CAS你知道吗？原子类AtomicInteger的ABA问题谈谈？

（1）CAS是什么？

 比较并交换

举例1,  CAS产生场景代码？

import java.util.concurrent.atomic.AtomicInteger;

public class CASDemo {
    public static void main(String[] args) {
        AtomicInteger atomicInteger=new AtomicInteger(5);//主物理内存
        //比较5
        System.out.println(atomicInteger.compareAndSet(5, 2019)+"\t current data（主物理内存） : "+ atomicInteger.get());
        //修改失败
        System.out.println(atomicInteger.compareAndSet(5, 1024)+"\t current data（主物理内存） : "+ atomicInteger.get());
    }
}

举例2,  CAS产生场景代码？原子引用

public class CAS_ABADemo {
    public static void main(String[] args) {

        //========================原子引用==========================
        User A = new User("A", 123);
        User B = new User("B", 456);
        //主物理内存 引用比较的是地址
        AtomicReference<User> atomicReference1 = new AtomicReference<>();
        atomicReference1.set(A);
        System.out.println(atomicReference1.compareAndSet(A, B) + "\t" + atomicReference1.get().toString());
        System.out.println(atomicReference1.compareAndSet(A, B) + "\t" + atomicReference1.get().toString());
    }
}
class User {
    String name;
    int age;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    public User() {
    }

    public User(String name, int age) {
        this.name = name;
        this.age = age;
    }

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

原子引用

参考博客：CopyOnWriteArrayList  写时复制----------https://www.cnblogs.com/huangjuncong/p/9160713.html

   参考博客：ReentrantLock 独占锁（可重入锁）----------https://www.cnblogs.com/takumicx/p/9338983.html

举例3，ABA问题的产生和解决方案代码场景。

public class ABADemo {
    static AtomicReference<Integer> atomicReference = new AtomicReference<>(100);
    static AtomicStampedReference<Integer> atomicStampedReference = new AtomicStampedReference<>(100, 1);
    public static void main(String[] args) {
        System.out.println("======ABA问题的产生======");

        new Thread(() -> {
            atomicReference.compareAndSet(100, 101);
            atomicReference.compareAndSet(101, 100);
        }, "t1").start();

        new Thread(() -> {
            try {
                TimeUnit.SECONDS.sleep(1);
            } catch (InterruptedException e) {
                e.printStackTrace();

            }
            System.out.println(atomicReference.compareAndSet(100, 2019) + "\t" + atomicReference.get().toString());
        }, "t2").start();

        try { TimeUnit.SECONDS.sleep(2); } catch (InterruptedException e) { e.printStackTrace(); }

        System.out.println("======ABA问题的解决======");
        new Thread(() -> {
            int stamp = atomicStampedReference.getStamp();
            System.out.println(Thread.currentThread().getName() + "\t第一次版本号： " + stamp);
            try { TimeUnit.SECONDS.sleep(1); } catch (InterruptedException e) { e.printStackTrace(); }
            atomicStampedReference.compareAndSet(100,101,
                    atomicStampedReference.getStamp(),atomicStampedReference.getStamp()+1);
            System.out.println(Thread.currentThread().getName() + "\t第二次版本号： " + atomicStampedReference.getStamp());
            atomicStampedReference.compareAndSet(101,100,
                    atomicStampedReference.getStamp(),atomicStampedReference.getStamp()+1);
            System.out.println(Thread.currentThread().getName() + "\t第三次版本号： " + atomicStampedReference.getStamp());
        }, "t3").start();

        new Thread(() -> {
            int stamp = atomicStampedReference.getStamp();
            System.out.println(Thread.currentThread().getName() + "\t第一次版本号： " + stamp);
            try { TimeUnit.SECONDS.sleep(3); } catch (InterruptedException e) { e.printStackTrace(); }
            boolean result=atomicStampedReference.compareAndSet(100,2019,
                    stamp,stamp+1);
            System.out.println(Thread.currentThread().getName()+"\t修改成功与否："+result+"  当前最新版本号"+atomicStampedReference.getStamp());
            System.out.println(Thread.currentThread().getName()+"\t当前实际值："+atomicStampedReference.getReference());
        }, "t4").start();
    }
}

ABA问题的产生和解决方案

（3）CAS实现原子操作的三大问题。

CAS虽然采用自旋的方式高效的解决了原子操作，但任然存在三个问题.

1. ABA问题。
2. 循环时间长开销大。
3. 只能保证一个共享变量的原子操作。