import java.time.*;
import java.util.concurrent.*;
import java.util.stream.*;

// *** before: ***
// a task held a thread of the operating system, so threads were counted
// and shared through a pool:
ExecutorService pool = Executors.newFixedThreadPool(200);
pool.submit(() -> System.out.println("task on " + Thread.currentThread()));
pool.shutdown();

// *** in version 21: ***
Thread vt = Thread.ofVirtual().start(() ->
        System.out.println("virtual " + Thread.currentThread()));
vt.join();

// one thread per task, and a hundred thousand of them is normal:
try (var executor = Executors.newVirtualThreadPerTaskExecutor()) {
    IntStream.range(0, 10_000).forEach(i ->
            executor.submit(() -> {
                Thread.sleep(Duration.ofMillis(10)); // the carrier is freed
                //here
                return i;
            }));
}   // close waits for every task

System.out.println(Thread.currentThread().isVirtual());