CLH Lock浅析

CLH Lock的定义：The CLH lock is a scalable, high performance, fairness and spin lock based on the list, the application thread spin only on a local variable, it constantly polling the precursor state, if it is found that the pre release lock end spin.

public class ClhSpinLock implements Lock{
    private final ThreadLocal<Node> prev;
    private final ThreadLocal<Node> node;
    private final AtomicReference<Node> tail = new AtomicReference<Node>(new Node());

    public ClhSpinLock() {
        this.node = new ThreadLocal<Node>() {
            protected Node initialValue() {
                return new Node();
            }
        };

        this.prev = new ThreadLocal<Node>() {
            protected Node initialValue() {
                return null;
            }
        };
    }
     public void lock() {
        final Node node = this.node.get();
        node.locked = true;
        Node pred = this.tail.getAndSet(node);
        this.prev.set(pred);
        // 自旋
        while (pred.locked);
    }

    public void unlock() {
        final Node node = this.node.get();
        node.locked = false;
        this.node.set(this.prev.get());
//        this.node.set(new Node());
    }
}

CLH 代码分析：
（1）pred node 的作用：其实完全可以把pred node去掉，然后在unlock方法中将this.node 赋值一个新的Node就行。而对this.node赋值prev node，就不用new了，效率更高。
（2）公平锁的体现：通过AtomicReference.getAndSet()将线程形成一个链表.只有等前一个线程释放了锁，后面的线程才能获取锁，确保了锁的公平性。
（3）是否可重入：不能重入，非重入锁。