AtomicReference、AtomicStampedReference、AtomicMarkableReference

这三个都是自JDK1.5开始加入到java.util.concurrent.atomic下面的。他们都可以在lock-free的情况下以原子的方式更新对象引用。

区别在于：AtomicStampedReference内部维护了一个[reference, integer] pairs的二元组。AtomicMarkableReference 内部维护了一个[reference, boolean] pairs的二元组。

以下部分源码片段摘自JDK1.8（保留了注释）:

• AtomicReference

public class AtomicReference<V> implements java.io.Serializable {
    private static final long serialVersionUID = -1848883965231344442L;

    private static final Unsafe unsafe = Unsafe.getUnsafe();
    private static final long valueOffset;

    //class初始化时执行
    static {
        try {
            valueOffset = unsafe.objectFieldOffset
                (AtomicReference.class.getDeclaredField("value"));
        } catch (Exception ex) { throw new Error(ex); }
    }

    //通过volatile关键字保证value值的可见性。
    private volatile V value;
    
    public AtomicReference(V initialValue) {
        value = initialValue;
    }
    
    public AtomicReference() {
    }
    
    //CAS操作
    /**
     * Atomically sets the value to the given updated value
     * if the current value {@code ==} the expected value.
     * @param expect the expected value
     * @param update the new value
     * @return {@code true} if successful. False return indicates that
     * the actual value was not equal to the expected value.
     */
    public final boolean compareAndSet(V expect, V update) {
        return unsafe.compareAndSwapObject(this, valueOffset, expect, update);
    }
}

• AtomicStampedReference

public class AtomicStampedReference<V> {
    //内部维护一个静态内部类
    //reference表示引用对象
    //stamp表示时间戳（版本号），int型
    private static class Pair<T> {
        final T reference;
        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);
        }
    }

    private volatile Pair<V> pair;

    /**
     * Creates a new {@code AtomicStampedReference} with the given
     * initial values.
     *
     * @param initialRef the initial reference
     * @param initialStamp the initial stamp
     */
    public AtomicStampedReference(V initialRef, int initialStamp) {
        pair = Pair.of(initialRef, initialStamp);
    }
    
    /**
     * Atomically sets the value of both the reference and stamp
     * to the given update values if the
     * current reference is {@code ==} to the expected reference
     * and the current stamp is equal to the expected stamp.
     *
     * @param expectedReference the expected value of the reference
     * @param newReference the new value for the reference
     * @param expectedStamp the expected value of the stamp
     * @param newStamp the new value for the stamp
     * @return {@code true} if successful
     */
    public boolean compareAndSet(V   expectedReference,
                                 V   newReference,
                                 int expectedStamp,
                                 int newStamp) {
        Pair<V> current = pair;
        //通过增加了stamp（版本号）的CAS操作，可以避免ABA问题，即更新始终是递增的，不会出现往复。
        return
            expectedReference == current.reference &&
            expectedStamp == current.stamp &&
            ((newReference == current.reference &&
              newStamp == current.stamp) ||
             casPair(current, Pair.of(newReference, newStamp)));
    }
    
    private boolean casPair(Pair<V> cmp, Pair<V> val) {
        return UNSAFE.compareAndSwapObject(this, pairOffset, cmp, val);
    }
}

• AtomicMarkableReference

public class AtomicMarkableReference<V> {
    //这里几乎和AtomicStampedReference相同，只是从 final int stamp 变成了 final boolean mark
    private static class Pair<T> {
        final T reference;
        final boolean mark;
        private Pair(T reference, boolean mark) {
            this.reference = reference;
            this.mark = mark;
        }
        static <T> Pair<T> of(T reference, boolean mark) {
            return new Pair<T>(reference, mark);
        }
    }

    private volatile Pair<V> pair;

    /**
     * Creates a new {@code AtomicMarkableReference} with the given
     * initial values.
     *
     * @param initialRef the initial reference
     * @param initialMark the initial mark
     */
    public AtomicMarkableReference(V initialRef, boolean initialMark) {
        pair = Pair.of(initialRef, initialMark);
    }
}