Java 25 Virtual Threads – Benchmarks, Internals & Real-World Pitfalls
Virtual Threads are the most impactful change to Java’s concurrency model in decades — but only if you understand their limits.
With Java 25, Project Loom is production-ready, but many teams still misuse it. If you’re new to Java 25 features, start with this Java 25 migration guide .
Why This Article Exists
After seeing multiple production issues caused by misunderstood Virtual Threads — especially around database pools and ThreadLocal usage — it became clear most guides ignore operational reality.
These problems are also common discussion points in Java 25 concurrency interviews .
1. What Are Virtual Threads (Really)?
Virtual Threads are JVM-managed lightweight threads designed to make blocking I/O scalable. They do not replace async frameworks or improve CPU throughput.
2. Platform Threads vs Virtual Threads
| Aspect | Platform | Virtual |
|---|---|---|
| Managed by | OS | JVM |
| Blocking I/O | Blocks OS thread | Unmounts from carrier |
| Scalability | Thousands | Millions |
3. Creating Virtual Threads
Thread.startVirtualThread(() -> {
blockingCall();
});
Executor-based usage integrates best with Structured Concurrency in Java 25 .
4. Benchmarks – What Improves (and What Doesn’t)
Virtual Threads drastically improve concurrency for I/O-bound workloads. They do not make slow systems fast.
5. What Still Pins Virtual Threads
- Synchronized blocks
- Blocking native calls
- Legacy file I/O
Always profile using Java Flight Recorder. Memory and pinning issues are covered in Java memory leak analysis .
6. Spring Boot + Virtual Threads
Virtual Threads allow Spring MVC apps to scale without reactive complexity. However, blocking DB calls still dominate latency.
For broader Spring tuning strategies, see Spring Boot performance tuning .
7. Database Connection Pool Pitfall
Virtual Threads do not increase database capacity. This limitation is common in system design interviews .
8. ThreadLocal vs Scoped Values
ThreadLocal usage is unsafe with Virtual Threads. Java 25 introduces Scoped Values as a replacement.
9. CPU-Bound Work
Encryption, compression, and heavy computation should still use bounded executors. Virtual Threads are not a replacement for CPU scheduling.
10. Production Readiness Checklist
- DB pools sized correctly
- No CPU-heavy work on virtual threads
- ThreadLocal removed or audited
- JFR enabled