JEP 491: Synchronize Virtual Threads without Pinning

🧠 Quick Summary: What Is JEP 491?

https://openjdk.org/jeps/491

In earlier Java versions, using synchronized blocks could pin a virtual thread to a platform thread, blocking scalability.

🔐 JEP 491 (Java 24) removes that limitation:

Virtual threads now don't get pinned during synchronized blocks on object monitors.

RUN :

SyncNoPinningDemo

This is very basic example where thread sleep 100ms within synchrronized block. Although there is 100 virtual threads per core prorgam should finish within 100-200ms

🔥 What happens before JDK 24:

• When a virtual thread enters a synchronized block, it may pin the carrier platform thread (i.e., the actual OS thread backing the virtual thread during execution).

• Blocking calls like Thread.sleep() inside a synchronized block would prevent the JVM from unmounting the virtual thread, because JVM assumes the monitor might need to be released immediately (e.g., if another thread is waiting).

• Therefore, the platform thread gets blocked during that sleep, even if there’s no actual contention.

• With 1000 virtual threads, and only one platform thread in the pool, the runtime would be:

  yamlCopyEdit1000 * 100ms = ~100 seconds

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✅ What changed in JDK 24 with JEP 491:

• The JVM was improved to detect that in non-contended synchronized blocks, pinning is unnecessary, and allows the virtual thread to yield the platform thread during blocking.

• So now, each sleeping virtual thread unmounts cleanly, and the one platform thread is reused.

• The result: it completes in just over 100 ms, not 100 seconds.