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What is the modern java.net.http.HttpClient (introduced in Java 11), and how does it improve on the older HttpURLConnection for making HTTP requests? How would you add custom HTTP headers (like an Authorization bearer token) to an outgoing request using HttpRequest.Builder? How would you configure HttpClient with custom timeouts, a proxy, and connection pooling behavior, and what's the difference between a connection timeout and a request/response timeout? How would you send a POST request with a JSON request body using HttpClient, and correctly parse a JSON response using a library like Jackson? What is the java.net.Socket and ServerSocket class pair, and how would you implement a basic TCP client-server communication using them at a lower level than HttpClient? How would you send URL-encoded form data (application/x-www-form-urlencoded) using HttpClient, as opposed to a JSON request body? How would you use InetAddress to perform DNS resolution (converting a hostname to an IP address) programmatically, and what's the difference between InetAddress.getByName() and getAllByName()? How does HttpClient handle automatic following of HTTP redirects (3xx responses), and how would you configure or disable this behavior? How would you implement a custom BodySubscriber or BodyHandler to stream a very large HTTP response body incrementally, rather than buffering the entire response into memory at once? How would you implement retry logic with exponential backoff for HTTP requests made via HttpClient, given the client doesn't provide built-in retry support natively?
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How would you make an asynchronous, non-blocking HTTP request using HttpClient's sendAsync() method, and how does the returned CompletableFuture integrate with the rest of an asynchronous pipeline?

Intermediate

Answer

client.sendAsync(request, bodyHandler) immediately returns a CompletableFuture<HttpResponse<T>> without blocking the calling thread, letting you compose the eventual response using CompletableFuture's fluent chaining methods (thenApply(), thenAccept(), exceptionally()) to process the result once it arrives, integrating naturally with other asynchronous work in a larger reactive or CompletableFuture-based application pipeline rather than dedicating a thread to block waiting for each individual HTTP call to complete.
HttpRequest request = HttpRequest.newBuilder()
    .uri(URI.create("https://api.example.com/data"))
    .build();

CompletableFuture<String> futureBody = client.sendAsync(request, HttpResponse.BodyHandlers.ofString())
    .thenApply(HttpResponse::body)
    .exceptionally(ex -> {
        System.err.println("Request failed: " + ex.getMessage());
        return "";
    });

futureBody.thenAccept(System.out::println);  // processes the result once it eventually arrives
Real-world example A dashboard service fetching data concurrently from five independent external APIs uses sendAsync() for all five requests simultaneously, combining their resulting CompletableFutures with CompletableFuture.allOf() to wait for all responses together, achieving far better overall latency than issuing five sequential blocking requests one after another.

Common follow-ups: How would you set a per-request timeout that's independent of the HttpClient's own default connection timeout?;What thread does the CompletableFuture's continuation callbacks (like thenApply) actually execute on by default?

Concurrency & Threads;Background Tasks & Hosted Services
All 15 Java Networking & HTTP Client questions