What are virtual threads in Java 21?

Question

Accepted Answer

**Virtual threads** — lightweight threads managed by the JVM, enabling millions of concurrent threads for I/O-bound workloads.

```java
// Create virtual thread
Thread.startVirtualThread(() -> {
    System.out.println("Hello from virtual thread");
});

// Virtual thread executor
try (var executor = Executors.newVirtualThreadPerTaskExecutor()) {
    IntStream.range(0, 10_000).forEach(i -> {
        executor.submit(() -> {
            Thread.sleep(Duration.ofSeconds(1));
            return i;
        });
    });
}

// Structured concurrency (Preview)
try (var scope = new StructuredTaskScope.ShutdownOnFailure()) {
    var user = scope.fork(() -> findUser(id));
    var order = scope.fork(() -> fetchOrder(id));
    scope.join();
    return new Response(user.resultNow(), order.resultNow());
}
```

**vs Platform threads:**
- Millions of virtual threads possible.
- Blocking a virtual thread releases the carrier thread.
- Ideal for I/O-bound workloads.

**Gotcha: synchronized pinning**
- Virtual threads inside synchronized blocks cannot unmount from carrier.
- Fix: use ReentrantLock instead of synchronized.

**Key benefits:**
- Simple blocking code with massive concurrency.
- No callback hell — write sequential code.
- Backward compatible — existing thread code works.

Java: What are virtual threads in Java 21?