java - 重入读写锁。读写优先

Question

我研究 ReentrantReadWriteLock

来自 java 文档的片段:

The thread will not acquire the read lock until after the oldest currently waiting writer thread has acquired and released the write lock

据我了解。

阅读时长- 1个时间单位

写入时长- 3个时间单位

1. 时间 0 - 获得写锁
2. 时间 1 - 读锁尝试读
3. 时间 2 - 写锁尝试写

因此我期望以下顺序:

1. 先写
2. 第二次写
3. 阅读

我的实验代码:

public class RWLockCalculator {
    static long initTime = System.currentTimeMillis();
    private static int calculatedValue = 0;
    private static ReadWriteLock lock = new ReentrantReadWriteLock();
    public void calculate(int value) {
        lock.writeLock().lock();
        try {           
            System.out.println("write lock acquired at "+ (System.currentTimeMillis()-RWLockCalculator.initTime));
            this.calculatedValue = 1;
            Thread.sleep(300);
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } finally {
            lock.writeLock().unlock();
        }
    }

    public int getCalculatedValue() {
        lock.readLock().lock();
        try {           
            System.out.println("read lock acquired at "+ (System.currentTimeMillis()-RWLockCalculator.initTime));
            Thread.sleep(100);
            return calculatedValue;
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
            return -1;
        } finally {
            lock.readLock().unlock();
        }
    }
}

class Test {
    public static void main(String[] args) throws InterruptedException {
        new WriteThread().start();
        Thread.sleep(100);
        new ReadThread().start();
        Thread.sleep(100);
        new WriteThread().start();

    }
}

class ReadThread extends Thread {
    @Override
    public void run() {
        System.out.println(new RWLockCalculator().getCalculatedValue() + ", " + (System.currentTimeMillis() - RWLockCalculator.initTime));
    }
}

class WriteThread extends Thread {
    @Override
    public void run() {
        new RWLockCalculator().calculate(99);
        System.out.println("I have written in  " + (System.currentTimeMillis() - RWLockCalculator.initTime));
    }
}

输出:

write lock acquired at 0
I have written in  300
read lock acquired at 300
1, 400
write lock acquired at 400
I have written in  700

因此我得到

1. 先写
2. 阅读
3. 第二次写

为什么我会得到这个结果？

是否可以打破 FIFO 顺序？

更新

请比较 java 文档中的两个兄弟片段(关于公平模式):

首先

尝试获取公平读锁(不可重入)的线程将在持有写锁或有等待写入线程的情况下阻塞。直到当前最旧的等待写线程获得并释放写锁后，该线程才会获得读锁。当然，如果一个正在等待的写者放弃等待，留下一个或多个读者线程作为队列中最长的等待者，写锁空闲，那么这些读者将被分配读锁。

第二个:

尝试获取公平写锁(不可重入)的线程将阻塞，除非读锁和写锁都是空闲的(这意味着没有等待线程)。 (请注意，非阻塞 ReentrantReadWriteLock.ReadLock.tryLock() 和 ReentrantReadWriteLock.WriteLock.tryLock() 方法不遵守此公平设置，如果可能，将获取锁，而不管等待线程如何。)

我不完全理解那里所写内容的含义但是我看到 ReentrantReadWriteLock 使用不同的策略来获取读锁和写锁。我建议如果 java 文档中的政治相同，则不会写两个缩进。

ReadLock 可以共享锁。只有一个区别吗？

Answer 1

首先，ReentrantReadWriteLock 应该在公平模式下创建，以施加特定的锁获取顺序:

private static ReadWriteLock lock = new ReentrantReadWriteLock(true);

然后，javadoc非常清楚地描述了您的案例:

When constructed as fair, threads contend for entry using an approximately arrival-order policy. When the currently held lock is released either the longest-waiting single writer thread will be assigned the write lock, or if there is a group of reader threads waiting longer than all waiting writer threads, that group will be assigned the read lock.

由于您的读取线程比第二个写入线程等待的时间更长，因此它在写入线程之前获取锁。