Best practice for using values returned in callbacks (RequestFactory)

In GWT and particularly the RequestFactory framework many callbacks are used to retrieve certain values

    

void callBack(final Integer userId) {

        final MainRequestFactory.ResourceRequest reqFactory = mainRequestFactory.resourceRequest();

        final com.google.web.bindery.requestfactory.shared.Request<Integer> req = reqFactory.getUserResourceId(userId);

        req.fire(new Receiver<Integer>() {

            @Override

            public void onSuccess(final Integer response) {

               System.out.println("My VALUE I want to return or assign to globally accessible variable = " + response); // MyValue

            }

        });

    }

However I cannot neither return MyValue or assign it to a (even static) field because the callBack is asynchronous. 

What is the best (a good one is fine enough too :) practice to process MyValue?

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