CAS机制与自旋锁

 1 public class CASDemo {
 2 
 3     public static void main(String[] args) {
 4 
 5         //初始值5
 6         AtomicInteger atomicInteger = new AtomicInteger(5);
 7 
 8         //和5比较,设置为10
 9         System.out.println("预期值:5,当前值:"+atomicInteger);
10         System.out.println("是否设置成功:"+atomicInteger.compareAndSet(5, 10));
11         //和5比较,设置为15
12         System.out.println("预期值:5,当前值:"+atomicInteger);
13         System.out.println("是否设置成功:"+atomicInteger.compareAndSet(5, 15));
14 
15         System.out.println("当前值:"+atomicInteger);
16     }
17 }

预期值:5,当前值:5
是否设置成功:true
预期值:5,当前值:10
是否设置成功:false
当前值:10

//va1为对象，var2为地址值，var4是要增加的值，var5为当前地址中最新的值
public final int getAndAddInt(Object var1, long var2, int var4) {
        int var5;
        do {
            var5 = this.getIntVolatile(var1, var2);
        } while(!this.compareAndSwapInt(var1, var2, var5, var5 + var4));

        return var5;
    }

 0 // ABA问题及解决方式
 1　public class ABADemo {
 2 
 3     private static AtomicReference<String> atomicReference = new AtomicReference<>("A");
 4     private static AtomicStampedReference<String> stampReference = new AtomicStampedReference<>("A",1);
 5 
 6     public static void main(String[] args){
 7         new Thread(()->{
 8             //获取到版本号
 9             int stamp = stampReference.getStamp();
10             System.out.println("t1获取到的版本号："+stamp);
11             try {
12                 //暂停1秒,确保t1,t2版本号相同
13                 TimeUnit.SECONDS.sleep(1);
14             } catch (InterruptedException e) {
15                 e.printStackTrace();
16             }
17             atomicReference.compareAndSet("A","B");
18             atomicReference.compareAndSet("B","A");
19 
20             stampReference.compareAndSet("A","B",stamp,stamp+1);
21             stampReference.compareAndSet("B","A",stamp+1,stamp+2);
22             System.out.println("t1线程ABA之后的版本号："+stampReference.getStamp());
23 
24         },"t1").start();
25 
26         new Thread(()->{
27             //获取到版本号
28             int stamp = stampReference.getStamp();
29             System.out.println("t2获取到的版本号："+stamp);
30             try {
31                 //暂停2秒,等待t1执行完成ABA
32                 TimeUnit.SECONDS.sleep(2);
33             } catch (InterruptedException e) {
34                 e.printStackTrace();
35             }
36             System.out.print("普通原子类无法解决ABA问题： ");
37             System.out.println(atomicReference.compareAndSet("A","C")+"\t"+atomicReference.get());
38             System.out.print("版本号的原子类解决ABA问题： ");
39             System.out.println(stampReference.compareAndSet("A","C",stamp,stamp+1)+"\t"+stampReference.getReference());
40 
41         },"t2").start();
42     }
43 }

t1获取到的版本号：1
t2获取到的版本号：1
t1线程ABA之后的版本号：3
普通原子类无法解决ABA问题： true    C
版本号的原子类解决ABA问题： false    A