Finding the current UI and page and vaadin session

There are many cases where you need a reference to the active UI, Page or VaadinServiceSession, for instance for showing notifications in a click listener. It is possible to get a reference to the component from the event and then a reference from the component to the UI but Vaadin also offers an easier way through two static methods:

UI.getCurrent()
Page.getCurrent()
VaadinSession.getCurrent()

For example when you want to show the name of the current UI class:

Button helloButton = new Button("Say Hello");
    helloButton.addClickListener(new ClickListener() {
        @Override
        public void buttonClick(ClickEvent event) {
            Notification.show("This UI is "
                    + UI.getCurrent().getClass().getSimpleName());
        }
    });

Similarly for VaadinServiceSession, for instance to find out if the application is running in production mode:

public void buttonClick(ClickEvent event) {
    String msg = "Running in ";
    msg += VaadinSession.getCurrent().getConfiguration()
            .isProductionMode() ? "production" : "debug";
    Notification.show(msg);
}

And finally similarly for Page. For instance adding a browser window resize listener can be added like this:

Page.getCurrent().addBrowserWindowResizeListener(
  new Page.BrowserWindowResizeListener() {
    @Override
    public void browserWindowResized(BrowserWindowResizeEvent event) {
      Notification.show("Browser resized to " + event.getWidth() + "x" + event.getHeight());
    }
});

Note that these are based on ThreadLocal so they won’t work in a background thread (or otherwise outside the standard request scope).