Synchronize Virtual Thread Without Pinning

Virtual threads in Java can get pinned to their carrier platform thread in several scenarios, negating their scalability benefits. The main pinning causes are: synchronized methods/blocks, native method calls, static initializer blocks, and custom class loaders. The post demonstrates how to detect pinning using Java Flight Recorder (JFR) and the -Djdk.tracePinnedThreads VM flag. The fix for synchronized blocks is to replace them with ReentrantLock from java.util.concurrent.locks, which is Loom-aware and does not cause pinning. JDK 24+ (via JEP-491) resolves the synchronized keyword pinning at the JVM level, with JMH benchmarks showing dramatically improved throughput (e.g., ~8x at 1000 concurrency) in JDK 25 vs JDK 21. Native methods, class loaders, and static initializers still cause pinning in all versions.