Java 8 Stamped Lock:为什么这段代码不会导致死锁? [英] Java 8 Stamped Lock: Why this piece of code doesnt result into a deadlock?

问题描述

在尝试理解Java 8中的乐观锁时，我遇到了以下代码. 此处的原始博客

In my attempt to understand Optimistic locking in Java 8,I came across the below piece of code. Original Blog Here.

如博客中所述，这段代码试图将读取锁转换为写入锁.如果将读取锁转换为写入锁失败，则代码会请求显式写入锁.

As explained in the blog, this piece of code is attempting to convert a read lock into a write lock.The code requests an explicit write lock if conversion of read lock to write lock failed.

这使我感到困惑当父线程已经持有读取锁时，如何期望授予显式写入锁?看起来读取锁似乎并没有被释放强制请求写锁定之前的时间点.以我的错误理解，线程将无限期地等待写锁，因为从不释放读锁而导致死锁.

It puzzles me How can it be expected that the explicit write lock get granted when the parent thread is already holding a read lock? It doesn't look like the read lock is getting released at any point before a write lock is forcefully requested. To my flawed understanding, the thread would wait infinitely for write lock as the read lock is never released creating a deadlock.

为什么这不会导致僵局?

ExecutorService executor = Executors.newFixedThreadPool(2);
StampedLock lock = new StampedLock();

executor.submit(() -> {
    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);
    }
});

stop(executor);

提前谢谢！

推荐答案

有帮助吗?从tryConvertToWriteLock

如果锁定状态与给定的标记相匹配，则自动执行以下操作之一.如果该图章表示持有写锁，则将其返回. 或者，如果有读锁(如果有写锁)，则释放读锁并返回写标记.或者，如果是乐观读，则仅在立即可用时才返回写标记.在所有其他情况下，此方法均返回零.

If the lock state matches the given stamp, atomically performs one of the following actions. If the stamp represents holding a write lock, returns it. Or, if a read lock, if the write lock is available, releases the read lock and returns a write stamp. Or, if an optimistic read, returns a write stamp only if immediately available. This method returns zero in all other cases.

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