
                confirmDialog.open();
                error.getThrowable().printStackTrace();
            });
        });
    }
}

[mvysny]

Sevice - UI is one to many mapping when there are multiple browser tabs.

That is true, but Vaadin can simply remember the UI instance in the ui.access() itself, for example within the new ErrorHandlingCommand() when it is constructed. Or in the FutureAccess class. If there is a will, there is a way 😉