Virtual threads

Definition

Virtual threads (Java 21) are very cheap threads managed by the JVM. When one waits, for example on a network call, it steps aside so another can run. You can run tens of thousands at once with simple blocking code.

Example code

10,000 tasks that each wait, run at the same time

import java.time.Duration;
import java.util.concurrent.Executors;
import java.util.concurrent.atomic.AtomicInteger;

class Demo {
    public static void main(String[] args) {
        AtomicInteger finished = new AtomicInteger();

        try (var executor = Executors.newVirtualThreadPerTaskExecutor()) {   // one virtual thread per task
            for (int i = 0; i < 10_000; i++) {
                executor.submit(() -> {
                    Thread.sleep(Duration.ofMillis(100));                      // waiting does not block a core
                    finished.incrementAndGet();
                    return null;
                });
            }
        }                                                                      // close() waits for all tasks

        System.out.println(finished.get() + " tasks finished");
    }
}

// Output:
// 10000 tasks finished