How to invoke a REST API Asynchronously

Dependencies:

<dependency>
   <groupId>org.apache.httpcomponents</groupId>
   <artifactId>httpclient</artifactId>
   <version>4.3.1</version>
</dependency>

<dependency>
    <groupId>org.apache.httpcomponents</groupId>
    <artifactId>httpasyncclient</artifactId>
    <version>4.0</version>
</dependency>
<dependency>
    <groupId>org.apache.httpcomponents</groupId>
    <artifactId>httpcore-nio</artifactId>
    <version>4.3</version>
</dependency>

Sample Code snippet:

private static void sendAsyncRequest(final HttpPost postRequest, 

FutureCallback futureCallback, CountDownLatch latch)
        throws IOException {
    CloseableHttpAsyncClient client;
    client = HttpAsyncClients.createDefault();
    client.start();
    client.execute(postRequest, futureCallback);
    try {
        latch.await();
    } catch (InterruptedException e) {
        log.error("Error occurred while calling end point - " + e);
    }
}

private void postRequest() throws IOException {
    final HttpPost postRequest = new HttpPost("www.google.com");
    final CountDownLatch latch = new CountDownLatch(1);
    FutureCallback<HttpResponse> futureCallback = 
new FutureCallback<HttpResponse>() {
        @Override public void completed(final HttpResponse response) {
            latch.countDown();
            if ((response.getStatusLine().getStatusCode() != 201)) {
                log.error("Error occurred while calling end point - " + response.getStatusLine().getStatusCode() +
                                  "; Error - " +
                                  response.getStatusLine().getReasonPhrase());
            } else {
                if (log.isDebugEnabled()) {
                    log.debug("Success Request - " + postRequest.getURI().getSchemeSpecificPart());
                }
            }
        }

        @Override public void failed(final Exception ex) {
            latch.countDown();
            log.error("Error occurred while calling end point - " 
+ postRequest.getURI().getSchemeSpecificPart() +
                              "; Error - " + ex);
        }

        @Override public void cancelled() {
            latch.countDown();
            log.warn("Operation cancelled while calling end point - " +
                             postRequest.getURI().getSchemeSpecificPart());
        }
    };
    sendAsyncRequest(postRequest, futureCallback, latch);
}