Migrating Existing Code to Virtual Threads: Pros and Pitfalls

Java 21 has introduced Virtual Threads, a powerful new feature that changes how we handle concurrency. While they make it easier to write scalable applications, moving your existing code to virtual threads is not just copy-paste. You need to understand what’s involved, what changes to make, and what issues to avoid while migrating existing code to Virtual Threads.

This guide will walk you through the benefits, challenges, and best practices when migrating from traditional platform threads to virtual threads.

☕ What Are Virtual Threads?

Before migrating, let’s recap what virtual threads are:

• Virtual threads are lightweight threads managed by the JVM, not the operating system.
• They allow your app to handle millions of concurrent tasks with very little memory.
• Introduced as part of Project Loom in Java 21.

✅ Why Migrate to Virtual Threads?

1. 🧵 More Threads, Less Overhead

You can create thousands or even millions of threads without crashing the app or exhausting memory.

2. 💤 No More Blocking Worries

Blocking operations (like Thread.sleep() or database calls) don’t block the underlying thread, improving efficiency.

3. 👩‍💻 Easier to Read Code

You can write code in a traditional, step-by-step (imperative) style instead of complex asynchronous callbacks.

🛠️ How to Migrate: Step-by-Step

Step 1: Switch Your Executors

Replace this:

With this:

Step 2: Update Thread Creation Code

Instead of:

Use

Step 3: Check for Blocking Operations

Go through your code and find:

• Network calls
• File I/O
• Sleep or wait logic

Virtual threads can handle blocking, but you’ll get best results when using non-native blocking APIs (like Java’s NIO or JDBC that doesn’t lock platform threads).

⚠️ Pitfalls to Avoid

Migrating sounds easy — but some parts of existing code can break or behave unexpectedly with virtual threads.

1. ❌ Using ThreadLocals Carelessly

Virtual threads are reused often. If you’re using ThreadLocal to store per-thread data, it might not behave the same.

👉 Use ScopedValue (a new feature introduced to work better with virtual threads).

2. 🧪 Native Code and Blocking I/O

If your code uses native libraries or calls native I/O (like some old file-handling or sockets), these may still block platform threads, reducing the benefit of virtual threads.

3. 🕵️ Logging and Monitoring

Your observability tools (logging, metrics, thread dumps) may not yet support virtual threads fully. Update your tooling!

4. 🧵 Thread Pool Misuse

Virtual threads don’t need a fixed-size pool. Using traditional fixed pools with virtual threads limits their benefits.

🧪 Testing After Migration

After converting your code:

1. Run load tests — try thousands of simultaneous tasks.
2. Monitor thread usage (jconsole, VisualVM)
3. Check for performance improvements
4. Make sure logs, traces, and debuggers still work properly

🧾 Example: Migrating a Web Request Handler

Before:

After

Result: You can handle 100,000+ requests without memory issues or slowdowns.

🎯 When Should You NOT Migrate?

• If you rely heavily on native threading or JNI
• If you use frameworks that aren’t yet compatible with virtual threads
• If your app is CPU-bound (virtual threads don’t speed up CPU-heavy tasks)

🧰 Tips Before Migrating

✅ Use Java 21 with --enable-preview

✅ Start with non-critical modules or test environments

✅ Avoid combining platform threads and virtual threads in the same logic unless you know what you’re doing

✅ Don’t use ThreadLocal unless necessary

🔚 Final Thoughts

Migrating to virtual threads can massively improve performance and scalability, especially for I/O-bound applications. But take the time to understand how your existing code works, and migrate gradually.

Virtual threads are not just a feature — they are a shift in how Java applications are built.

Reference : Virtual Threads