Virtual Threads

1. Introduction

Traditional Java threads are platform threads, which are mapped to operating system threads.

Platform threads are powerful, but they are relatively expensive.

Problems with large numbers of platform threads:

• high memory usage
• thread creation overhead
• limited scalability for blocking workloads

To improve this, modern Java introduced:

Virtual Threads

Virtual threads make it possible to create very large numbers of lightweight threads more efficiently.

They are part of Java's Project Loom work and are available in modern Java versions.

They became a standard feature in Java 21.

2. What is a Virtual Thread

A virtual thread is a lightweight thread managed by the JVM rather than being directly tied one-to-one with an operating system thread.

Important idea:

• many virtual threads can be scheduled onto a smaller number of platform threads

This makes them much cheaper to create and use, especially for applications with many blocking tasks.

3. Why Virtual Threads Matter

In server applications, many tasks spend time waiting for:

• database responses
• network I/O
• file operations

With platform threads, each waiting task occupies an expensive OS thread.

With virtual threads:

• blocking code becomes more scalable
• we can keep the simple thread-per-task model
• asynchronous style is not always necessary just for scalability

This is a major improvement for highly concurrent applications.

4. Creating a Virtual Thread

Java provides simple APIs for virtual threads.

Example:

public class Main {

    public static void main(String[] args) throws InterruptedException {

        Thread thread = Thread.ofVirtual().start(() -> {
            System.out.println("Running in virtual thread");
        });

        thread.join();

    }

}

Here the thread is virtual, not a normal platform thread.

5. Thread Per Task Executor

A common virtual-thread style is using an executor that creates one virtual thread per task.

Example:

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

public class Main {

    public static void main(String[] args) {

        try (ExecutorService executor = Executors.newVirtualThreadPerTaskExecutor()) {
            for (int i = 1; i <= 5; i++) {
                int taskId = i;
                executor.submit(() ->
                    System.out.println("Task " + taskId + " on " +
                        Thread.currentThread())
                );
            }
        }

    }

}

This style is very natural and easy to read.

6. Virtual Threads vs Platform Threads

Virtual threads do not make the CPU magically faster, but they make concurrency more scalable for the right workloads.

7. Best Use Cases for Virtual Threads

Virtual threads are especially useful for:

• web servers
• request-per-thread architectures
• database-backed services
• network applications
• tasks that block frequently

They are ideal when code naturally waits often.

8. Virtual Threads and Blocking Code

One major advantage is that normal blocking code can often remain readable:

String data = service.fetch();
save(data);

With older models, developers often used complex callback-based or reactive code to avoid blocking too many platform threads.

Virtual threads make synchronous-looking code far more scalable.

9. Important Caution: CPU-Bound Work

Virtual threads are not a solution for every problem.

For CPU-heavy tasks:

• the CPU is still the bottleneck
• too many runnable tasks can still hurt performance

So virtual threads are best for blocking and high-concurrency workloads, not for magically accelerating heavy calculations.

10. Pinning and Native Blocking

In some situations, a virtual thread may become pinned to a platform thread.

This can happen with certain cases such as:

• blocking while holding a monitor with synchronized
• native calls
• operations that prevent flexible scheduling

This does not mean virtual threads are bad, but it means developers should still understand blocking behavior and monitor real workloads.

11. Do Old Concurrency Rules Still Matter

Yes.

Virtual threads do not remove the need for:

• thread safety
• synchronization
• safe shared-state design
• proper interruption handling

A virtual thread is still a thread.

Race conditions, deadlocks, and visibility issues are still possible if shared data is handled poorly.

12. Best Practices

• use virtual threads mainly for high-concurrency blocking workloads
• keep using proper synchronization for shared mutable state
• prefer thread-per-task style where it improves clarity
• avoid assuming virtual threads help CPU-bound work
• measure real performance in production-like conditions

13. Summary

Virtual threads are lightweight JVM-managed threads that make it easier to run huge numbers of concurrent tasks efficiently.

They are especially valuable for server-side and I/O-heavy applications because they allow developers to keep simple blocking code while improving scalability.

Although they reduce the cost of threads, they do not remove the need for correct concurrency design. Used properly, virtual threads are one of the most important modern improvements in Java concurrency.

Written By: Shiva Srivastava