🚀 “How are Virtual Threads different from Thread Pools?”
😵 “Are they OS threads or JVM threads?”
🙃 “Should I still use CompletableFuture?”
🤯 “How do I even use them in real-time microservices?”
🧠 What are Virtual Threads?
Virtual Threads (introduced in Java 21 as stable 🎉) are lightweight threads managed by the JVM instead of the OS kernel.
👉 They look like normal threads, but don’t hog OS resources like traditional threads.
🧠 What is the OS Kernel?
🏛️ OS Kernel = The Brain of the Operating System
It’s the core part of your OS (Windows, Linux, Mac) that:
- Manages memory 🧠
- Schedules threads 🕒
- Talks to hardware 💻
- Handles I/O operations 📨
When you create a traditional thread in Java, the JVM asks the OS Kernel to create a real OS-level thread.
🖼️ Imagine This...
┌───────────────────────────┐
│ Your Java Application │
└────────────┬──────────────┘
│
▼
┌─────────────────────┐
│ JVM (Java) │
└────────────┬────────┘
▼
┌────────────────┐
│ OS Kernel │ <-- 👑 THE BOSS
└────────────────┘
👆 In traditional threading, JVM tells the OS Kernel:
“Hey, I need a thread!”
The OS Kernel:
“Okay, here's an expensive, heavyweight OS thread 🏋️♂️.”
🧚 Virtual Threads: JVM says "No thanks!" to the Kernel
In Virtual Threads, the JVM says:
“I’ll handle my own threads inside me! I don’t need the OS kernel for each tiny task.”
So, the JVM runs its own mini scheduler, called the "user-mode scheduler", to manage thousands or even millions of tiny threads.
- ✅ No OS-level thread for each Java task
- ✅ No need to block real system threads for I/O
- ✅ Just use a few carrier threads and juggle tasks smartly
🎨 Diagram: Virtual Threads vs Traditional Threads
| Feature | Traditional Threads 🏋️ | Virtual Threads 🧚 |
|---|---|---|
| Managed by | OS Kernel 🖥️ | JVM Scheduler ☕ |
| Memory per thread | ~1MB stack | ~few KB stack |
| Blocking I/O | Expensive | Cheap (handled async) |
| Thread creation | Limited (few thousand max) | Millions possible! |
| Uses Thread class? | ✅ Yes | ✅ Yes (surprise!) |
| Useful for | CPU-bound 🧠 | I/O-bound 📨 |
🏛️ Architecture View (Virtual Thread)
┌──────────────────────────────┐
│ Application Code │
└──────────────────────────────┘
│
▼
Traditional: ┌────────────┐
↳ Thread --> │ OS Thread │ --> Expensive! 🔥
└────────────┘
Virtual: ┌───────────────┐
↳ Thread --> │ JVM Scheduler │ --> Lightweight! ☁️
└───────────────┘
🤔 Are Virtual Threads Inside or Outside JVM?
➡️ Virtual Threads are purely JVM-level constructs.
They don’t map 1:1 to OS threads. Instead, JVM uses a small pool of carrier OS threads under the hood to run millions of virtual threads via scheduling.
📌 OS thread ≠ Java thread anymore. JVM does the magic.
🔍 Virtual Threads vs CompletableFuture
| Feature | Virtual Threads 🧚 | CompletableFuture 🚀 |
|---|---|---|
| Programming model | Imperative (normal Thread.sleep) | Functional, async-style (thenApply) |
| Easy to debug? | ✅ Super easy | ❌ Stack traces can be complex |
| Use case | Replace thread pools, simple code | Reactive chains, parallel tasks |
| Scheduling | Handled by JVM | Uses thread pools (ForkJoinPool) |
| Exception handling | Traditional try/catch | Use .exceptionally, hard for freshers |
👉 Think of Virtual Threads as: “Asynchronous made Synchronous”
🔧 Real World Code Comparison
🚫 Traditional Thread Pool (Before Java 21)
ExecutorService executor = Executors.newFixedThreadPool(10);
executor.submit(() -> {
// Some I/O task
fetchDataFromDB();
});
✅ Virtual Threads (Java 21+)
ExecutorService executor = Executors.newVirtualThreadPerTaskExecutor();
executor.submit(() -> {
fetchDataFromDB(); // Looks blocking, but isn't costly
});
🎉 No CompletableFuture, no callback hell. Just plain readable code!
