AQS基于无锁化的CAS机制实现高性能加锁

 /**
     * Sync object for non-fair locks
     */
    static final class NonfairSync extends Sync {
        private static final long serialVersionUID = 7316153563782823691L;
        /**
         * Performs lock.  Try immediate barge, backing up to normal
         * acquire on failure.
         */
        final void lock() {
            if (compareAndSetState(0, 1))
                //设置当前加锁线程
                setExclusiveOwnerThread(Thread.currentThread());
            else
                acquire(1);
        }
        protected final boolean tryAcquire(int acquires) {
            return nonfairTryAcquire(acquires);
        }
    }

if (compareAndSetState(0, 1))：对state变量做操作，代表了锁的一个状态。看一下state是否是0？如果是0的话，代表没人加过锁，此时我就可以加锁，把这个state设置为1。

 protected final boolean compareAndSetState(int expect, int update) {
        // See below for intrinsics setup to support this
        return unsafe.compareAndSwapInt(this, stateOffset, expect, update);
    }

compareAndSetState(0, 1)：相当于是在尝试加锁，底层原来是基于Unsafe来实现的，JDK内部使用的API，指针操作，基于cpu指令实现原子性的CAS，Atomic原子类底层也是基于Unsafe来实现的CAS操作。如果替换成功了，就会返回true，加锁失败就返回false。
当加锁成功后就要setExclusiveOwnerThread(Thread.currentThread());

/**
     * The current owner of exclusive mode synchronization.
     */
    private transient Thread exclusiveOwnerThread;
   protected final void setExclusiveOwnerThread(Thread thread) {
        exclusiveOwnerThread = thread;
    }