javademos/lanes/src/PinningDemo.java

import java.util.ArrayList;
import java.util.List;

/**
 * Virtual threads and synchronized: until JEP 491 (Java 24) a virtual thread that blocked inside a synchronized
 * block kept its carrier thread (was "pinned"). With ONE carrier, four virtual threads that each sleep 200 ms inside
 * synchronized on their own lock take about 800 ms if pinned (one at a time) and about 200 ms if not (all at once).
 * Run by scripts/lanes.sh with -Djdk.virtualThreadScheduler.parallelism=1 -Djdk.virtualThreadScheduler.maxPoolSize=1
 * Chapter: docs/14-lanes-21-to-25.md
 */
public class PinningDemo {
    public static void main(String[] args) throws Exception {
        System.out.println("java.version = " + System.getProperty("java.version"));
        List<Thread> threads = new ArrayList<>();
        long t0 = System.nanoTime();
        for (int i = 0; i < 4; i++) {
            Object lock = new Object();
            threads.add(Thread.ofVirtual().start(() -> {
                synchronized (lock) {
                    try { Thread.sleep(200); } catch (InterruptedException ignored) { }
                }
            }));
        }
        for (Thread t : threads) t.join();
        long ms = (System.nanoTime() - t0) / 1_000_000;
        System.out.println("4 virtual threads, 1 carrier, sleep(200) inside synchronized: "
                + (ms >= 700 ? "SERIALISED (pinned): about 4 x 200 ms" : "OVERLAPPED (not pinned): about 200 ms"));
    }
}