Java developers have been writing reactive code for years to solve a concurrency problem that the platform itself could never handle well. That might finally be changing. With Java 21 and Project Loom, Virtual Threads bring lightweight concurrency natively to the JVM — and the question is no longer whether reactive works. It does. The real question is whether it should still be the default choice.
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The Problem
Platform Threads and Their Limits
Traditional Java threads — known as platform threads — are mapped 1:1 to OS kernel threads. This design is simple but fundamentally limited at scale:
Limited scalability — you can realistically create only a few thousand threads before the system buckles
High memory overhead — each thread consumes ~1 MB of stack memory
Expensive context switching — the OS must save and restore thread state on every switch
Thread starvation on I/O waits — a thread blocked on a database response is completely idle but still consumes resources
