AtomicStampedReference

Pair

获取当前reference值和版本号stamp

CAS操作

unsafe

完整案例

AtomicMarkableReference

AtomicStampedReference和AtomicMarkableReference是解决CAS存在ABA问题的两种方案，他们俩的实现原理大致相同，弄明白AtomicStampedReference的实现原理，AtomicMarkableReference就迎刃而解了。

AtomicStampedReference
AtomicStampedReference是JUC的atomic包里提供的一个类，官方解释该类的作用是:

An AtomicStampedReference maintains an object reference along with an integer "stamp", that can be updated atomically.
Implementation note: This implementation maintains stamped references by creating internal objects representing "boxed" [reference, integer] pairs.

意思是: AtomicStampedReference维护了一个reference对象和一个int类型的stamp, 能提供原子性更新的功能。

具体的原理: 每次CAS前，先获取当AtomicStampedReference对象里的Pair里的stamp和reference, 该stamp为版本号,reference为期望的对象值，如果获取到的版本号为期望的版本号并且获取到的reference值为期望值，就是说当前线程获取到的版本号在CAS前没有被其他线程修改且reference也没有修改，那么当前线程才能进行CAS操作，如果版本号不是期望值，那么就不执行更新操作。

Pair

Pair是AtomicStampedReference和AtomicMarkableReference类里维护的一个静态内部类。

   private static class Pair {
       // 存放的对象，采用泛型
        final T reference;
      // stamp类似于版本号的功能
        final int stamp;
        private Pair(T reference, int stamp) {
            this.reference = reference;
            this.stamp = stamp;
        }
       //提供初始化Pair对象的方法
        static  Pair of(T reference, int stamp) {
            return new Pair(reference, stamp);
        }
    }

AtomicStampedReference类和AtomicMarkableReference类里都含有一个volatile修饰的Pair成员变量，它的作用是用来存放当前的Pair值。

    private volatile Pair pair;

因为CAS操作时，我们需要用expected值与当前值做比较。

获取当前reference值和版本号stamp

给一个数组容量为1的数组给get()方法作为参数，版本号会存放到数组里，返回值为当前的reference值

int[] stampHolder = new int[1];
// 1. 给一个空数组给atomicStampedReference
int value = (int) atomicStampedReference.get(stampHolder);

就是把当前的pair值给拿出来，然后取出stamp赋值给数组并将reference进行返回即可。

  public V get(int[] stampHolder) {
     // 获取当前pair对象
        Pair pair = this.pair;
      // 赋值
        stampHolder[0] = pair.stamp;
        return pair.reference;
    }

CAS操作

CAS操作有4个参数:

   public boolean compareAndSet(V   expectedReference,
                                 V   newReference,
                                 int expectedStamp,
                                 int newStamp) {
        Pair current = pair;
        return
           // 比较期望值
            expectedReference == current.reference &&
            expectedStamp == current.stamp &&
         // 如果当前值与新值相等并且版本号一致，那么直接return, 不执行cas操作
            ((newReference == current.reference &&
              newStamp == current.stamp) ||
          // 执行CAS操作
             casPair(current, Pair.of(newReference, newStamp)));
    }

casPair方法的底层就是调用的unsafe提供的compareAndSwapObject方法。

    private boolean casPair(Pair cmp, Pair val) {
        return UNSAFE.compareAndSwapObject(this, pairOffset, cmp, val);
    }

unsafe

第一步获取到unsafe就能执行CAS操作，获取到unsafe变量

    private static final sun.misc.Unsafe UNSAFE = sun.misc.Unsafe.getUnsafe();

第二步获取到共享变量pair的偏移量

 private static final long pairOffset =
        objectFieldOffset(UNSAFE, "pair", AtomicStampedReference.class);

根据UNSAFE对象和字段field获取到字段在对象里的偏移量, 偏移量可以用sun.misc.Unsafe类里提供一个native方法objectFieldOffset(Field field)方法来获取。

 static long objectFieldOffset(sun.misc.Unsafe UNSAFE,
                                  String field, Class klazz) {
        try {
            return UNSAFE.objectFieldOffset(klazz.getDeclaredField(field));
        } catch (NoSuchFieldException e) {
            // Convert Exception to corresponding Error
            NoSuchFieldError error = new NoSuchFieldError(field);
            error.initCause(e);
            throw error;
        }
    }

