Java 26 Adds Full HTTP/3 Client Support: Faster, Safer Connections with QUIC

Java 26 updates the HttpClient API to support the HTTP/3 protocol, allowing libraries and applications to interact with HTTP/3 servers with minimal code modifications. The implementation has several limitations: it does not provide a native QUIC protocol API, does not support third-party security socket providers, does not offer a server-side HTTP/3 implementation, and does not update the legacy java.net.URL API which remains limited to HTTP/1.1.

What Is HTTP/3

HTTP/3 replaces the TCP transport layer with QUIC (Quick UDP Internet Connections), delivering significant improvements in performance, reliability, and security. QUIC merges the transport handshake with the TLS 1.3 handshake into a single round trip, reducing connection establishment latency — especially beneficial on high-latency and mobile networks. QUIC streams are independent; packet loss affects only the impacted stream while others continue uninterrupted. Most modern browsers already support HTTP/3 and automatically enable it when servers advertise support.

Practical Examples

2.1 Declaring HTTP/3

To send requests over HTTP/3, you must explicitly enable the protocol. Set the protocol version on the HttpClient so all requests default to HTTP/3:

var client = HttpClient.newBuilder()
    .version(HttpClient.Version.HTTP_3)
    .build();

You can also set the preferred version on a single HttpRequest:

var request = HttpRequest.newBuilder(URI.create("https://api.pack.com/"))
    .version(HttpClient.Version.HTTP_3)
    .GET()
    .build();

No further changes are required. If the target server does not support HTTP/3, the request transparently falls back to HTTP/2 or HTTP/1.1.

2.2 Protocol Discovery

Because HTTP/3 runs over UDP-based QUIC while HTTP/1.1 and HTTP/2 use TCP, the client cannot upgrade an existing connection. The HttpOption.H3_DISCOVERY option controls how the client discovers HTTP/3 support, with three modes:

ANY (default) : The client uses its own algorithm, attempting both QUIC-based HTTP/3 and TLS/TCP-based HTTP, preferring whichever connects first.

HTTP_3_URI_ONLY : The client directly tries HTTP/3 on the host and port from the request URI, without using the Alternative Services mechanism. This succeeds only if the server already listens for HTTP/3 on that port.

ALT_SVC : The client relies solely on HTTP Alternative Services (RFC 7838) to discover HTTP/3. The server responds to an initial HTTP/1.1 or HTTP/2 request with an Alt-Svc header (or HTTP/2 ALTSVC frame) advertising an h3 endpoint; subsequent requests then use HTTP/3.

Example using HTTP_3_URI_ONLY to connect directly without waiting for Alt-Svc advertisement:

public HttpResponse<String> fetch(String url) throws Exception {
  HttpClient client = HttpClient.newBuilder()
      .version(HttpClient.Version.HTTP_3)
      .build();

  HttpRequest request = HttpRequest.newBuilder(URI.create(url)).GET()
      .setOption(HttpOption.H3_DISCOVERY, HttpOption.Http3DiscoveryMode.HTTP_3_URI_ONLY)
      .build();

  return client.send(request, BodyHandlers.ofString());
}

2.3 Verifying a Valid HTTP/3 Endpoint

The article tests against https://cloudflare-quic.com/, a public Cloudflare endpoint for HTTP/3 experimentation. The complete runnable example:

public static void main(String[] args) {
  HttpClient client = HttpClient.newBuilder()
      .version(HttpClient.Version.HTTP_3)
      .build();

  HttpResponse<String> response = null;
  try {
    response = fetch("https://cloudflare-quic.com/");
  } catch (Exception e) {
    e.printStackTrace();
  }
  System.err.println(response.statusCode() + "@" + response.version());
}

public static HttpResponse<String> fetch(String url) throws Exception {
  HttpClient client = HttpClient.newBuilder()
      .version(HttpClient.Version.HTTP_3)
      .build();

  HttpRequest request = HttpRequest.newBuilder(URI.create(url)).GET()
      .setOption(HttpOption.H3_DISCOVERY, HttpOption.Http3DiscoveryMode.HTTP_3_URI_ONLY).build();

  return client.send(request, BodyHandlers.ofString());
}

Output confirms successful HTTP/3 communication:

200@HTTP_3