In order to send requests asynchronously, the HTTP Client API relies on CompletableFeature, as discussed in Chapter 11, Concurrency – Deep Dive, and the sendAsync() method, as follows:

HttpClient client = HttpClient.newHttpClient();

HttpRequest request = HttpRequest.newBuilder()
  .uri(URI.create("https://reqres.in/api/users/2"))
  .build();

client.sendAsync(request, HttpResponse.BodyHandlers.ofString())
  .thenApply(HttpResponse::body)
  .exceptionally(e -> "Exception: " + e)
  .thenAccept(System.out::println)
  .get(30, TimeUnit.SECONDS); // or join()

Alternatively, let's assume that, while waiting for the response, we want to execute other tasks as well:

HttpClient client = HttpClient.newHttpClient();

HttpRequest request = HttpRequest.newBuilder()
  .uri(URI.create("https://reqres.in/api/users/2"))
  .build();

CompletableFuture<String> response
    = client.sendAsync(request, HttpResponse.BodyHandlers.ofString())
  .thenApply(HttpResponse::body)
  .exceptionally(e -> "Exception: " + e);

while (!response.isDone()) {
  Thread.sleep(50);
  System.out.println("Perform other tasks 
    while waiting for the response ...");
}

String body = response.get(30, TimeUnit.SECONDS); // or join()
System.out.println("Body: " + body);