jdk-24/test/jdk/java/lang/Thread/virtual/ActivateSpareCarrier.java

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/**
 * @test
 * @bug 8292240
 * @summary Test the scenario where a blocking operation pins a virtual thread to its
 *   carrier thread (cT1) and doesn't activate a spare. Subsequent blocking operations
 *   that pin a virtual thread to cT1 should attempt to activate a spare.
 * @requires vm.continuations
 * @run main/othervm
 *     -Djdk.virtualThreadScheduler.parallelism=1
 *     -Djdk.virtualThreadScheduler.maxPoolSize=2 ActivateSpareCarrier 100
 */

import java.time.Duration;
import java.util.Comparator;
import java.util.List;
import java.util.concurrent.ForkJoinWorkerThread;
import java.util.stream.Collectors;

public class ActivateSpareCarrier {

    private static final int DEFAULT_ITERTAIONS = 10_000;

    private static final Object LOCK = new Object();

    public static void main(String[] args) throws Exception {
        int iterations;
        if (args.length == 0) {
            iterations = DEFAULT_ITERTAIONS;
        } else {
            iterations = Integer.parseInt(args[0]);
        }
        for (int i = 0; i < iterations; i++) {
            test(i);
        }
    }

    /**
     * This method creates 3 virtual threads:
     * - thread1 blocks in Object.wait, activating a spare carrier thread
     * - thread2 is started and runs on the spare carrier thread
     * - thread1 is notified causing it to re-adjust the release count and terminate
     * - thread3 is started and should run on the one active thread
     *
     * This method need invoked at least twice in the same VM.
     */
    private static void test(int i) throws Exception {
        System.out.printf("---- %d ----%n", i);

        // thread1 blocks in wait, this triggers a tryCompensate to activate a spare thread
        Thread thread1 = Thread.ofVirtual().unstarted(() -> {
            System.out.println(Thread.currentThread());
            synchronized (LOCK) {
                try {
                    LOCK.wait();
                } catch (InterruptedException e) { }
            }
        });
        System.out.printf("starting waiter thread #%d%n", thread1.threadId());
        thread1.start();

        // wait for thread1 to block in Object.wait
        while (thread1.getState() != Thread.State.WAITING) {
            Thread.sleep(10);
        }

        // start another virtual thread, it should run on the spare carrier thread
        startAndJoinVirtualThread();

        // notify thread1, this releases the blocker
        synchronized (LOCK) {
            LOCK.notifyAll();
        }
        joinThread(thread1);

        // start another virtual thread after counts have been re-adjusted
        startAndJoinVirtualThread();
    }

    /**
     * Start a virtual thread and wait for it to terminate.
     */
    private static void startAndJoinVirtualThread() throws InterruptedException {
        Thread thread = Thread.ofVirtual().unstarted(() -> {
            System.out.println(Thread.currentThread());
        });
        System.out.format("starting #%d%n", thread.threadId());
        thread.start();
        joinThread(thread);
    }

    /**
     * Wait for the give thread to terminate with diagnostic output if the thread does
     * not terminate quickly.
     */
    private static void joinThread(Thread thread) throws InterruptedException {
        long tid = thread.threadId();
        System.out.printf("Waiting for #%d to terminate%n", tid);
        boolean terminated = thread.join(Duration.ofSeconds(2));
        if (!terminated) {
            System.out.printf("#%d did not terminate quickly, continue to wait...%n", tid);
            printForkJoinWorkerThreads();
            thread.join();
        }
        System.out.printf("#%d terminated%n", tid);
    }

    /**
     * Print the list of ForkJoinWorkerThreads and their stack traces.
     */
    private static void printForkJoinWorkerThreads() {
        List<Thread> threads = Thread.getAllStackTraces().keySet().stream()
                .filter(t -> t instanceof ForkJoinWorkerThread)
                .sorted(Comparator.comparingLong(Thread::threadId))
                .collect(Collectors.toList());
        System.out.println("ForkJoinWorkerThreads:");
        for (Thread t : threads) {
            System.out.printf("    %s%n", t);
            StackTraceElement[] stack = t.getStackTrace();
            for (StackTraceElement e : stack) {
                System.out.printf("      %s%n", e);
            }
        }
    }
}