package com;

import sun.misc.Unsafe;

import java.lang.reflect.Field;

/**

 * Created by yangyu on 16/11/24.

 */

public class TestUnsafe {

    public static void main(String[] args) {

        Node node = new Node();

        /**

         * 通过CAS方法更新node的next属性

         * 原子操作

         */

        boolean flag = node.casNext(null,new Node());

        System.out.println(flag);

    }

    private static class Node{

        volatile Node next;

        /**

         * 使用Unsafe CAS方法

         * @param cmp 目标值与cmp比较，如果相等就更新返回true；如果不相等就不更新返回false；

         * @param val 需要更新的值；

         * @return

         */

        boolean casNext(Node cmp, Node val) {

            /**

             * compareAndSwapObject(Object var1, long var2, Object var3, Object var4)

             * var1 操作的对象

             * var2 操作的对象属性

             * var3 var2与var3比较，相等才更新

             * var4 更新值

             */

            return UNSAFE.compareAndSwapObject(this, nextOffset, cmp, val);

        }

        private static final sun.misc.Unsafe UNSAFE;

        private static final long nextOffset;

        static {

            try {

                UNSAFE = getUnsafe();

                Class<?> k = Node.class;

                nextOffset = UNSAFE.objectFieldOffset

                        (k.getDeclaredField("next"));

            } catch (Exception e) {

                throw new Error(e);

            }

        }

        /**

         * 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 f = Unsafe.class.getDeclaredField("theUnsafe");

                f.setAccessible(true);

                return (Unsafe)f.get(null);

            } catch (Exception e) {

                return null;

            }

        }

    }

}