Java Synchronization in Virtual Threads Without Pinning (JDK 24)

Introduction

With the release of JDK 24, Java's virtual threads (introduced in JDK 19 and stabilized in JDK 21) continue to revolutionize concurrency in Java applications. However, developers face challenges when synchronizing virtual threads, particularly avoiding "pinning"—a situation where a virtual thread gets stuck to a platform (OS) thread, negating its lightweight nature.

This article explores virtual threads, the pitfalls of synchronization with thread pinning, and how JDK 24 provides solutions for synchronization without pinning.

What Are Virtual Threads?

Virtual threads are a lightweight alternative to traditional platform threads (also known as carrier threads). Unlike platform threads, which directly map to OS threads, virtual threads are managed by the JVM and multiplexed over a smaller pool of carrier threads. This enables Java applications to scale efficiently with thousands or even millions of concurrent tasks without exhausting system resources.

Key Features of Virtual Threads:

? Lightweight – Virtual threads do not consume significant system memory, allowing thousands to run concurrently.

? Efficient Blocking – Unlike platform threads, virtual threads allow blocking I/O operations without wasting CPU resources.

? Automatic Scheduling – The JVM efficiently schedules virtual threads across available carrier threads.

Example of Virtual Thread Creation in Java 21+

try (var executor = Executors.newVirtualThreadPerTaskExecutor()) {
    executor.submit(() -> {
        System.out.println("Running in a virtual thread: " + Thread.currentThread());
    });
}
        

In the above example, a virtual thread is created for each task, running independently and efficiently.

Thread Pinning in Synchronization: The Problem

A common challenge with virtual threads is thread pinning, which occurs when a virtual thread becomes stuck to a carrier thread due to certain blocking mechanisms, preventing it from being rescheduled efficiently.

Why Does Pinning Happen?

Virtual threads rely on carrier threads for execution, but some operations, particularly those involving synchronized blocks or methods, can cause a virtual thread to pin to a specific carrier thread, reducing scalability.

Example of Pinning in Java (Before JDK 24)

class SharedResource {
    synchronized void performTask() {  // ?? Causes pinning!
        System.out.println(Thread.currentThread() + " is executing performTask");
        try { Thread.sleep(1000); } catch (InterruptedException e) { Thread.currentThread().interrupt(); }
    }
}

public class PinningExample {
    public static void main(String[] args) {
        SharedResource resource = new SharedResource();
        Thread.startVirtualThread(resource::performTask);
    }
}
        

Here, the synchronized method locks the monitor, forcing the virtual thread to stay pinned to a carrier thread while waiting, reducing concurrency efficiency.

JDK 24: Synchronization Without Pinning

JDK 24 introduces enhancements to synchronized blocks, ensuring that virtual threads no longer get pinned to carrier threads during synchronization. Instead of blocking a carrier thread, the JVM parks the virtual thread, allowing other tasks to execute in the meantime.

How JDK 24 Fixes This Issue

?? Monitor Parking – Instead of keeping a virtual thread pinned, JDK 24 allows it to detach while waiting for a lock.

?? Non-blocking Lock Management – Synchronization no longer prevents carrier threads from handling other virtual threads.

Improved Synchronization in JDK 24 (No Pinning)

class SharedResource {
    synchronized void performTask() {
        System.out.println(Thread.currentThread() + " is executing performTask");
        try { Thread.sleep(1000); } catch (InterruptedException e) { Thread.currentThread().interrupt(); }
    }
}

public class NoPinningExample {
    public static void main(String[] args) {
        SharedResource resource = new SharedResource();
        Thread.startVirtualThread(resource::performTask);
    }
}
        

Even though this example looks the same as before, in JDK 24, the JVM now parks the virtual thread instead of keeping it pinned, making synchronization efficient and scalable.

Alternative Synchronization Approaches for Virtual Threads

In addition to the improvements in JDK 24, there are other ways to synchronize data safely without blocking virtual threads:

1?? Using ReentrantLock (Non-blocking)

import java.util.concurrent.locks.ReentrantLock;

class SharedResource {
    private final ReentrantLock lock = new ReentrantLock();

    void performTask() {
        lock.lock();
        try {
            System.out.println(Thread.currentThread() + " is executing performTask");
            Thread.sleep(1000);
        } catch (InterruptedException e) {
            Thread.currentThread().interrupt();
        } finally {
            lock.unlock();
        }
    }
}
        

? Benefit: Unlike synchronized, ReentrantLock allows fine-grained locking, which improves fairness and avoids unnecessary blocking.

2?? Using ThreadLocal to Avoid Shared State

If a shared resource is not essential, using ThreadLocal helps avoid locking entirely.

class ThreadLocalExample {
    private static final ThreadLocal<Integer> threadLocalValue = ThreadLocal.withInitial(() -> 0);

    void increment() {
        threadLocalValue.set(threadLocalValue.get() + 1);
        System.out.println(Thread.currentThread() + " value: " + threadLocalValue.get());
    }
}
        

? Benefit: Each virtual thread gets its own copy of the variable, eliminating the need for synchronization.

Conclusion: The Future of Virtual Threads and Synchronization

JDK 24 is a game changer for Java developers working with high-concurrency applications. By eliminating synchronization-induced pinning, it allows developers to confidently use virtual threads in scenarios where shared state is involved.

Key Takeaways:

?? Virtual threads in Java provide massive scalability and efficient thread management.

?? Pinning occurs when a virtual thread is forced to stay on a carrier thread, reducing efficiency.

?? JDK 24 fixes pinning by allowing virtual threads to park instead of blocking carrier threads.

?? Alternatives like ReentrantLock and ThreadLocal can also help manage synchronization effectively.

With these improvements, Java is now better equipped than ever to handle concurrent programming at scale.

What are your thoughts on Java’s virtual threads and synchronization changes in JDK 24? Let’s discuss in the comments!