Java9HttpClient挂起

我正在试验HTTP/2客户jdk 9-ea+171.代码取自此示例:

HttpClient client = HttpClient.newHttpClient();

HttpRequest request = HttpRequest.newBuilder()
    .uri(new URI("https://www.google.com/"))
    .build();

HttpResponse respOnse= client.send(request, HttpResponse.BodyHandler.asString());

但客户端永远挂在最后一行.请指教如何解决？

调试显示它在方法中无限等待waitUntilPrefaceSent().