java实现无锁队列

拼搏现实的明天。 2022-10-09 03:17 113阅读 0赞

# 写作目的 #

说到无锁，其实就是用cas，不过我在百度上搜java实现无锁队列的文章其实不多，所以自己用cas和volatile实现一下，线程安全那是必须的。

## 无锁队列 ##

package untils;
    
    import java.lang.reflect.Field;
    import java.util.concurrent.atomic.AtomicInteger;
    
    import lombok.Data;
    import sun.misc.Unsafe;
    
    /**
     * Created on 2021-06-23
     */
    public class NoLockQueue {
    
    
        private static final Unsafe unsafe;
    
        //头节点
        volatile Node head;
        //尾节点
        volatile Node tail;
    
        //头节点偏移量
        private static final Long headOffset;
        //尾节点偏移量
        private static final Long tailOffset;
    
    
        //当前队列的长度
        private AtomicInteger length = new AtomicInteger(0);
    
        //队列允许的最大长度
        private int maxSize = 0;
    
    
        static {
            try {
                //获取成员变量
                Field field = Unsafe.class.getDeclaredField("theUnsafe");
                //设置为可访问
                field.setAccessible(true);
                //是静态字段,用null来获取Unsafe实例
                unsafe = (Unsafe) field.get(null);
                //设置头节点变量在类中的偏移值
                headOffset = unsafe.objectFieldOffset(NoLockQueue.class.getDeclaredField("head"));
                //设置尾节点变量在类中的偏移值
                tailOffset = unsafe.objectFieldOffset(NoLockQueue.class.getDeclaredField("tail"));
            } catch (Exception e) {
                System.out.println(e.getLocalizedMessage());
                throw new Error(e);
            }
        }
    
        public NoLockQueue() {
    
            this.maxSize = Integer.MAX_VALUE;
            //初始化节点
            head = tail = new Node();
        }
    
    
        public NoLockQueue(int maxSize) {
            this.maxSize = maxSize;
            //初始化节点
            head = tail = new Node();
        }
    
    
        /**
         * 入队
         */
        public void enQueue(int value) {
    
            //创建新节点
            Node newNode = new Node();
            newNode.setValue(value);
    
    
            while (true) {
    
                //获取尾节点
                Node oldTail = this.tail;
                if (length.get() < maxSize && oldTail.casNext(null, newNode)) {
                    System.out.println(Thread.currentThread().getName() + "进队列：" + value);
                    unsafe.compareAndSwapObject(this, tailOffset, oldTail, newNode);
                    length.incrementAndGet();
                    break;
                }
    
            }
    
    
        }
    
    
        /**
         * 出队
         */
        public void dequeue() {
    
            while (true) {
                //如果没有数据
                if (length.get() <= 0) {
                    continue;
                }
    
                //获取头节点
                Node oldHead = this.head;
                Node oldNext = oldHead.getNext();
    
                if (unsafe.compareAndSwapObject(this, headOffset, head, oldNext)) {
                    System.out.println(Thread.currentThread().getName() + "出队列：" + head.getValue());
                    length.decrementAndGet();
                    break;
                }
    
    
            }
    
    
        }
    
    
    }
    
    @Data
    class Node {
    
        //Unsafe类
        private static final Unsafe unsafe;
    
        //next变量的偏移量
        private static final Long nextOffset;
    
    
        //值
        private volatile int value;
    
        //next节点
        private volatile Node next;
    
        static {
            try {
                //获取成员变量
                Field field = Unsafe.class.getDeclaredField("theUnsafe");
                //设置为可访问
                field.setAccessible(true);
                //是静态字段,用null来获取Unsafe实例
                unsafe = (Unsafe) field.get(null);
                //获取state变量在类中的偏移值
                nextOffset = unsafe.objectFieldOffset(Node.class.getDeclaredField("next"));
            } catch (Exception e) {
                System.out.println(e.getLocalizedMessage());
                throw new Error(e);
            }
        }
    
    
        /**
         * cas的方式设置next的值
         * @param before 期望的值
         * @param after 修改的值
         * @return
         */
        public boolean casNext(Node before, Node after) {
            return unsafe.compareAndSwapObject(this, nextOffset, before, after);
        }
    
