原子类的 ABA 问题

原子引用

public class AtomicReferenceDemo {
    public static void main(String[] args) {
        User cuzz = new User("cuzz", 18);
        User faker = new User("faker", 20);
        AtomicReference<User> atomicReference = new AtomicReference<>();
        atomicReference.set(cuzz);
        System.out.println(atomicReference.compareAndSet(cuzz, faker)); // true
        System.out.println(atomicReference.get()); // User(userName=faker, age=20)
    }
}

ABA问题的产生

/**
 * 当有一个值从 A 改为 B 又改为 A，这就是 ABA 问题
 **/
public class ABADemo {
    private static AtomicReference<Integer> atomicReference = new AtomicReference<>(100);

    public static void main(String[] args) {
        new Thread(() -> {
            atomicReference.compareAndSet(100, 101);
            atomicReference.compareAndSet(101, 100);
        }).start();

        new Thread(() -> {
            // 保证上面线程先执行
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            atomicReference.compareAndSet(100, 2019);
            System.out.println(atomicReference.get()); // 2019
        }).start();
    }
}

时间戳原子引用

/**
 * 我们先保证两个线程的初始版本为一致，后面修改是由于版本不一样就会修改失败
 **/
public class ABADemo2 {
    private static AtomicStampedReference<Integer> atomicStampedReference = new AtomicStampedReference<>(100, 1);

    public static void main(String[] args) {
        new Thread(() -> {
            int stamp = atomicStampedReference.getStamp();
            System.out.println(Thread.currentThread().getName() + " 的版本号为：" + stamp);
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            atomicStampedReference.compareAndSet(100, 101, atomicStampedReference.getStamp(), atomicStampedReference.getStamp() + 1 );
            atomicStampedReference.compareAndSet(101, 100, atomicStampedReference.getStamp(), atomicStampedReference.getStamp() + 1 );
        }).start();

        new Thread(() -> {
            int stamp = atomicStampedReference.getStamp();
            System.out.println(Thread.currentThread().getName() + " 的版本号为：" + stamp);
            try {
                Thread.sleep(3000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            boolean b = atomicStampedReference.compareAndSet(100, 2019, stamp, stamp + 1);
            System.out.println(b); // false
            System.out.println(atomicStampedReference.getReference()); // 100
        }).start();
    }
}