J.U.C源码阅读笔记（一）AQS

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AQS(AbstractQueuedSynchronizer)这个类提供了一个框架用来实现阻塞锁和一些同步工具类。比如ReentrantLock、ReadWriteLock、Semaphore、CountDownLatch、CyclicBarrier等。

AQS

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public abstract class AbstractQueuedSynchronizer
    extends AbstractOwnableSynchronizer
    implements java.io.Serializable

介绍

这个类通过维护一个volatile修饰的变量state表示锁的获取情况以及一个FIFO队列用来封装要阻塞的线程。
线程通过CAS来修改state来获取锁，如果获取失败，会将当前线程封装成队列节点入队或者返回失败(tryLock())。

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while(synchronization state does not allow acquire){

    enqueue current thread if not already queued;

    possibly block current thread;

}

队列中的每个节点都封装了一个没有获取到锁而被阻塞的线程，队列中的节点有多种状态,比如SIGNAL代表当前节点的后继节点被阻塞了，当释放锁时需要唤醒后继节点中的线程。CANCELLED表示当前节点中的线程超时或者被中断了，此时就会将节点的waitStatus改为CANCELLED。保证出队和入队线程安全用的锁是CLH锁，CLH是一种基于链表的高性能自旋锁。作者介绍说CLH锁相对于MCS锁比较容易处理节点的超时和取消状态。并且出队和入队由于没有锁操作,效率会更高一些。

However, they appeared more amenable than MCS for use in the synchronizer framework because they are more easily adapted to handle cancellation and timeouts, so were chosen as a basis.

Among the advantages of CLH locks are that enqueuing and dequeuing are fast, lock-free, and obstruction free (even under contention, one thread will always win an insertion race so will make progress); that detecting whether any threads are waiting is also fast (just check if head is the same as tail); and that release status is decentralized, avoiding some memory contention

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/**
* Status field, taking on only the values:
*   SIGNAL:     The successor of this node is (or will soon be)
*               blocked (via park), so the current node must
*               unpark its successor when it releases or
*               cancels. To avoid races, acquire methods must
*               first indicate they need a signal,
*               then retry the atomic acquire, and then,
*               on failure, block.
*   CANCELLED:  This node is cancelled due to timeout or interrupt.
*               Nodes never leave this state. In particular,
*               a thread with cancelled node never again blocks.
*   CONDITION:  This node is currently on a condition queue.
*               It will not be used as a sync queue node
*               until transferred, at which time the status
*               will be set to 0. (Use of this value here has
*               nothing to do with the other uses of the
*               field, but simplifies mechanics.)
*   PROPAGATE:  A releaseShared should be propagated to other
*               nodes. This is set (for head node only) in
*               doReleaseShared to ensure propagation
*               continues, even if other operations have
*               since intervened.
*   0:          None of the above
*
* The values are arranged numerically to simplify use.
* Non-negative values mean that a node doesn't need to
* signal. So, most code doesn't need to check for particular
* values, just for sign.
*
* The field is initialized to 0 for normal sync nodes, and
* CONDITION for condition nodes.  It is modified using CAS
* (or when possible, unconditional volatile writes).
*/

参考

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