@Override
    public URL select(List<URL> urls, String serviceId, String tag, String requestKey) {
        URL url = null;
        if (urls.size() > 1) {
            String key = tag == null ? serviceId : serviceId + "|" + tag;
            url = doSelect(urls, key);
        } else if (urls.size() == 1) {
            url = urls.get(0);
        }
        return url;
    }
@Test
    public void testSelectWithEmptyList() throws Exception {
        List<URL> urls = new ArrayList<>();
        URL url = loadBalance.select(urls, "serviceId", "tag", null);
        Assert.assertNull(url);
    }