Are Virtual Threads a Replacement for ExecutorService?

Introduction

Imagine you’re running a busy coffee shop. In the "old" way of doing things (Platform Threads), every customer who walks in requires a dedicated waiter to stand with them from the moment they order until their drink is ready. If the espresso machine takes three minutes, the waiter just stands there, staring at it, unable to help anyone else. To serve 1,000 customers at once, you’d need 1,000 waiters, which would bankrupt you!

In Java programming, we’ve faced this same bottleneck for decades. But with the arrival of Java 21, we have Virtual Threads.

The big question everyone is asking: Are Virtual Threads a replacement for ExecutorService? The short answer is no, but they fundamentally change how we use it. This blog will help you understand how these two powers combine to make your applications faster and lighter.

Core Concepts

What is the ExecutorService?

The ExecutorService is like a manager. Its job is to take a list of tasks and decide which "worker" (thread) will handle them. Traditionally, these workers were Platform Threads—heavy, expensive, and limited by the operating system.

What are Virtual Threads?

Virtual Threads are like "Ghost Waiters." They don't take up much space. When a Virtual Thread waits for a database or a network call, it "parks" itself, letting the underlying system do other work. When the data is ready, the Virtual Thread pops back into existence to finish the job.

Key Use Cases & Benefits:

• High Throughput: You can literally run millions of virtual threads on a single laptop.
• Simplicity: You can write "blocking" code that looks simple but performs like complex asynchronous code.
• Better Resource Usage: No more wasting megabytes of memory on idle thread stacks.

Is it a replacement? No. ExecutorService is the manager, and Virtual Threads are the new type of worker. Instead of replacing the manager, we are just giving the manager a much better team.

Code Examples

To use Virtual Threads effectively in Java 21, we use a new factory method in the Executors class. This allows us to keep the familiar ExecutorService interface while utilizing the power of Virtual Threads.

Example 1: The Modern Way (Virtual Thread Per Task)

This example simulates a web server handling multiple requests simultaneously without a fixed pool size.

import java.util.concurrent.Executors;
import java.util.concurrent.ExecutorService;
import java.util.stream.IntStream;

public class VirtualThreadDemo {
    public static void main(String[] args) {
        // We use the ExecutorService as the manager, 
        // but tell it to use Virtual Threads as workers.
        try (ExecutorService executor = Executors.newVirtualThreadPerTaskExecutor()) {

            IntStream.range(0, 10_000).forEach(i -> {
                executor.submit(() -> {
                    long threadId = Thread.currentThread().threadId();
                    System.out.println("Processing task " + i + " on Virtual Thread: " + threadId);

                    // Simulate a blocking network call
                    try {
                        Thread.sleep(100); 
                    } catch (InterruptedException e) {
                        Thread.currentThread().interrupt();
                    }

                    return i;
                });
            });
        } // The try-with-resources automatically shuts down the executor

        System.out.println("All 10,000 tasks submitted successfully!");
    }
}

Example 2: Complete End-to-End Setup (Simple Web Task)

Let's see how this would look if you were calling an endpoint to fetch data.

The Code:

import java.util.concurrent.Executors;
import java.util.concurrent.CompletableFuture;

public class ApiSimulator {
    public static void main(String[] args) {
        // Create an executor that handles tasks using Virtual Threads
        var executor = Executors.newVirtualThreadPerTaskExecutor();

        System.out.println("Starting API Simulation...");

        // Simulate an incoming request
        CompletableFuture.runAsync(() -> {
            System.out.println("Working on request...");
            try {
                Thread.sleep(500); // Simulate I/O delay
                System.out.println("Data fetched from Database successfully!");
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }, executor).join();

        executor.close();
    }
}

How to test (Conceptual):
If this were wrapped in a Spring Boot 3.2+ or Helidon application, you would trigger it via CURL.

Request:

curl -X GET http://localhost:8080/api/fast-process

Response:

{
  "status": "Success",
  "message": "Processed using Virtual Threads",
  "thread_type": "VirtualThread"
}

Best Practices

When you learn Java's new concurrency model, keep these tips in mind to avoid common pitfalls:

1. Don't Pool Virtual Threads: We used to pool threads because they were expensive to create. Virtual Threads are cheap. Just create a new one for every task using newVirtualThreadPerTaskExecutor().
2. Avoid ThreadLocal Heavily: Since you might have millions of threads, putting large objects in ThreadLocal can still exhaust your memory.
3. Avoid "Pinning": If you use synchronized blocks with long-running I/O, you might "pin" the virtual thread to the carrier thread, defeating the purpose. Use ReentrantLock instead.
4. Write Simple Code: You no longer need complex callbacks or reactive programming (like WebFlux) for simple I/O tasks. Plain, synchronous code is back in style!

Conclusion

So, are Virtual Threads a replacement for ExecutorService? Definitely not. Instead, they are the fuel that makes ExecutorService incredibly powerful for modern Java programming.

By moving away from fixed thread pools and embracing a "thread-per-task" model with Virtual Threads, you can build applications that scale to massive heights with very little code complexity. It’s the best of both worlds: the simplicity of old-school Java with the performance of modern asynchronous systems.

For more details, check out the official Oracle Java 21 documentation.

Call to Action

Are you planning to migrate your existing thread pools to Virtual Threads? Or are you sticking with the traditional FixedThreadPool for now? Let us know in the comments below! If you have any questions about implementation, feel free to ask.