code: public HttpExecutor getHttpExecutor(BodyType bodyType) {
      if (httpClient == null) {
        synchronized (HttpExecutorFactory.class) {
          if (httpClient == null) {
            httpClient = HttpClients.createDefault();
          }
        }
      }
      return getHttpExecutor(bodyType, httpClient);
    }
docs: 获取http executor，如果没设置{@link #httpClient}，则自动创建一个http client