StampedLock

该类自 JDK 8 加入，是为了进一步优化读性能，它的特点是在使用读锁、写锁时都必须配合【戳】使用加解读锁


long stamp = lock.readLock();
lock.unlockRead(stamp);

加解写锁


long stamp = lock.writeLock();
lock.unlockWrite(stamp);

乐观读，StampedLock 支持 tryOptimisticRead() 方法（乐观读），读取完毕后需要做一次戳校验如果校验通过，表示这期间确实没有写操作，数据可以安全使用，如果校验没通过，需要重新获取读锁，保证数据安全。


long stamp = lock.tryOptimisticRead();
// 验戳
if(!lock.validate(stamp)){
// 锁升级
}

提供一个数据容器类内部分别使用读锁保护数据的 read() 方法，写锁保护数据的 write() 方法


@Slf4j(topic = "c.DataContainerStamped")
class DataContainerStamped {
    private int data;
    private final StampedLock lock = new StampedLock();

    public DataContainerStamped(int data) {
        this.data = data;
    }

    public int read(int readTime) {
        long stamp = lock.tryOptimisticRead();
        log.debug("optimistic read locking...{}", stamp);
        sleep(readTime);
        if (lock.validate(stamp)) {
            log.debug("read finish...{}, data:{}", stamp, data);
            return data;
        }
        // 锁升级 - 读锁
        log.debug("updating to read lock... {}", stamp);
        try {
            stamp = lock.readLock();
            log.debug("read lock {}", stamp);
            sleep(readTime);
            log.debug("read finish...{}, data:{}", stamp, data);
            return data;
        } finally {
            log.debug("read unlock {}", stamp);
            lock.unlockRead(stamp);
        }
    }

    public void write(int newData) {
        long stamp = lock.writeLock();
        log.debug("write lock {}", stamp);
        try {
            sleep(2);
            this.data = newData;
        } finally {
            log.debug("write unlock {}", stamp);
            lock.unlockWrite(stamp);
        }
    }
}

测试读-读可以优化


@Slf4j(topic = "c.TestStampedLock")
public class TestStampedLock {
    public static void main(String[] args) {
        DataContainerStamped dataContainer = new DataContainerStamped(1);
        new Thread(() -> {
            dataContainer.read(1);
        }, "t1").start();
        sleep(0.5);
        new Thread(() -> {
            dataContainer.read(0);
        }, "t2").start();
    }
}

输出结果，可以看到实际没有加读锁


12:48:27.518 c.DataContainerStamped [t1] - optimistic read locking...256
12:48:28.018 c.DataContainerStamped [t2] - optimistic read locking...256
12:48:28.018 c.DataContainerStamped [t2] - read finish...256, data:1
12:48:28.536 c.DataContainerStamped [t1] - read finish...256, data:1

测试读-写时优化读补加读锁


@Slf4j(topic = "c.TestStampedLock")
public class TestStampedLock {
    public static void main(String[] args) {
        DataContainerStamped dataContainer = new DataContainerStamped(1);
        new Thread(() -> {
            dataContainer.read(1);
        }, "t1").start();
        sleep(0.5);
        new Thread(() -> {
            dataContainer.write(100);
        }, "t2").start();
    }
}

输出结果


15:57:00.219 c.DataContainerStamped [t1] - optimistic read locking...256
15:57:00.717 c.DataContainerStamped [t2] - write lock 384
15:57:01.225 c.DataContainerStamped [t1] - updating to read lock... 256
15:57:02.719 c.DataContainerStamped [t2] - write unlock 384
15:57:02.719 c.DataContainerStamped [t1] - read lock 513
15:57:03.719 c.DataContainerStamped [t1] - read finish...513, data:1000
15:57:03.719 c.DataContainerStamped [t1] - read unlock 513

注意 StampedLock 不支持条件变量 StampedLock 不支持可重入