🔥 Real-time Example: Spring Boot REST API
Let's say you’re building a REST API that calls DB + Redis + External API.
🧟♂️ ThreadPool Style:
@GetMapping("/user")
public CompletableFuture<User> getUser() {
return CompletableFuture.supplyAsync(() -> userService.getUserDetails());
}
🧚 Virtual Thread Style:
@GetMapping("/user")
public User getUser() {
return userService.getUserDetails(); // Blocking? Who cares. It’s virtual!
}
➡️ Benefit: No need to switch programming paradigm. Cleaner logs, easier debugging, fewer threads blocking.
❓ Why should I enable Virtual Threads for a normal REST call? It’s just a request/response, right?
It seems like a normal REST call is simple—but under the hood, it often involves:
- ⏳ Calling external REST APIs
- 🛢️ Querying databases
- 💾 Accessing caches like Redis
All of these operations are blocking I/O tasks. If you're using a fixed-size thread pool, threads doing these tasks will sit idle while waiting for responses—wasting system resources.
💡 With Virtual Threads, the JVM can pause and resume these I/O tasks efficiently—freeing up the underlying OS threads for other work.
Conclusion: Even basic REST APIs benefit from Virtual Threads if they perform blocking I/O.
❓ Will my REST controller automatically run on a Virtual Thread?
🛑 No, not by default! REST controllers use platform threads (OS-backed) unless you explicitly configure Spring Boot to use Virtual Threads.
To enable them in Spring Boot 3.2 or later, use:
# application.yaml
server:
virtual-threads:
enabled: true
✅ Once enabled, all incoming HTTP requests will be handled using Virtual Threads under the hood—so your synchronous code remains clean and resource-efficient
🧠 What are Virtual Threads?
Virtual Threads (introduced in Java 21 as stable 🎉) are lightweight threads managed by the JVM instead of the OS kernel.
👉 They look like normal threads, but don’t hog OS resources like traditional threads.
⚠️ Do's & Don'ts
- ✅ Do use Virtual Threads for I/O-heavy apps (REST calls, DB queries)
- ❌ Don’t use Virtual Threads if your logic is CPU-heavy (matrix multiplication etc.)
- ✅ Do migrate ExecutorService to Executors.newVirtualThreadPerTaskExecutor()
- ❌ Don’t manually .start() millions of new Thread() objects
🧪 Sample Implementation
public class VirtualThreadDemo {
public static void main(String[] args) throws Exception {
ExecutorService executor = Executors.newVirtualThreadPerTaskExecutor();
for (int i = 0; i < 10000; i++) {
executor.submit(() -> {
System.out.println("Thread: " + Thread.currentThread());
Thread.sleep(100); // This is okay!
return null;
});
}
executor.shutdown();
}
}
☕ Run this and see 10,000 tasks running with just a few carrier threads behind the scenes!
🛑 What You Can’t Do
- You can’t do CPU-heavy stuff with millions of threads
- Virtual threads don’t help with deadlocks
- You still need synchronization (they don’t magically fix shared state problems)
💡 Interview Questions You May Face
Basic:
- ❓ What is a Virtual Thread?
- ❓ How does it differ from a traditional thread?
- ❓ Is it inside the JVM or OS-level?
- ❓ How do you create a Virtual Thread?
Intermediate:
- ❓ How does Virtual Thread reduce memory usage?
- ❓ Can we use Thread.sleep() inside Virtual Thread?
- ❓ Compare Virtual Threads vs CompletableFuture.
Advanced:
- ❓ Can I use Virtual Threads in Spring Boot today?
- ❓ What happens if Virtual Threads do blocking DB calls?
- ❓ Is Virtual Thread an alternative to Reactive Programming?
🎁 Wrapping Up
👨💻 Virtual Threads are here to change the way Java handles concurrency.
They bring the simplicity of synchronous code with the power of async under the hood.
- 💥 Replace ugly thread pool code
- 💥 Ditch CompletableFuture chains
- 💥 Write code like a boss — clean, readable, and millions of tasks without breaking the JVM
Comments
Post a Comment