javademos/scoped-values/src/InheritanceRequiresStructuredTaskScope.java

import java.util.concurrent.StructuredTaskScope;

/**
 * The single easiest wrong assumption about Scoped Values: that binding one makes it visible to
 * ANY child thread, the way InheritableThreadLocal does. It does not. A binding is only visible
 * to code running on the SAME thread, or to subtasks forked from a {@code StructuredTaskScope}
 * opened while the binding is active. A plain {@code Thread.ofVirtual().start(...)} or
 * {@code new Thread(...)} -- even started from inside the bound block -- sees an UNBOUND value.
 *
 * Compile and run with {@code --enable-preview --release 25}: StructuredTaskScope (JEP 505) is
 * still a preview API on JDK 25 (its sixth preview; finalized later, per docs/12-lanes-25-to-29.md
 * and docs/output/82-structured-not-final.txt in this repo) even though ScopedValue (JEP 506)
 * sitting right next to it is fully final. Mixing a final API with a preview one in the same
 * example is exactly the kind of thing that's easy to get wrong by skimming release notes.
 *
 * Chapter: docs/17-scoped-values-migration.md
 */
public class InheritanceRequiresStructuredTaskScope {

    static final ScopedValue<String> REQUEST_ID = ScopedValue.newInstance();

    public static void main(String[] args) throws Exception {
        ScopedValue.where(REQUEST_ID, "req-42").run(() -> {
            try {
                plainVirtualThreadDoesNotInherit();
                plainPlatformThreadDoesNotInherit();
                structuredTaskScopeForkDoesInherit();
            } catch (Exception e) {
                throw new RuntimeException(e);
            }
        });
    }

    static void plainVirtualThreadDoesNotInherit() throws InterruptedException {
        String[] outcome = new String[1];
        Thread t = Thread.ofVirtual().start(() -> outcome[0] = readOrReport());
        t.join();
        System.out.println("Thread.ofVirtual().start(...) from inside the bound block: " + outcome[0]);
        Check.that(outcome[0].equals("UNBOUND"),
                "a plain virtual thread started from inside where().run() does NOT see the binding -- REQUEST_ID.get() threw NoSuchElementException");
    }

    static void plainPlatformThreadDoesNotInherit() throws InterruptedException {
        String[] outcome = new String[1];
        Thread t = new Thread(() -> outcome[0] = readOrReport());
        t.start();
        t.join();
        System.out.println("new Thread(...) from inside the bound block: " + outcome[0]);
        Check.that(outcome[0].equals("UNBOUND"), "a plain platform Thread doesn't see the binding either -- this isn't a virtual-thread-specific rule");
    }

    @SuppressWarnings("preview")
    static void structuredTaskScopeForkDoesInherit() throws Exception {
        String[] outcome = new String[1];
        try (var scope = StructuredTaskScope.open()) {
            scope.fork(() -> {
                outcome[0] = readOrReport();
                return null;
            });
            scope.join();
        }
        System.out.println("StructuredTaskScope.fork(...) from inside the bound block: " + outcome[0]);
        Check.that(outcome[0].equals("req-42"),
                "a subtask forked from a StructuredTaskScope opened while REQUEST_ID is bound DOES see \"req-42\"");
    }

    static String readOrReport() {
        try {
            return REQUEST_ID.get();
        } catch (java.util.NoSuchElementException e) {
            return "UNBOUND";
        }
    }
}