CAS引起的ABA問題以及解決

用AtomicInteger舉例說明：

public class TestATomicABA {
    private static AtomicInteger atomicInteger = new AtomicInteger(0);
    public static void main(String[] args) {
        new Thread(() -> {
            boolean b = atomicInteger.compareAndSet(0, 2009);
            System.out.println("****" + b + "\t" + atomicInteger.get());
            boolean b1 = atomicInteger.compareAndSet(2009, 0);
            System.out.println("####" + b1 + "\t" + atomicInteger.get());
        }, "t1").start();

        new Thread(() -> {
            //保證t1線程發生一次ABA問題
            try {
                TimeUnit.SECONDS.sleep(1);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            boolean b = atomicInteger.compareAndSet(0, 1);
            System.out.println("&&&&" + b + "\t" + atomicInteger.get());
        }, "t2").start();
    }
}

打印出來的結果如下：

****true	2009
####true	0
&&&&true	1

說明產生了ABA問題。

解決方式如下：

public class TestATomicABA {
    private static AtomicStampedReference<Integer> stampedReference = new AtomicStampedReference<>(100, 1);

    public static void main(String[] args) {
        new Thread(() -> {
            //確保線程t2拿到了初始版本號
            try {
                TimeUnit.SECONDS.sleep(1);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            int stamp = stampedReference.getStamp();
            System.out.println("第一次獲取版本號：" + stamp);
            boolean b = stampedReference.compareAndSet(100, 101, stamp, stamp + 1);
            System.out.println("第一次更新值：" + b + "\t value=" + stampedReference.getReference());
            int stamp1 = stampedReference.getStamp();
            System.out.println("第二次獲取版本號：" + stamp1);
            boolean b1 = stampedReference.compareAndSet(101, 100, stamp1, stamp1 + 1);
            System.out.println("第一次更新值：" + b1 + "\t value=" + stampedReference.getReference());

            System.out.println("發生了一次ABA問題");
        }, "t1").start();


        new Thread(() -> {
            int stamp = stampedReference.getStamp();
            //模擬t2線程響應時間較長，t1執行了一次ABA
            try {
                TimeUnit.SECONDS.sleep(3);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            System.out.println(Thread.currentThread().getName() + "\t" + stamp);
            boolean b = stampedReference.compareAndSet(100, 102, stamp, stamp + 1);
            System.out.println(Thread.currentThread().getName() + "\t" + b + "\t stampedReference=" + stampedReference.getReference());

        }, "t2").start();
    }

}

打印結果如下：

第一次獲取版本號：1
第一次更新值：true	 value=101
第二次獲取版本號：2
第一次更新值：true	 value=100
發生了一次ABA問題
t2	1
t2	false	 stampedReference=100

可以看到t2線程執行失敗了，返回了false。

補充一個小知識點，原子引用更新，這個類 類似於原子整形，它是針對自定義的一些類對象需要做原子更新操作的情況。

public class Person {
    private String name;
    private Integer age;

    public Person(String name, Integer age) {
        this.name = name;
        this.age = age;
    }

    @Override
    public String toString() {
        return "Person{" +
                "name='" + name + '\'' +
                ", age=" + age +
                '}';
    }
}

public class TestAtomicReference {
    private static AtomicReference<Person> atomicReference = new AtomicReference<>();

    public static void main(String[] args) {
        Person zhangsan = new Person("zhangsan", 22);
        atomicReference.set(zhangsan);
        boolean b = atomicReference.compareAndSet(zhangsan, new Person("lisi", 44));

        System.out.println("b=" + b + "\t" + atomicReference.get());
    }
}

打印結果如下：

b=true	Person{name='lisi', age=44}