StampedLock §

StampedLock是比ReadWriteLock更宽松的一种锁。ReadWriteLock是悲观锁，不允许在有线程写入时读取。StampedLock是乐观锁，认为在大多数情况下，写入时不会有读取操作，允许上写锁时也允许读取行为，通过额外的代码判断读取数据的正确性。

public class Point {
    private final StampedLock stampedLock = new StampedLock();
 
    private double x;
    private double y;
 
    public void move(double deltaX, double deltaY) {
        long stamp = stampedLock.writeLock(); // 获取写锁
        try {
            x += deltaX;
            y += deltaY;
        } finally {
            stampedLock.unlockWrite(stamp); // 释放写锁
        }
    }
 
    public double distanceFromOrigin() {
        long stamp = stampedLock.tryOptimisticRead(); // 获得一个乐观读锁
        // 注意下面两行代码不是原子操作
        // 假设x,y = (100,200)
        double currentX = x;
        // 此处已读取到x=100，但x,y可能被写线程修改为(300,400)
        double currentY = y;
        // 此处已读取到y，如果没有写入，读取是正确的(100,200)
        // 如果有写入，读取是错误的(100,400)
        if (!stampedLock.validate(stamp)) { // 检查乐观读锁后是否有其他写锁发生
            stamp = stampedLock.readLock(); // 获取一个悲观读锁
            try {
                currentX = x;
                currentY = y;
            } finally {
                stampedLock.unlockRead(stamp); // 释放悲观读锁
            }
        }
        return Math.sqrt(currentX * currentX + currentY * currentY);
    }
}

StampedLock是不可重入锁