Java底层类和源码分析系列-AtomicStampedReference解决ABA问题

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在原子类持续累加或累减时，比如AtomicInteger的incrementAndGet时，是不存在ABA问题的，但compareAndSet或者updateAndGet是可能存在ABA问题，像AtomicBoolean或AtomicLong等这样的变量在多线程修改时，也都存在ABA的问题。
为了理解ABA，下面的例子，针对线程1来说，第一次的A也就是1和第二次的A是另外修改过的1，实际上并不是同一个A（1）。

public static void main(String[] args) {AtomicInteger atomicInteger = new AtomicInteger(1);new Thread(()->{int value = atomicInteger.get();System.out.println("thread 1 read value: " + value);// 阻塞1sLockSupport.parkNanos(1000000000L);if (atomicInteger.compareAndSet(value, 3)) {System.out.println("thread 1 update from " + value + " to 3");} else {System.out.println("thread 1 update fail!");}}).start();new Thread(()->{int value = atomicInteger.get();System.out.println("thread 2 read value: " + value);if (atomicInteger.compareAndSet(value, 2)) {System.out.println("thread 2 update from " + value + " to 2");// do sthvalue = atomicInteger.get();System.out.println("thread 2 read value: " + value);if (atomicInteger.compareAndSet(value, 1)) {System.out.println("thread 2 update from " + value + " to 1");}}}).start();}

为了解决这个问题，引入了AtomicStampedReference。

内部类

    private static class Pair<T> {/*** 目标对象引用*/final T reference;/*** 整形标记*/final int stamp;private Pair(T reference, int stamp) {this.reference = reference;this.stamp = stamp;}static <T> Pair<T> of(T reference, int stamp) {return new Pair<T>(reference, stamp);}}

属性

private volatile Pair<V> pair;
private static final sun.misc.Unsafe UNSAFE = sun.misc.Unsafe.getUnsafe();
private static final long pairOffset =objectFieldOffset(UNSAFE, "pair", AtomicStampedReference.class);

构造方法

       public AtomicStampedReference(V initialRef, int initialStamp) {pair = Pair.of(initialRef, initialStamp);}

compareAndSet()方法

public boolean compareAndSet(V   expectedReference,V   newReference,int expectedStamp,int newStamp) {// 获取当前的（元素值，版本号）对Pair<V> current = pair;return// 引用没变expectedReference == current.reference &&// 版本号没变expectedStamp == current.stamp &&// 新引用等于旧引用((newReference == current.reference &&// 新版本号等于旧版本号newStamp == current.stamp) ||// 构造新的Pair对象并CAS更新
         casPair(current, Pair.of(newReference, newStamp)));
}
private boolean casPair(Pair<V> cmp, Pair<V> val) {// 调用Unsafe的compareAndSwapObject()方法CAS更新pair的引用为新引用return UNSAFE.compareAndSwapObject(this, pairOffset, cmp, val);
}

解决上个ABA的问题的版本：

private static void testStamp() {AtomicStampedReference<Integer> atomicStampedReference = new AtomicStampedReference<>(1, 1);new Thread(()->{int[] stampHolder = new int[1];int value = atomicStampedReference.get(stampHolder);int stamp = stampHolder[0];System.out.println("thread 1 read value: " + value + ", stamp: " + stamp);// 阻塞1sLockSupport.parkNanos(1000000000L);if (atomicStampedReference.compareAndSet(value, 3, stamp, stamp + 1)) {System.out.println("thread 1 update from " + value + " to 3");} else {System.out.println("thread 1 update fail!");}}).start();new Thread(()->{int[] stampHolder = new int[1];int value = atomicStampedReference.get(stampHolder);int stamp = stampHolder[0];System.out.println("thread 2 read value: " + value + ", stamp: " + stamp);if (atomicStampedReference.compareAndSet(value, 2, stamp, stamp + 1)) {System.out.println("thread 2 update from " + value + " to 2");// do sthvalue = atomicStampedReference.get(stampHolder);stamp = stampHolder[0];System.out.println("thread 2 read value: " + value + ", stamp: " + stamp);if (atomicStampedReference.compareAndSet(value, 1, stamp, stamp + 1)) {System.out.println("thread 2 update from " + value + " to 1");}}}).start();}