Virtual Threads

Introduced in Java 21, Virtual Threads (Project Loom) are lightweight threads mapped dynamically to carrier OS threads. They enable high-throughput applications using simple thread-per-request blocking APIs.

Platform vs. Virtual Threads

• Platform Threads: 1-to-1 mapping with native OS threads. Resource-intensive (allocating 1MB stack by default). Limited to a few thousand instances.
• Virtual Threads: Mapped to a ForkJoinPool of carrier OS threads. Stack frames are stored on the JVM heap. Virtual threads are cheap to create, meaning you should never pool them; just create a new thread for each task.
• Thread Pinning: Occurs when a virtual thread is pinned to its carrier thread, preventing other virtual threads from running. Pinning is caused by executing code inside a synchronized block or calling native code (JNI). Use ReentrantLock instead of synchronized to prevent pinning.

Code Walkthrough

This program demonstrates creating and executing virtual threads.

import java.util.concurrent.Executors;
public class VirtualThreadDemo {
    public static void main(String[] args) throws Exception {
        // Create an executor that spawns a new virtual thread for each task
        try (var executor = Executors.newVirtualThreadPerTaskExecutor()) {
            for (int i = 0; i < 10000; i++) {
                executor.submit(() -> {
                    // Simulates non-blocking blocking I/O: JVM yields OS thread automatically
                    Thread.sleep(100); 
                    return null;
                });
            }
        } // Executor close blocks until all tasks complete
        System.out.println("All tasks finished!");
    }
}

Interview-Relevant Information