Virtual threads (Project Loom, Java 21) vs. platform threads
A platform Thread is a thin wrapper around one real OS thread -- expensive enough that thread pools exist
specifically to avoid creating too many. A virtual thread is a JVM-managed lightweight thread; you can spin
up hundreds of thousands of them, and a blocking call (like Thread.sleep) on a virtual thread doesn't
block an OS thread underneath it -- the JVM parks it and reuses the carrier thread for other virtual threads.
java
Thread platformThread = Thread.ofPlatform().unstarted(() -> {});
Thread virtualThread = Thread.ofVirtual().unstarted(() -> {});
System.out.println("platform thread isVirtual(): " + platformThread.isVirtual());
System.out.println("virtual thread isVirtual(): " + virtualThread.isVirtual());
The practical payoff: 10,000 concurrent "blocking" sleeps finish together, not sequentially -- something that would exhaust a platform thread pool almost immediately:
java
import java.util.concurrent.*;
int numTasks = 10_000;
var latch = new CountDownLatch(numTasks);
long start = System.nanoTime();
try (ExecutorService executor = Executors.newVirtualThreadPerTaskExecutor()) {
for (int i = 0; i < numTasks; i++) {
executor.submit(() -> {
try { Thread.sleep(20); } catch (InterruptedException ignored) {}
latch.countDown();
});
}
latch.await();
}
long elapsedMs = (System.nanoTime() - start) / 1_000_000;
System.out.println(numTasks + " virtual threads, each \"blocking\" for 20ms, all finished in ~" + elapsedMs
+ "ms -- not the ~200 seconds a small platform-thread pool running them sequentially would need");