private CloseableHttpClient getHttpClient(Site site) {
        if (site == null) {
            return httpClientGenerator.getClient(null);
        }
        String domain = site.getDomain();
        CloseableHttpClient httpClient = httpClients.get(domain);
        if (httpClient == null) {
            synchronized (this) {
                httpClient = httpClients.get(domain);
                if (httpClient == null) {
                    httpClient = httpClientGenerator.getClient(site);
                    httpClients.put(domain, httpClient);
                }
            }
        }
        return httpClient;
    }
The http downloader based on HttpClient.

@author code4crafter@gmail.com <br>
@since 0.1.0
webmagic-core/src/main/java/us/codecraft/webmagic/downloader/HttpClientDownloader.java