完整案例

用AtomicStampedReference初始化当前reference值为1，版本号为1，定义2个线程，线程1的目标是将reference值更新为3，线程2的目标是将reference值更新为2，那么如果2个线程同时更新，那么必定有其中1个线程更新失败，因为另外一个线程更改了版本号和reference值。

package com.example.jucdemo.atomic;

import java.util.concurrent.atomic.AtomicStampedReference;

import lombok.extern.slf4j.Slf4j;


@Slf4j
public class AtomicStampedReferenceTest {

    public static void main(String[] args) {
        // 定义AtomicStampedReference, 版本号为1, 为了方便对象的值为1,Pair.reference值为1, Pair.stamp为1
        AtomicStampedReference atomicStampedReference = new AtomicStampedReference(1,1);

        new Thread(()->{
            int[] stampHolder = new int[1];
            // 1. 给一个空数组给atomicStampedReference
            int reference = (int) atomicStampedReference.get(stampHolder);
            int stamp = stampHolder[0];
            // 2. 获取到当前版本号，值为stampHolder[0]
            log.debug("Thread1 read value: " + reference + ", stamp: " + stamp);
            // 阻塞1s
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            // Thread1通过CAS操作修改reference值为3，每次修改可以通过stamp+1保证版本唯一性
            if (atomicStampedReference.compareAndSet(reference, 3,stamp,stamp+1)) {
                log.debug("Thread1 update from " + reference + " to 3");
                // do something
            } else {
                log.debug("Thread1 update fail!");
            }
        },"Thread1").start();

        new Thread(()->{
            int[] stampHolder = new int[1];
            int reference = (int)atomicStampedReference.get(stampHolder);
            int stamp = stampHolder[0];
            log.debug("Thread2 read value: " + reference+ ", stamp: " + stamp);
            // Thread2通过CAS修改reference值为2
            if (atomicStampedReference.compareAndSet(reference, 2,stamp,stamp+1)) {
                log.debug("Thread2 update from " + reference + " to 2");
                // do something
                reference = (int) atomicStampedReference.get(stampHolder);
                stamp = stampHolder[0];
                log.debug("Thread2 read value: " + reference+ ", stamp: " + stamp);
                // Thread2通过CAS修改value值为1
                if (atomicStampedReference.compareAndSet(reference, 1,stamp,stamp+1)) {
                    log.debug("Thread2 update from " + reference + " to 1");
                }
            }
        },"Thread2").start();
    }
}

打印结果:

22:58:56.548 [Thread1] DEBUG com.example.jucdemo.atomic.AtomicStampedReferenceTest - Thread1 read value: 1, stamp: 1
22:58:56.548 [Thread2] DEBUG com.example.jucdemo.atomic.AtomicStampedReferenceTest - Thread2 read value: 1, stamp: 1
22:58:56.550 [Thread2] DEBUG com.example.jucdemo.atomic.AtomicStampedReferenceTest - Thread2 update from 1 to 2
22:58:56.550 [Thread2] DEBUG com.example.jucdemo.atomic.AtomicStampedReferenceTest - Thread2 read value: 2, stamp: 2
22:58:56.550 [Thread2] DEBUG com.example.jucdemo.atomic.AtomicStampedReferenceTest - Thread2 update from 2 to 1
22:58:57.551 [Thread1] DEBUG com.example.jucdemo.atomic.AtomicStampedReferenceTest - Thread1 update fail!

Process finished with exit code 0

由打印结果可知，线程1更新失败，因为线程1在执行CAS前有个sleep操作，那么线程2会先去更新reference的值为2，版本号为2，当线程1重新唤醒执行的时候，会发现版本号发生变化了，那么CAS操作就不会进行了！

AtomicMarkableReference

AtomicMarkableReference与AtomicStampedReference的区别是Pair内部类维护的类型不同。

    private static class Pair {
        final T reference;
        final boolean mark;
        private Pair(T reference, boolean mark) {
            this.reference = reference;
            this.mark = mark;
        }
        static  Pair of(T reference, boolean mark) {
            return new Pair(reference, mark);
        }
    }

采用boolean类型的值替换掉了int stamp, 其他功能的实现方式参考AtomicStampedReference即可。

AtomicStampedReference和AtomicMarkableReference详解

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