The AddOn abstraction provides a simple and generic way how to extend existing HttpServer functionality. The interface looks very simple:

/**
 * The {@link HttpServer} addon interface, responsible for adding
 * features like WebSockets, Comet to HttpServer.
 */
public interface AddOn {
    /**
     * The method, which will be invoked by {@link HttpServer} in order to
     * initialize the AddOn on the passed {@link NetworkListener}.
     * Most of the time the AddOn implementation will update the passed
     * {@link NetworkListener}'s {@link FilterChainBuilder} by adding custom
     * {@link Filter}(s), which implement AddOn's logic.
     * 
     * @param networkListener the {@link NetworkListener} the addon is being
     *          initialized on.
     * @param builder the {@link FilterChainBuilder},
     *          representing the {@link NetworkListener} logic.
     */
    public void setup(NetworkListener networkListener,
            FilterChainBuilder builder);
}

So basically custom AddOn should implement only one method, and most of the time AddOn just inserts a custom logic Filter into the given FilterChainBuilder. Here is example of WebSocketAddOn:

/**
 * WebSockets {@link AddOn} for the {@link org.glassfish.grizzly.http.server.HttpServer}.
 */
public class WebSocketAddOn implements AddOn {

    @Override
    public void setup(final NetworkListener networkListener,
            final FilterChainBuilder builder) {

        // Get the index of HttpCodecFilter in the HttpServer filter chain
        final int httpCodecFilterIdx = builder.indexOfType(HttpCodecFilter.class);

        if (httpCodecFilterIdx >= 0) {
            // Insert the WebSocketFilter right after HttpCodecFilter
            builder.add(httpCodecFilterIdx + 1, new WebSocketFilter());
        }
    }
}