Java Virtual Threads in 2025: Scalable I/O Without Async Hell (and the Real Limits)

Note This article uses Java 21 as the baseline. Virtual threads are stable in Java 21 (JEP 444), so no preview flags are needed here. Originally published on engnotes.dev: https://engnotes.dev/blog/project-loom/virtual-threads-revolution-part-1 This is a shortened version with the same core code and takeaways. If you have worked on Java backends long enough, you have probably seen the same problem show up again and again. The business logic is not especially hard. The traffic is. Then the thread math starts. How many threads, how much memory, how much waiting, how much tuning before the whole thing starts feeling awkward. That is the pain virtual threads address. The Old Problem Platform threads are expensive. Each one uses real OS resources and carries a meaningful memory cost. For blocking I/O workloads, that puts a ceiling on concurrency much earlier than most teams want. This is the usual shape: public class PlatformThreadPoolServer { private static final int PORT = 8080; private s