java HttpClient 비동기식 리소스 요청 방법

package demo;
import java.util.concurrent.CountDownLatch;
import org.apache.http.HttpResponse;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.impl.nio.client.DefaultHttpAsyncClient;
import org.apache.http.nio.client.HttpAsyncClient;
import org.apache.http.nio.concurrent.FutureCallback;
import org.apache.http.nio.reactor.IOReactorException;
public class Main {
 /**
  * @param args
  * @throws IOReactorException 
  * @throws InterruptedException 
  */
 public static void main(String[] args) throws IOReactorException, InterruptedException {
  final HttpAsyncClient httpclient = new DefaultHttpAsyncClient();
  httpclient.start();
  HttpGet[] requests = new HttpGet[] {
    new HttpGet("http://www.apache.org/"),
    new HttpGet("https://www.verisign.com/"),
    new HttpGet("http://www.google.com/")
  };
  final CountDownLatch latch = new CountDownLatch(requests.length);
  try {
   for (final HttpGet request: requests) {
    httpclient.execute(request, new FutureCallback<HttpResponse>() {
     public void completed(final HttpResponse response) {
      latch.countDown();
      System.out.println(request.getRequestLine() + "->" + response.getStatusLine());
     }
     public void failed(final Exception ex) {
      latch.countDown();
      ex.printStackTrace();
     }
     public void cancelled() {
      latch.countDown();
     }
    });
   }
   System.out.println("Doing...");
  }finally {
   latch.await();
   httpclient.shutdown();
  }
  System.out.println("Done");
 }
}