Java虚拟线程（Virtual Threads）与ConcurrentHashMap的唤醒异常问题

在使用Java虚拟线程（VT）结合ConcurrentHashMap时，遇到了异常问题：当在computeIfAbsent方法内部获取锁时，虚拟线程可能永远无法被唤醒。该问题的出现与虚拟线程在载体线程上的调度方式相关，且可轻松复现此场景。

我知道ConcurrentHashMap内部使用synchronized关键字，而虚拟线程与synchronized的兼容性不佳，但官方并未提及需避免使用ConcurrentHashMap，这一点令人遗憾。

复现代码 #

class Scratch {
    public static void main(String[] args) throws InterruptedException {
        var lock = new ReentrantLock();
        var map = new ConcurrentHashMap<Integer, String>();

        var callables = new ArrayList<Callable<Void>>();
        for (int i = 0; i < 32 /* more than threads in FJ pool */; i++) {
            callables.add(() -> {
                System.out.println("Spawned thread " + Thread.currentThread().threadId());
                map.computeIfAbsent(1, k -> {
                    System.out.println("on lock() " + Thread.currentThread().threadId());
                    lock.lock();
                    try {
                        System.out.println("within lock() " + Thread.currentThread().threadId());
                    } finally {
                        lock.unlock();
                    }
                    return "";
                });
                System.out.println("Exiting " + Thread.currentThread().threadId());
                return null;
            });
        }

        new Thread(() -> {
            System.out.println("locking " + Thread.currentThread().threadId());
            lock.lock();
            try {
                Thread.sleep(2000);
            } catch (InterruptedException e) {
                throw new RuntimeException(e);
            }
            System.out.println("unlocking " + Thread.currentThread().threadId());
            lock.unlock();
            System.out.println("unlocked " + Thread.currentThread().threadId());
        }).start();

        while (!lock.isLocked()) {
            Thread.sleep(10);
        }

        // it works with Executors.newCachedThreadPool()
        try (var es = Executors.newVirtualThreadPerTaskExecutor()) {
            es.invokeAll(callables);
        }
        System.out.println("done");
    }
}

内容的提问来源于stack exchange，提问作者milan