AQS队列同步器

class Mutex implements Lock, java.io.Serializable {

    // 静态内部类
    // Our internal helper class
    private static class Sync extends AbstractQueuedSynchronizer {
        // Reports whether in locked state
        // 判断是否处于占用状态
        protected boolean isHeldExclusively() {
            return getState() == 1;
        }

        // Acquires the lock if state is zero
        public boolean tryAcquire(int acquires) {
            assert acquires == 1; // Otherwise unused
            if (compareAndSetState(0, 1)) {
                setExclusiveOwnerThread(Thread.currentThread());	//AQS中已定义的方法，设置当前拥有独占访问权限的是哪个线程
                return true;
            }
            return false;
        }

        // Releases the lock by setting state to zero
        protected boolean tryRelease(int releases) {
            assert releases == 1; // Otherwise unused
            if (getState() == 0) throw new IllegalMonitorStateException();
            setExclusiveOwnerThread(null);
            setState(0);
            return true;
        }

        // Provides a Condition
        Condition newCondition() {
            return new ConditionObject();
        }

        // Deserializes properly
        private void readObject(ObjectInputStream s)
                throws IOException, ClassNotFoundException {
            s.defaultReadObject();
            setState(0); // reset to unlocked state
        }
    }

    // 使用Sync完成相关的同步操作
    // The sync object does all the hard work. We just forward to it.
    private final Sync sync = new Sync();

    public void lock() {
        sync.acquire(1);
    }

    public boolean tryLock() {
        return sync.tryAcquire(1);
    }

    public void unlock() {
        sync.release(1);
    }

    public Condition newCondition() {
        return sync.newCondition();
    }

    public boolean isLocked() {
        return sync.isHeldExclusively();
    }

    public boolean hasQueuedThreads() {
        return sync.hasQueuedThreads();
    }

    public void lockInterruptibly() throws InterruptedException {
        sync.acquireInterruptibly(1);
    }

    public boolean tryLock(long timeout, TimeUnit unit)
            throws InterruptedException {
        return sync.tryAcquireNanos(1, unit.toNanos(timeout));
    }
}