Vaadin 7中的外部脚本(CORS)问题

@WebServlet(urlPatterns = "/*", name = "MyUIServlet", asyncSupported = true)
@VaadinServletConfiguration(ui = MyUI.class, productionMode = false)
public static class MyUIServlet extends VaadinServlet {
    @Override
    protected void servletInitialized() throws ServletException {
        super.servletInitialized();
        getService().addSessionInitListener(new SessionInitListener() {
            @Override
            public void sessionInit(SessionInitEvent event) throws ServiceException {
                event.getSession().addBootstrapListener(new BootstrapListener() {
                    @Override
                    public void modifyBootstrapPage(BootstrapPageResponse response) {
                        response.getDocument().head().appendElement("script")
                                .attr("src", "https://example.com/external-script.js")
                                .attr("crossorigin", "anonymous");
                    }

                    @Override
                    public void modifyBootstrapFragment(BootstrapFragmentResponse response) {
                    }
                });
            }
        });
    }
}

JavaScript.getCurrent().execute("var xhr = new XMLHttpRequest();\n" +
        "xhr.open('GET', 'https://example.com/external-script.js', true);\n" +
        "xhr.withCredentials = true;\n" +
        "xhr.send();");