[mperor]

What you'll learn

• Why many virtual threads blocking inside synchronized used to exhaust carrier threads, and how JEP 491 fixes that in JDK 24+
• The one pinning case JEP 491 does not fix: a native (JNI) frame still pins the carrier
• That a thread blocked entering a contended synchronized block ignores interrupt(), unlike ReentrantLock#lockInterruptibly

Why it matters

Picking synchronized vs. java.util.concurrent locks for virtual-thread code used to be a real capacity-planning decision; JEP 491 removes most of that cost, but interruptibility, tryLock, fairness and multiple Conditions are still reasons to reach for ReentrantLock.

Note: this sandbox's network policy blocks every JDK 24/25 distribution host, so ./gradlew could not run locally. I validated the new tests by compiling and running them standalone against a local JDK 21 + JUnit 6.1: they compile cleanly, and testManyVirtualThreadsBlockingInsideSynchronizedDoNotPinCarriers fails on JDK 21 exactly as predicted (elapsed time matches the pinned-carrier estimate almost exactly, confirming real pinning). CI has since run on JDK 25 and is green (306 tests, +3, 0 failures), confirming the fix actually works there.

Read more

JEP 491: Synchronize Virtual Threads without Pinning

Quiz

Before JDK 24, 1000 virtual threads each enter their own (uncontended) synchronized block and call Thread.sleep(80) inside it, on a machine with 8 carrier threads. Roughly how long does the whole batch take, and why?

Answer

Roughly 1000 / 8 * 80 ms ≈ 10 seconds. Blocking inside synchronized pinned each virtual thread to its carrier thread, so only 8 could be "sleeping" at once; the rest queued up waiting for a free carrier. On JDK 24+ (JEP 491) the carrier is released instead, so all 1000 run close to concurrently and the batch finishes in about 80 ms.

Closes #97

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Test Results

306 tests  +3   303 ✅ +3   5s ⏱️ -1s
 84 suites ±0     3 💤 ±0 
 84 files   ±0     0 ❌ ±0 

Results for commit a8298de. ± Comparison against base commit a9c068a.

This pull request skips 1 and un-skips 1 tests.

pl.mperor.lab.java.design.pattern.creational.singleton.SingletonTest ‑ testThreadSafe()

pl.mperor.lab.java.design.pattern.creational.singleton.SingletonTest ‑ testSerializationSafe()

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