    
    }

## 测试类 ##

package untils;
    
    import java.util.Random;
    import java.util.concurrent.ExecutorService;
    import java.util.concurrent.Executors;
    import java.util.concurrent.TimeUnit;
    
    /**
     * Created on 2021-06-23
     */
    public class Main {
        public static void main(String[] args) {
    
            NoLockQueue queue = new NoLockQueue(10);
            ExecutorService executorService = Executors.newFixedThreadPool(20);
            Random random = new Random();
            
            for (int i = 0; i < 10; i++) {
                executorService.submit(new Runnable() {
                    @Override
                    public void run() {
                        queue.enQueue(random.nextInt());
                    }
                });
            }
            
            //        //判断入队的顺序
            //        System.out.println("----------------");
            //        try { TimeUnit.SECONDS.sleep(2); } catch (Exception e) { e.printStackTrace(); } finally { }
            //        Node p = queue2.head.getNext();
            //        while (p != null){
            //            System.out.println(p.getValue());
            //            p = p.getNext();
            //        }
            //
            //        System.out.println("----------------");
    
    
            for (int i = 0; i < 10; i++) {
                executorService.submit(new Runnable() {
                    @Override
                    public void run() {
                        queue.dequeue();
                    }
                });
            }
    
    
            try {
                TimeUnit.SECONDS.sleep(5);
            } catch (Exception e) {
                e.printStackTrace();
            } finally {
            }
            executorService.shutdown();
    
    
        }
    }

# 小插曲 #

## unsafe类的获取 ##

其实当时参考的是AtomicInteger里获取unsafe方法

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

但是报错了，报错的原因竟然是 双亲委派模型

[https://www.cnblogs.com/yanwu0527/p/13036330.html][https_www.cnblogs.com_yanwu0527_p_13036330.html]

那怎么获取unsafe类呢，如下所示，固定格式

//获取成员变量
                Field field = Unsafe.class.getDeclaredField("theUnsafe");
                //设置为可访问
                field.setAccessible(true);
                //是静态字段,用null来获取Unsafe实例
                Unsafe unsafe = (Unsafe) field.get(null);

## 打印顺序不对，影响了代码的正确性 ##

/**
         * 出队
         */
        public void dequeue() {
            while (true) {
                //如果没有数据
                if (length.get() <= 0) {
                    continue;
                }
                //获取头节点
                Node oldHead = this.head;
                Node oldNext = oldHead.getNext();
    
                if (unsafe.compareAndSwapObject(this, headOffset, head, oldNext)) {
                    //正确的位置
                    //System.out.println(Thread.currentThread().getName() + "出队列：" + head.getValue());
                    length.decrementAndGet();
                    //错误的位置
                    System.out.println(Thread.currentThread().getName() + "出队列：" + head.getValue());
                    break;
                }
            }
        }

一开始我在错误的位置打印，发现入队和出队的顺序不一样，后来换了一个位置试了一下，好了。最后还是分析一下为什么吧。

比如此时此刻 队列里有2个元素A和B。现在2个线程按照下面的顺序执行，其实理论上出队顺序是没有问题的，只不过后面的先打印了，给了一种先出队的错觉。

# 收获 #

其实[https://blog.csdn.net/meiyongdesan/article/details/106414902][https_blog.csdn.net_meiyongdesan_article_details_106414902] 这个里面使用AtomicReference实现的，主要想用他的cas;但是我感觉有些绕，所以就自己用unsafe类实现cas。

断断续续写了一天才写了一个demo,其实不亏，至少写出来了。

# 参考 #

[https://blog.csdn.net/meiyongdesan/article/details/106414902][https_blog.csdn.net_meiyongdesan_article_details_106414902]

[https://www.jianshu.com/p/cda24891f9e4][https_www.jianshu.com_p_cda24891f9e4]

[https://www.cnblogs.com/yanwu0527/p/13036330.html][https_www.cnblogs.com_yanwu0527_p_13036330.html]

ConcurrentLinkedQueue源码

[https_www.cnblogs.com_yanwu0527_p_13036330.html]: https://www.cnblogs.com/yanwu0527/p/13036330.html
[https_blog.csdn.net_meiyongdesan_article_details_106414902]: https://blog.csdn.net/meiyongdesan/article/details/106414902
[https_www.jianshu.com_p_cda24891f9e4]: https://www.jianshu.com/p/cda24891f9e4