Virtual Threads in Java 21

For years we wrapped blocking I/O in thread pools and reactive frameworks to avoid exhausting OS threads. Virtual threads flip that: they are managed by the JVM, cost almost nothing, and let you write straightforward blocking code.

Spawn freely

A virtual thread parks instead of blocking an OS thread when it hits I/O. That means one thread per task is suddenly a reasonable design.

try (var executor = Executors.newVirtualThreadPerTaskExecutor()) {
    for (var request : requests) {
        executor.submit(() -> {
            var response = httpClient.send(request);  // blocking is fine
            store(response);
        });
    }
}

Enable them in Spring Boot

One property switches Tomcat and the task executor to virtual threads.

spring.threads.virtual.enabled=true

Takeaways

• Use virtual threads for I/O-bound work, not CPU-bound number crunching.
• Avoid pooling them — create one per task; they are disposable.
• Watch for synchronized blocks around I/O, which can pin the carrier thread.

Reach for ReentrantLock instead of synchronized when guarding blocking sections, so virtual threads can unmount cleanly.