javademos/recap26/src/Http3Fallback.java

import com.sun.net.httpserver.HttpsConfigurator;
import com.sun.net.httpserver.HttpsServer;
import java.io.FileInputStream;
import java.net.InetSocketAddress;
import java.net.URI;
import java.net.http.HttpClient;
import java.net.http.HttpOption;
import java.net.http.HttpRequest;
import java.net.http.HttpResponse;
import java.security.KeyStore;
import javax.net.ssl.KeyManagerFactory;
import javax.net.ssl.SSLContext;
import javax.net.ssl.TrustManagerFactory;

/**
 * JEP 517 (Java 26): HTTP/3 for the HTTP Client API. The sandbox cannot reach a real HTTP/3 server (no UDP egress),
 * so this shows the half that is checkable everywhere: a client that PREFERS HTTP/3 talking to a server that does not
 * speak it (the JDK's own HttpsServer, HTTP/1.1) quietly falls back, and says so in response.version().
 * args: keystore path, password
 * Chapter: docs/11-recap-26.md
 */
public class Http3Fallback {
    public static void main(String[] args) throws Exception {
        KeyStore ks = KeyStore.getInstance("PKCS12");
        try (var in = new FileInputStream(args[0])) { ks.load(in, args[1].toCharArray()); }
        KeyManagerFactory kmf = KeyManagerFactory.getInstance("SunX509"); kmf.init(ks, args[1].toCharArray());
        TrustManagerFactory tmf = TrustManagerFactory.getInstance("SunX509"); tmf.init(ks);
        SSLContext ctx = SSLContext.getInstance("TLS"); ctx.init(kmf.getKeyManagers(), tmf.getTrustManagers(), null);

        HttpsServer server = HttpsServer.create(new InetSocketAddress("127.0.0.1", 0), 0);
        server.setHttpsConfigurator(new HttpsConfigurator(ctx));
        server.createContext("/", ex -> { byte[] b = "hello".getBytes(); ex.sendResponseHeaders(200, b.length); ex.getResponseBody().write(b); ex.close(); });
        server.start();
        int port = server.getAddress().getPort();
        try (HttpClient client = HttpClient.newBuilder().sslContext(ctx).version(HttpClient.Version.HTTP_3).build()) {
            HttpRequest req = HttpRequest.newBuilder(URI.create("https://localhost:" + port + "/")).GET().build();
            HttpResponse<String> r = client.send(req, HttpResponse.BodyHandlers.ofString());
            System.out.println("client asked for : " + client.version());
            System.out.println("server answered  : " + r.version() + " " + r.statusCode() + " " + r.body());

            HttpRequest strict = HttpRequest.newBuilder(URI.create("https://localhost:" + port + "/")).GET()
                    .setOption(HttpOption.H3_DISCOVERY, HttpOption.Http3DiscoveryMode.HTTP_3_URI_ONLY).build();
            try {
                client.send(strict, HttpResponse.BodyHandlers.ofString());
                System.out.println("HTTP_3_URI_ONLY  : unexpectedly succeeded");
            } catch (Exception e) {
                System.out.println("HTTP_3_URI_ONLY  : " + e.getClass().getName() + ": " + e.getMessage());
            }
        } finally {
            server.stop(0);
        }
    }
}