MPSCLinkedQueue的两种实现

public class MpscLinkedQueue7<E> extends MpscLinkedQueue<E> {
    @SuppressWarnings("unchecked")
    protected final LinkedQueueNode<E> xchgProducerNode(LinkedQueueNode<E> newVal) {
        Object oldVal;
        do {
            oldVal = producerNode;
        } while(!UNSAFE.compareAndSwapObject(this, P_NODE_OFFSET, oldVal, newVal));
        return (LinkedQueueNode<E>) oldVal;
    }
}

public class MpscLinkedQueue8<E> extends MpscLinkedQueue<E> {
    @SuppressWarnings("unchecked")
    protected final LinkedQueueNode<E> xchgProducerNode(LinkedQueueNode<E> newVal) {
        return (LinkedQueueNode<E>) UNSAFE.getAndSetObject(this, P_NODE_OFFSET, newVal);
    }
}

public final Object getAndSetObject(Object o, long offset, Object newValue) {
            Object v;
            do {
                v = getObjectVolatile(o, offset);
            } while (!compareAndSwapObject(o, offset, v, newValue));
            return v;
        }

public abstract class MpscLinkedQueue<E> extends BaseLinkedQueue<E> {

    /**
     * Construct the implementation based on availability of getAndSet intrinsic.
     *
     * @return the right queue for you!
     */
    public static <E> MpscLinkedQueue<E> newMpscLinkedQueue() {
        if (UnsafeAccess.SUPPORTS_GET_AND_SET) {
            return new MpscLinkedQueue8<E>();
        }
        else {
            return new MpscLinkedQueue7<E>();
        }
    }
    protected MpscLinkedQueue() {
        consumerNode = new LinkedQueueNode<E>();
        xchgProducerNode(consumerNode);// this ensures correct construction: StoreLoad
    }

    protected abstract LinkedQueueNode<E> xchgProducerNode(LinkedQueueNode<E> nextNode);

public class UnsafeAccess {
    public static final boolean SUPPORTS_GET_AND_SET;
    public static final Unsafe UNSAFE;
    static {
        Unsafe instance;
        try {
            final Field field = Unsafe.class.getDeclaredField("theUnsafe");
            field.setAccessible(true);
            instance = (Unsafe) field.get(null);
        } catch (Exception ignored) {
            // Some platforms, notably Android, might not have a sun.misc.Unsafe
            // implementation with a private `theUnsafe` static instance. In this
            // case we can try and call the default constructor, which proves
            // sufficient for Android usage.
            try {
                Constructor<Unsafe> c = Unsafe.class.getDeclaredConstructor();
                c.setAccessible(true);
                instance = c.newInstance();
            } catch (Exception e) {
                SUPPORTS_GET_AND_SET = false;
                throw new RuntimeException(e);
            }
        }

        boolean getAndSetSupport = false;
        try {
            Unsafe.class.getMethod("getAndSetObject", Object.class, Long.TYPE,Object.class);
            getAndSetSupport = true;
        } catch (Exception ignored) {
        }

        UNSAFE = instance;
        SUPPORTS_GET_AND_SET = getAndSetSupport;
    }
}