Support for suspend of an HTTP request a la Servlet 3.0. My current thoughts are to have a very simple API

public interface AsynchronousResponse {
void setResponse(Response response);
}

public @interface Suspend {
long value();
}

@GET
public void get(@Suspend(300) AsynchronousResponse response) {
...
}

AsynchronousResponse.setResponse() will throw an exception if the request is no longer valid.