究竟是UNSAFE.compareAndSwapObject的作用

Question

我在阅读JDK ConcurrentLinkedQueue时发现UNSAFE.compareAndSwapObject非常奇怪.(CLQ类是来自ConcurrentLinkedQueue的副本,以便于调试......)

当我向ConcurrentLinkedQueue提供第一项时.

• 在 p.casNext(null, newNode) head == tail == p == t ref之前的同一个对象,就像这样.

  http://i.stack.imgur.com/8dCO0.png

• 之后进入casNext UNSAFE.compareAndSwapObject(this, nextOffset, cmp, val);

  p.next按预期引用newNode,就像这样.

  http://i.stack.imgur.com/mNtUN.png

• 走出去提供

  一切都变得奇怪......我可以理解为什么p.next ref伤口改为p,如何自动引导newNode ...

  http://i.stack.imgur.com/Pvvcy.png

代码:ConcurrentLinkedQueue.class offer()

public boolean offer(E e) {
    checkNotNull(e);
    final Node<E> newNode = new Node<E>(e);

    for (Node<E> t = tail, p = t;;) {
        Node<E> q = p.next;
        if (q == null) {
            // p is last node
            if (p.casNext(null, newNode)) {
                // Successful CAS is the linearization point
                // for e to become an element of this queue,
                // and for newNode to become "live".
                if (p != t) // hop two nodes at a time
                    casTail(t, newNode);  // Failure is OK.
                return true;
            }
            // Lost CAS race to another thread; re-read next
        }
        else if (p == q)
            // We have fallen off list.  If tail is unchanged, it
            // will also be off-list, in which case we need to
            // jump to head, from which all live nodes are always
            // reachable.  Else the new tail is a better bet.
            p = (t != (t = tail)) ? t : head;
        else
            // Check for tail updates after two hops.
            p = (p != t && t != (t = tail)) ? t : q;
    }
}

Answer 1

它的操作可能类似于下面的伪代码，但用所谓的本机代码 (C/C++) 编写，通常比 Java 快得多：

boolean compareAndSwapObject(Object obj, long fieldId, Object expectedValue, Object newValue) {
    synchronized (globalExchangeLock) {
        if (obj.fieldArray.get(fieldId) == expectedValue) {
            obj.fieldArray.set(fieldId, newValue);
            return true;
        }
    }
    return false;
}

private static final Object globalExchangeLock = new Object();

然而，传递fieldId并不复杂，也许应该被缓存，就像下面的 Java 代码（这次不是伪代码）：

boolean compareAndSwapObject(Object obj, long fieldId, Object expectedValue, Object newValue) {
    synchronized (globalExchangeLock) {
        if (obj.fieldArray.get(fieldId) == expectedValue) {
            obj.fieldArray.set(fieldId, newValue);
            return true;
        }
    }
    return false;
}

private static final Object globalExchangeLock = new Object();

我的 Android 项目使用的是：

/*
 * Written by Stefan Zobel and released to the
 * public domain, as explained at
 * http://creativecommons.org/publicdomain/zero/1.0/
 */
package java8.util.concurrent;

import java.lang.reflect.Field;

import sun.misc.Unsafe;

final class UnsafeAcc {

    static final Unsafe unsafe;

    static {
        try {
            Field field = null;
            try {
                field = Unsafe.class.getDeclaredField("theUnsafe");
            } catch (NoSuchFieldException ignored) {
                // For older Androids.
                field = Unsafe.class.getDeclaredField("THE_ONE");
            }
            field.setAccessible(true);
            unsafe = (Unsafe) field.get(null);
        } catch (Exception e) {
            throw new Error(e);
        }
    }

    private UnsafeAcc() {
    }
}