java - StampedLock.writeLock() 是否释放当前线程持有的读锁？

Question

http://winterbe.com/posts/2015/04/30/java8-concurrency-tutorial-synchronized-locks-examples/包含此代码:

StampedLock lock = new StampedLock();
long stamp = lock.readLock();
try {
  if (count == 0) {
    stamp = lock.tryConvertToWriteLock(stamp);
    if (stamp == 0L) {
      System.out.println("Could not convert to write lock");
      stamp = lock.writeLock();
    }
    count = 23;
  }
  System.out.println(count);
} finally {
  lock.unlock(stamp);
}

作者写道:

Calling tryConvertToWriteLock() doesn't block but may return a zero stamp indicating that no write lock is currently available. In that case we call writeLock() to block the current thread until a write lock is available.

关于 tryConvertToWriteLock() 失败的情况，我是否遗漏了什么，或者作者获取了一个读锁，然后是一个写锁，并且从未释放读锁？

此外，lock.writeLock() 不会死锁等待当前线程释放其读锁吗？

Answer 1

我认为本文提供的代码示例存在错误。根据https://docs.oracle.com/javase/9/docs/api/java/util/concurrent/locks/StampedLock.html

Like Semaphore, but unlike most Lock implementations, StampedLocks have no notion of ownership. Locks acquired in one thread can be released or converted in another.

此外，他们还提供了以下示例代码:

void moveIfAtOrigin(double newX, double newY) { // upgrade
    // Could instead start with optimistic, not read mode
    long stamp = sl.readLock();
    try {
        while (x == 0.0 && y == 0.0) {
            long ws = sl.tryConvertToWriteLock(stamp);
            if (ws != 0L) {
                stamp = ws;
                x = newX;
                y = newY;
                break;
            }
            else {
                sl.unlockRead(stamp);
                stamp = sl.writeLock();
            }
        }
    } finally {
        sl.unlock(stamp);
    }
}

注意他们如何在获取写锁之前释放读锁。