Virtual Threads in Java: Concurrency Just Got Easier

Introduction

What if you could handle thousands of concurrent requests using simple for loops and try-catch blocks — without dealing with thread pool exhaustion or convoluted reactive code?

That’s the promise of virtual threads, a revolutionary feature introduced in Java 21 under Project Loom. Traditional Java concurrency required heavyweight platform threads, complex thread pool management, and often brittle async logic. Virtual threads flip that model — they are lightweight, cheap to spawn, and allow you to write blocking code that scales.

In this article, we’ll walk through:

• What virtual threads are and how they differ from platform threads
• Why this matters for modern applications
• Code examples with and without virtual threads
• How to use them with Spring Boot and real-world HTTP servers
• Pros, pitfalls, and what’s next for Java concurrency

What Are Virtual Threads?

A virtual thread is a new type of Java thread that isn’t backed by a one-to-one OS thread. Instead, it’s managed by the Java Virtual Machine (JVM) itself.

Think of it like this:

• Platform Thread (traditional): 1 Java thread ↔ 1 OS thread
• Virtual Thread: 1 Java thread ↔ JVM-managed “task”, scheduled on a small number of platform threads

This allows you to create thousands (even millions) of concurrent tasks with minimal memory and CPU overhead.

Key benefits:

• Near-zero cost to create
• No need to manage thread pools
• Can block without blocking the OS

Example: Traditional vs Virtual Threads

Before (Platform Threads)

ExecutorService executor = Executors.newFixedThreadPool(100);
for (int i = 0; i < 1000; i++) {
    executor.submit(() -> {
        try {
            Thread.sleep(1000); // Simulating I/O
        } catch (InterruptedException e) {
            Thread.currentThread().interrupt();
        }
    });
}

Only 100 threads at a time will run due to thread pool limits. Others wait.

After (Virtual Threads)

ExecutorService executor = Executors.newVirtualThreadPerTaskExecutor();
for (int i = 0; i < 1000; i++) {
    executor.submit(() -> {
        try {
            Thread.sleep(1000); // Still blocking — but it's cheap now!
        } catch (InterruptedException e) {
            Thread.currentThread().interrupt();
        }
    });
}

All 1000 tasks run concurrently, and you don’t need to worry about thread pool sizing.

Use Case: Virtual Threads in Web Servers

Imagine a traditional thread-per-request model for a web server. With platform threads, you can only handle a few hundred to a few thousand concurrent users — because each request ties up a full OS thread.

With virtual threads, you can go back to the thread-per-request model but scale to tens of thousands of users.

Example with a Simple HTTP Server:

var server = HttpServer.create(new InetSocketAddress(8080), 0);
server.createContext("/hello", exchange -> {
    Thread.sleep(500); // Simulate delay
    byte[] response = "Hello, Virtual Threads!".getBytes();
    exchange.sendResponseHeaders(200, response.length);
    try (var os = exchange.getResponseBody()) {
        os.write(response);
    }
});
server.setExecutor(Executors.newVirtualThreadPerTaskExecutor());
server.start();

This example uses Java’s built-in HttpServer, but the same concept applies to Spring Boot (more on that below).

Virtual Threads + Spring Boot 3.x

As of now, Spring doesn’t fully integrate with virtual threads out of the box — but support is growing.

You can experiment by customizing the TaskExecutor used in Spring MVC:

@Bean
public TaskExecutor applicationTaskExecutor() {
    return new ConcurrentTaskExecutor(Executors.newVirtualThreadPerTaskExecutor());
}

Caution: Some Spring internals, like WebClient or reactive flows, may not benefit directly from virtual threads. This is still an evolving area.

Common Pitfalls to Avoid

- Blocking in non-thread-safe code
- Assuming virtual threads are “always better”
- Using old thread debugging tools

When Should You Use Virtual Threads?

Great for:

• High-concurrency servers (chat apps, APIs)
• Background processing jobs
• I/O-heavy tasks (file, DB, network)

Not ideal for:

• CPU-intensive parallel tasks (prefer platform threads with ForkJoinPool)
• Real-time systems where precise scheduling is key

Final Thoughts

Virtual threads are the biggest shift in Java concurrency since the introduction of java.util.concurrent. They make it possible to write synchronous-looking code that scales like asynchronous code — without the complexity.

Whether you’re building cloud-native microservices, a high-performance web server, or processing pipelines — it’s time to give virtual threads a spin.

Java’s concurrency model just got a whole lot easier — and your next application can be faster, leaner, and simpler because of it.