Virtual Threads (Project Loom)

Java 21 brings one of the most awaited concurrency updates – Virtual Threads from Project Loom. Designed to dramatically reduce the complexity and resource cost of multithreaded programming, virtual threads enable scalable, high-throughput applications without complicated thread management.

🚀 What are Virtual Threads?

Virtual threads are lightweight threads managed by the Java Virtual Machine (JVM), not the OS. Unlike platform (or "native") threads, virtual threads use minimal memory and can scale to millions of concurrent tasks.

• ✅ Lightweight
• ✅ Non-blocking friendly
• ✅ Familiar API (same Thread class)

✨ Why They Matter

Traditional Java threads are costly in memory (1MB+ per thread) and limited in scalability. With virtual threads, you can handle massive I/O-bound workloads using simple, synchronous-style code.

🛠️ Using Virtual Threads

🔹 Basic Example

public class VirtualThreadExample {
    public static void main(String[] args) {
        Runnable task = () -> {
            System.out.println("Running on: " + Thread.currentThread());
        };

        Thread.startVirtualThread(task);
    }
}

This uses Thread.startVirtualThread() to create a virtual thread. It's as easy as working with normal threads.

🔹 Spawning Thousands

public class VirtualThreadMassive {
    public static void main(String[] args) throws InterruptedException {
        for (int i = 0; i < 10_000; i++) {
            Thread.startVirtualThread(() -> {
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            });
        }

        Thread.sleep(2000);
        System.out.println("Done");
    }
}

You can spawn tens or hundreds of thousands of virtual threads without choking the JVM.

⚖️ Comparison: Platform vs Virtual Threads

Feature	Platform Thread	Virtual Thread
Backed By	OS thread	JVM-managed
Memory Overhead	~1MB	~a few KB
Blocking APIs	Expensive	Handled efficiently
Use Case	CPU-bound tasks	I/O-bound & massive concurrency

📦 Thread Executors with Virtual Threads

ExecutorService executor = Executors.newVirtualThreadPerTaskExecutor();

executor.submit(() -> {
    System.out.println("Hello from virtual thread!");
});
executor.shutdown();

Java 21 introduces Executors.newVirtualThreadPerTaskExecutor() – ideal for apps like web servers or reactive pipelines.

🚧 Gotchas

• Avoid thread-local-heavy code – virtual threads may not persist on the same carrier thread.
• Blocking native code (e.g., database drivers) may still cause thread pinning.
• Proper structured concurrency is recommended for managing lifecycle.

🎯 Use Cases

• REST APIs handling thousands of requests
• Real-time chat systems
• Batch processing
• Message-driven microservices

🧪 Testing Virtual Threads

You can test virtual thread usage via:

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

🔚 Conclusion

Virtual Threads in Java 21 simplify concurrency without sacrificing performance. They’re ideal for modern server-side development, enabling you to write straightforward, blocking-style code that scales like async.

Try them out in your next Spring Boot service or REST API to boost scalability with minimal complexity!