我对CAS的一些理解和用法

前言：之前也一直看到CAS的一些操作，知道CAS是一种乐观锁，通过比较和替换来更新值。今天在看ConcurrentHashMap的时候，发现里面涉及很多的CAS操作，于是有了本文，代码中都有注释。

public class TestCAS {
          
   
    public static void main(String[] args) {
          
   
//        Node node = new Node(4);
//        boolean flag = node.casNext(null, new Node(5));
//        System.out.println(flag + ", val = " + node.next.value);

        Node[] nodes = new Node[5];
        for (int i = 0; i < 5; i++) {
          
   
            nodes[i] = new Node(i + 10);
        }

        Node node = Node.get(nodes, 0);
        System.out.println("node val = " + node.value); //输出 nodes[0] = 10;

        Node expect = nodes[0];
        Node update = new Node(23);
        boolean res = Node.casTabAt(nodes, 0, expect, update);//比较nodes[0]和expect是否相等，如果相等则替换nodes[0]为update
        System.out.println("res = " + res + ", nodes[0].val = " + nodes[0].value);//这里nodes[0]和expect相等，则替换nodes[0]为update，输出为23
    }

    private static class Node {
          
   
        int value;
        volatile Node next;
        static final sun.misc.Unsafe UNSAFE;
        static final long nextOffset;
        private static final int ABASE;
        private static final int ASHIFT;

        public Node(int val) {
          
   
            value = val;
        }

        static {
          
   
            try {
          
   
                UNSAFE = getUnsafe();
                Class<?> k = Node.class;
                nextOffset = UNSAFE.objectFieldOffset(k.getDeclaredField("next")); //Node 对象的变量 next 内存地址
                ABASE = UNSAFE.arrayBaseOffset(Node[].class);//可以获取数组第一个元素的偏移地址
                //可以获取数组的转换因子，也就是数组中元素的增量地址。将arrayBaseOffset与arrayIndexScale配合使用， 可以定位数组中每个元素在内存中的位置
                int scale = UNSAFE.arrayIndexScale(Node[].class);
                System.out.println("ABASE = " + ABASE + ", scale = " + scale);
                if ((scale & (scale - 1)) != 0)
                    throw new Error("array index scale not a power of two");
                ASHIFT = 31 - Integer.numberOfLeadingZeros(scale);
            } catch (Exception e) {
          
   
                throw new Error(e);
            }
        }

        //利用 unsafe 来获取数组中对应index的值
        public static Node get(Node[] tab, int index) {
          
   
            return (Node) UNSAFE.getObject(tab, ((long) index << ASHIFT) + ABASE);
        }

        /**
         * cas next 变量
         *
         * @param cmp
         * @param val
         * @return
         */
        boolean casNext(Node cmp, Node val) {
          
   
            /**
             * compareAndSwapObject(Object obj, long offset, Object expect, Object update)
             * obj 操作的对象
             * offset 对象的地址偏移量
             * expect offset 所保存的对象和 expect 比较，相等才更新对象的值为update
             * update 更新的值
             */
            return UNSAFE.compareAndSwapObject(this, nextOffset, cmp, val);
        }

        static boolean casTabAt(Node[] tab, int index, Node expect, Node update) {
          
   
            return UNSAFE.compareAndSwapObject(tab, ((long) index << ASHIFT) + ABASE, expect, update);
        }

        /**
         * 反射获取Unsafe的方法
         * 获取了以后就可以愉快的使用CAS啦
         *
         * @return
         */
        public static Unsafe getUnsafe() {
          
   
            try {
          
   
                Field field = Unsafe.class.getDeclaredField("theUnsafe");
                field.setAccessible(true);
                return (Unsafe) field.get(null);
            } catch (Exception e) {
          
   
                return null;
            }
        }
    }
}

欢迎指教！