Extensions usually consist of a pair of s like components do. Hence, they can use the regular shared state and RPC mechanisms to communicate between the client and the server. Extensions may or may not have a UI. They can create and display widgets on the client side, but are not part of the regular layout hierarchy.

We will rewrite the add-on as an extension. The Refresher causes the client to “ping” the server at regular intervals, allowing the server to keep the client up-to-date if the application state is changed eg. by a background thread (because of the way Vaadin works, the server cannot itself initiate communication.)

We start by writing the barebones server-side class for our extension:

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Next, the Refresher needs an RPC interface to ping the server and a shared state to keep track of the interval. These are rather trivial:

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The client-side connector is just like a component connector except that we inherit from AbstractExtensionConnector, not . We do not write a client-side widget at all, because the Refresher does not have a UI.

We create a Timer instance that calls the refresh RPC method when run. In , we know that either the interval, enabled state, or both have changed, so we always cancel a possible currently-running timer and schedule a new one if we’re enabled. We also remember to cancel the timer when the extension is detached.

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Finally, we add an event listener interface and some accessor methods to Refresher. There is nothing extension-specific in the following code: