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？
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
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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。
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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.
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