code: public void setHttpClient(HttpClient httpClient) {
  		Assert.isInstanceOf(CloseableHttpClient.class, httpClient, "'httpClient' is not of type CloseableHttpClient");
  		this.httpClient = (CloseableHttpClient) httpClient;
  	}
docs: Set the {@code HttpClient} used for
  <p>
  As of Spring Framework 4.0, the given client is expected to be of type
  {@link CloseableHttpClient} (requiring HttpClient 4.3+).