javademos/lanes/src/StructuredHub.java

import java.util.concurrent.StructuredTaskScope;

/**
 * The structured concurrency example from ankurm.com/java-21-to-25-lts-features. JEP 505 in Java 25 is the FIFTH
 * PREVIEW, not a final feature: without --enable-preview javac refuses, on 25, 26 and 27 alike.
 * Chapter: docs/14-lanes-21-to-25.md
 */
public class StructuredHub {
    record Dashboard(String user, String order) {}

    static String fetchUser(int id) throws InterruptedException { Thread.sleep(20); return "user-" + id; }
    static String fetchOrder(int id) throws InterruptedException { Thread.sleep(30); return "order-for-" + id; }

    static Dashboard fanOut(int userId) throws Exception {
        try (var scope = StructuredTaskScope.open()) {
            var user = scope.fork(() -> fetchUser(userId));
            var order = scope.fork(() -> fetchOrder(userId));
            scope.join();
            return new Dashboard(user.get(), order.get());
        }
    }

    static String firstSuccess() throws Exception {
        try (var scope = StructuredTaskScope.open(StructuredTaskScope.Joiner.<String>anySuccessfulResultOrThrow())) {
            scope.fork(() -> { Thread.sleep(200); return "slow"; });
            scope.fork(() -> "fast");
            return scope.join();
        }
    }

    public static void main(String[] args) throws Exception {
        System.out.println("java.version = " + System.getProperty("java.version"));
        Dashboard d = fanOut(7);
        Check.that(d.equals(new Dashboard("user-7", "order-for-7")), "fan-out: both results joined: " + d);
        Check.that(firstSuccess().equals("fast"), "anySuccessfulResultOrThrow(): the first result wins and the slow subtask is cancelled");
    }
}