Platform Threads vs Virtual Threads

TL;DR

• Platform Threads (Classic Threads) are managed by the OS. They are heavy, expensive, and limited in number.
• Virtual Threads are managed by the JVM. They are cheap, fast to create, and virtually limitless.
• Virtual threads are not a replacement for Platform threads; they run on top of them.

Concept

Platform Threads (OS Threads)

Every time you call new Thread().start() in Java (pre-Java 21), you ask the OS to allocate a thread.

• Cost: ~1MB-2MB of RAM per thread stack.
• Context Switching: Managed by the OS kernel, which is a slow and expensive operation requiring system calls.
• Limit: Typically a few thousand before the OS begins thrashing or runs out of memory.

Virtual Threads

A Virtual Thread is entirely in the JVM’s “User Space”.

• Cost: A few hundred bytes on the Java Heap. The JVM manages its call stack dynamically, growing and shrinking it as needed.
• Context Switching: When a virtual thread blocks, the JVM (not the OS) copies its stack to the heap and suspends it. This takes nanoseconds.
• Limit: You can easily run millions of them.

Examples

public class ThreadComparison {
    public static void main(String[] args) {
        
        // --- PLATFORM THREAD ---
        // Heavyweight. Do not create 100,000 of these.
        Thread platformThread = Thread.ofPlatform()
                                      .name("classic-thread-1")
                                      .start(() -> doWork());
                                      
        // --- VIRTUAL THREAD ---
        // Lightweight. Go ahead and create 1,000,000 of these.
        Thread virtualThread = Thread.ofVirtual()
                                     .name("virtual-thread-1")
                                     .start(() -> doWork());
                                     
        // Check what kind of thread you are currently on
        boolean isVirtual = Thread.currentThread().isVirtual();
    }
    
    public static void doWork() {
        try { Thread.sleep(100); } catch (Exception e) {}
    }
}

Interview Questions

Q: Can a Virtual Thread be a Daemon thread?
A: Yes, in fact, all Virtual Threads are daemon threads by default, and you cannot change them to non-daemon threads. This means that if your main application thread finishes, the JVM will instantly shut down, terminating all running Virtual Threads without waiting for them to complete.

Q: Do Virtual Threads have priorities?
A: No. The Thread.setPriority() method has no effect on Virtual Threads. The JVM treats all Virtual Threads equally. If you require strict priority-based scheduling, you must use Platform Threads.