CAS你知道吗？原子类AtomicInteger的ABA问题谈谈？，原子共面问题(1)CAS是什么？

比较并交换

举例1,  CAS产生场景代码？

importjava.util.concurrent.atomic.AtomicInteger;public classCASDemo {public static voidmain(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 classCAS_ABADemo {public static voidmain(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 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());

}

}classUser {

String name;intage;publicString getName() {returnname;

}public voidsetName(String name) {this.name =name;

}public intgetAge() {returnage;

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

}publicUser() {

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

}

@OverridepublicString 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 classABADemo {static AtomicReference atomicReference = new AtomicReference<>(100);static AtomicStampedReference atomicStampedReference = new AtomicStampedReference<>(100, 1);public static voidmain(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.

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