private CloseableHttpClient getCloseableHttpClient() {
        if (closeableHttpClient == null) {
            synchronized (this) {
                if (closeableHttpClient == null) {
                    PoolingHttpClientConnectionManager connectionManager = new PoolingHttpClientConnectionManager();
                    closeableHttpClient = HttpClients.custom().setConnectionManager(connectionManager).build();
                }
            }
        }
        return closeableHttpClient;
    }
@author <a href=mailto:zhiyuan.lzy@antfin.com>zhiyuan.lzy</a>
core-impl/client/src/main/java/com/alipay/sofa/rpc/client/http/RpcHttpClient.java