In Java 21, a virtual thread stays mounted on a carrier thread while waiting to acquire a lock of the synchronized block.

ThreadPinningSynchronizedTest.Java

package ca.bazlur.modern.concurrency.c02;

import java.util.List;
import java.util.stream.IntStream;

public class ThreadPinningSynchronizedTest {

	private static final Object lock = new Object();

	public static void main(String[] args) {

		// Give some time for JFR ready
		giveSomeTimeForJFRInit();

		List<Thread> threads = IntStream.range(0, 10)
				.mapToObj(index -> Thread.ofVirtual().unstarted(() -> {
					System.out.printf("BEFORE %d - %s%n", index, Thread.currentThread());

					// HERE!
					// A virtual thread stays mounted on a carrier while waiting for the lock.
					// But not because the virtual thread is pinned to the carrier,
					// but because Java 21 `synchronized` does not know how to deal with virtual threads.
					synchronized (lock) {
						// No blocking but takes long time
						getFibonacci(40);
					}

					System.out.printf("AFTER  %d - %s%n", index, Thread.currentThread());
				}))
				.toList();

		threads.forEach(Thread::start);

		threads.forEach(thread -> {
			try {
				thread.join();
			} catch (InterruptedException e) {
				Thread.currentThread().interrupt();
			}
		});
	}

	// n-th Fibonacci number
	private static long getFibonacci(int n) {
		if (n <= 1) {
			return n;
		}
		return getFibonacci(n - 1) + getFibonacci(n - 2);
	}

	private static void giveSomeTimeForJFRInit() {
		try {
			Thread.sleep(1000);
		} catch (InterruptedException e) {
			Thread.currentThread().interrupt();
			throw new RuntimeException(e);
		}
	}
}




/*
The result using JDK 21

BEFORE 0 - VirtualThread[#31]/runnable@ForkJoinPool-1-worker-1
BEFORE 1 - VirtualThread[#32]/runnable@ForkJoinPool-1-worker-3
BEFORE 2 - VirtualThread[#33]/runnable@ForkJoinPool-1-worker-4
BEFORE 3 - VirtualThread[#34]/runnable@ForkJoinPool-1-worker-6
BEFORE 4 - VirtualThread[#35]/runnable@ForkJoinPool-1-worker-5
BEFORE 6 - VirtualThread[#37]/runnable@ForkJoinPool-1-worker-2
BEFORE 5 - VirtualThread[#36]/runnable@ForkJoinPool-1-worker-7
BEFORE 8 - VirtualThread[#39]/runnable@ForkJoinPool-1-worker-9
BEFORE 9 - VirtualThread[#40]/runnable@ForkJoinPool-1-worker-8
BEFORE 7 - VirtualThread[#38]/runnable@ForkJoinPool-1-worker-10
AFTER  0 - VirtualThread[#31]/runnable@ForkJoinPool-1-worker-1
AFTER  7 - VirtualThread[#38]/runnable@ForkJoinPool-1-worker-10
AFTER  9 - VirtualThread[#40]/runnable@ForkJoinPool-1-worker-8
AFTER  8 - VirtualThread[#39]/runnable@ForkJoinPool-1-worker-9
AFTER  5 - VirtualThread[#36]/runnable@ForkJoinPool-1-worker-7
AFTER  6 - VirtualThread[#37]/runnable@ForkJoinPool-1-worker-2
AFTER  4 - VirtualThread[#35]/runnable@ForkJoinPool-1-worker-5
AFTER  3 - VirtualThread[#34]/runnable@ForkJoinPool-1-worker-6
AFTER  2 - VirtualThread[#33]/runnable@ForkJoinPool-1-worker-4
AFTER  1 - VirtualThread[#32]/runnable@ForkJoinPool-1-worker-3

가상 스레드별 BEFORE/AFTER 가 동일하다.
즉, 가상 스레드가 캐리어 스레드를 점유하고 자원을 낭비하면서 락 획득을 기다린다.

All virtual threads stay mounted on the initially mounted carrier threads.
So the virtual threads does not release the carrier thread and waste the resource while waiting for the lock.
*/