CLH Lock & MCS Lock

共性： 都是公平锁，先进先出，通过自旋轮询状态

区别： CLH锁持有前个节点状态并轮询，MCS持有后一个节点状态并轮询。

0，节点类 QNode

package com.test.learn;

public class LockNode {
    /**
     * 标志锁的状态
     */
    volatile boolean locked = false;

    /**
     * 用于保存后个节点
     */
    volatile LockNode next = null;
}

1，CLHLock

package com.test.learn;

import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicReference;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;

public class CLHLock implements Lock {

    // 提供CAS方式修改地址引用
    private AtomicReference tail;
    // 保存前一个节点引用
    private ThreadLocal myPre;
    // 当前节点
    private ThreadLocal myNode;

    public CLHLock() {
        tail = new AtomicReference<>(new LockNode());
        myNode = ThreadLocal.withInitial(LockNode::new);
        myPre = ThreadLocal.withInitial(() -> null);
    }

    @Override
    public void lock() {
        // L1.将自身节点状态标志为true(已上锁)
        LockNode node = myNode.get();
        node.locked = true;
        // L2.获取前个节点
        LockNode pre = tail.getAndSet(node);
        myPre.set(pre);
        while (pre.locked) {
            // L3.自旋等待前个节点释放锁,即前一个节点执行unLock(U2处)
        }
    }

    @Override
    public void unlock() {
        // 获取当前节点
        LockNode qnode = myNode.get();
        // 通过状态标志为锁已释放
        qnode.locked = false;
        // 回收前驱节点
        myNode.set(myPre.get());
    }

    @Override
    public void lockInterruptibly() throws InterruptedException {
        // TODO
    }

    @Override
    public boolean tryLock() {
        // TODO
        return false;
    }

    @Override
    public boolean tryLock(long time, TimeUnit unit) throws InterruptedException {
        // TODO
        return false;
    }

    @Override
    public Condition newCondition() {
        // TODO
        return null;
    }
}

2，MCSLock

package com.test.learn;

import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicReference;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;

public class MCSLock implements Lock {

    // 提供CAS方式修改地址引用
    private AtomicReference tail;
    // 保存当前节点
    private ThreadLocal myNode;

    public MCSLock() {
        // tail和CLH有区别，初始值为null
        tail = new AtomicReference<>();
        myNode = ThreadLocal.withInitial(LockNode::new);
    }

    @Override
    public void lock() {
        LockNode qnode = myNode.get();
        // L1,获取前节点
        LockNode pre = tail.getAndSet(qnode);
        // L2.不存在前一个节点则直接获取锁了，否则自旋前节点的状态
        if (null != pre) {
            qnode.locked = true;
            // L3 设置后续节点
            pre.next = qnode;
            while (qnode.locked) {
                // 自旋前节点的状态
            }
        }
    }

    @Override
    public void unlock() {
        LockNode qnode = myNode.get();
        if (qnode.next == null) {
            // 为什么需要CAS判断是否更新成功？因为多线程下可能新增了新的节点。
            if (tail.compareAndSet(qnode, null)) {
                return;
            }
            // 循环到等到next有值后退出,也即后一个节点执行L3之后
            while (qnode.next == null) {
            }
        }
        // 解锁时候主动更新后节点状态（和CLH的主要差距点)
        qnode.next.locked = false;
        qnode.next = null;
    }


    @Override
    public void lockInterruptibly() throws InterruptedException {
        // TODO
    }

    @Override
    public boolean tryLock() {
        // TODO
        return false;
    }

    @Override
    public boolean tryLock(long time, TimeUnit unit) throws InterruptedException {
        // TODO
        return false;
    }

    @Override
    public Condition newCondition() {
        // TODO
        return null;
    }
}

3, 使用

package com.test.learn;

import java.util.concurrent.CountDownLatch;
import java.util.concurrent.CyclicBarrier;
import java.util.concurrent.locks.Lock;

public class LockMain {

    private static final int THEAD_CNT = 10;

    private static Lock getLock() {
        // return new MCSLock();
        return new CLHLock();
    }

    public static void main(String[] args) throws Exception {
        CyclicBarrier cb = new CyclicBarrier(THEAD_CNT);
        CountDownLatch latch = new CountDownLatch(THEAD_CNT);

        Lock lock = getLock();
        for (int i = 0; i < THEAD_CNT; i++) {
            Thread th = new Thread(() -> {
                try {
                    cb.await();
                    System.out.println(Thread.currentThread().getName());
                    lock.lock();
                    System.out.println(Thread.currentThread().getName() + " Got the Lock!");
                    Thread.sleep(100);
                    lock.unlock();
                    System.out.println(Thread.currentThread().getName() + " Released the Lock!");
                    latch.countDown();
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }, "Thread:" + i);
            th.start();
        }

        long start = System.currentTimeMillis();
        latch.await();
        System.out.println("Duration:" + (System.currentTimeMillis() - start));
    }
}