Java利用StampedLock实现读写锁的方法详解

long stamp = lock.writeLock();
try {
    ....
} finally {
    lock.unlockWrite(stamp);
}

long stamp = lock.readLock();
try {
    ....
} finally {
    lock.unlockRead(stamp);	    
}

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

@Slf4j
@Data
public class Point {
    private double x, y;
    private final StampedLock sl = new StampedLock();

    void move(double deltaX, double deltaY) throws InterruptedException {
        //涉及对共享资源的修改，使用写锁-独占操作
        long stamp = sl.writeLock();
        log.info("writeLock lock success");
        Thread.sleep(500);
        try {
            x  = deltaX;
            y  = deltaY;
        } finally {
            sl.unlockWrite(stamp);
            log.info("unlock write lock success");
        }
    }

    /**
     * 使用乐观读锁访问共享资源
     * 注意：乐观读锁在保证数据一致性上需要拷贝一份要操作的变量到方法栈，并且在操作数据时候可能其他写线程已经修改了数据，
     * 而我们操作的是方法栈里面的数据，也就是一个快照，所以最多返回的不是最新的数据，但是一致性还是得到保障的。
     *
     * @return
     */
    double distanceFromOrigin() throws InterruptedException {
        long stamp = sl.tryOptimisticRead();    // 使用乐观读锁
        log.info("tryOptimisticRead lock success");
        // 睡一秒中
        Thread.sleep(1000);
        double currentX = x, currentY = y;      // 拷贝共享资源到本地方法栈中
        if (!sl.validate(stamp)) {              // 如果有写锁被占用，可能造成数据不一致，所以要切换到普通读锁模式
            log.info("validate stamp error");
            stamp = sl.readLock();
            log.info("readLock success");
            try {
                currentX = x;
                currentY = y;
            } finally {
                sl.unlockRead(stamp);
                log.info("unlock read success");
            }
        }
        return Math.sqrt(currentX * currentX   currentY * currentY);
    }

    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);  //读锁转换为写锁
                if (ws != 0L) {
                    stamp = ws;
                    x = newX;
                    y = newY;
                    break;
                } else {
                    sl.unlockRead(stamp);
                    stamp = sl.writeLock();
                }
            }
        } finally {
            sl.unlock(stamp);
        }
    }
}

@Test
public void testStamped() throws InterruptedException {
    Point point = new Point();
    point.setX(1);
    point.setY(2);
    // 线程0 执行了乐观读
    Thread thread0 = new Thread(() -> {
        try {
            // 乐观读
            point.distanceFromOrigin();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }, "thread-0");
    thread0.start();

    Thread.sleep(500);
    // 线程1 执行写锁
    Thread thread1 = new Thread(() -> {
        // 乐观读
        try {
            point.move(3, 4);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }, "thread-1");
    thread1.start();

    thread0.join();
    thread1.join();
}