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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 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? 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? 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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Java › Java Networking & HTTP Client

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?

Intermediate

Answer

HttpRequest.BodyPublishers.ofString(jsonString) constructs the request body, combined with setting the appropriate Content-Type: application/json header, sends a JSON payload; on the response side, HttpResponse.BodyHandlers.ofString() retrieves the raw JSON response body as a String, which you then deserialize into a Java object using a JSON library like Jackson's ObjectMapper, since HttpClient itself has no built-in JSON serialization/deserialization capability -- it deliberately operates purely at the HTTP transport level, leaving content-format handling to a separate, dedicated library.
ObjectMapper mapper = new ObjectMapper();
String jsonBody = mapper.writeValueAsString(new CreateUserRequest("Alice", "alice@example.com"));

HttpRequest request = HttpRequest.newBuilder()
    .uri(URI.create("https://api.example.com/users"))
    .header("Content-Type", "application/json")
    .POST(HttpRequest.BodyPublishers.ofString(jsonBody))
    .build();

HttpResponse<String> response = client.send(request, HttpResponse.BodyHandlers.ofString());
User createdUser = mapper.readValue(response.body(), User.class);
Real-world example A client library calling a REST API serializes a Java DTO to JSON via Jackson before constructing the HttpRequest's body, then deserializes the JSON response back into a typed Java object after receiving it, cleanly separating HttpClient's pure HTTP transport responsibilities from Jackson's dedicated JSON serialization responsibilities.

Common follow-ups: Why did the JDK team deliberately choose NOT to include built-in JSON support directly in HttpClient?;How would you handle a non-2xx response status code gracefully when parsing the response body, given an error response might have a different JSON structure than a success response?

RESTful Web APIs & Controllers;Serialization & Deserialization
All 15 Java Networking & HTTP Client questions