private List<Proxy> getDefaultProxies() {
        try {
            ProxySelector defaultProxySelector = ProxySelector.getDefault();
            return defaultProxySelector.select(new URI(getPingUrl()));
        } catch (URISyntaxException e) {
            throw new IllegalStateException(e);
        }
    }
Pings {@link HttpProxyCacheServer} to make sure it works.
VideoCache/src/main/java/com/yc/videocache/Pinger.java