Black Friday, a million concurrent requests – and somewhere in the backend, a simple new Thread(). OutOfMemoryError guaranteed! Platform threads cost you 1MB of RAM each and limit your application to a maximum of 10,000 threads.

With Java 21+, everything changes: Virtual Threads only need 1KB, are created in a microsecond, and you can easily start a million of them. I'll show you how to harness this power using a real-world rule engine. Together, we'll build a system that executes 100,000 business rules in parallel – without thread pools, without caching headaches.

You'll learn about the three pillars of modern Java concurrency: virtual threads (final since Java 21) for massive parallelisation, scoped values (final in Java 25) as a safe alternative to ThreadLocal, and structured concurrency for elegant task management. In live coding, you will see how these APIs interact.

After this talk, you will know exactly when virtual threads make sense (I/O-bound) and when they don't (CPU-intensive), how to migrate existing code with tools like OpenRewrite, and what pitfalls to watch out for. Bonus: a working GitHub GIST for experimenting

Jfokus

A conference talk covering Java 21's concurrency improvements: virtual threads, scope values, and structured concurrency. Virtual threads allow running millions of lightweight threads (1KB vs 1MB for platform threads) without exhausting OS resources, making them ideal for IO-heavy, high-concurrency applications. Scope values replace ThreadLocal for sharing immutable context across virtual threads efficiently. Structured concurrency via StructuredTaskScope provides clean lifecycle management and error handling for grouped async tasks. The talk includes live demos, migration advice using OpenRewrite recipes to identify ThreadLocal candidates for conversion, and best practices like never pooling virtual threads and avoiding them for CPU-intensive work.

More Action, More Overview – From 4k to 1M Threads, by Merlin Bögershausen