Pinning and Best Practices

A virtual thread is pinned to its carrier (cannot unmount) when it blocks inside a synchronized block or a native method. In Java 21 this wastes a carrier thread and can starve others. (Java 24, JEP 491, removed the synchronized pinning, but Java 21 LTS still has it.)

// Java 21: pins the carrier during the HTTP call
synchronized (this) {
    response = httpClient.send(request, BodyHandlers.ofString());
}

// Better on Java 21: ReentrantLock lets the virtual thread unmount
lock.lock();
try {
    response = httpClient.send(request, BodyHandlers.ofString());
} finally {
    lock.unlock();
}

java -Djdk.tracePinnedThreads=full -jar app.jar

Thread[#29,ForkJoinPool-1-worker-1,5,CarrierThreads]
    java.base/java.lang.VirtualThread.parkOnCarrierThread(VirtualThread.java:...)
    com.shop.PriceClient.fetch(PriceClient.java:42) <== monitors:1

• Do not pool virtual threads. Use newVirtualThreadPerTaskExecutor().
• Limit access to scarce resources (DB connections, rate-limited APIs) with a Semaphore or the connection pool, not by limiting threads.
• Avoid long synchronized blocks around I/O on Java 21; use ReentrantLock.
• Keep ThreadLocal values small; consider ScopedValue (preview in 21).
• Do not use them for CPU-heavy work; use a fixed pool sized to the cores or parallel streams.