Java 通过AQS实现数据组织

引言

从本篇文章开始，我们将介绍 Java AQS 的实现方式，本文先介绍 AQS 的内部数据是如何组织的，后面的文章中再分别介绍 AQS 的各个部门实现。

AQS

通过前面的介绍，大家一定看出来了，上述的各种类型的锁和一些线程控制接口（CountDownLatch 等），最终都是通过 AQS 来实现的，不同之处只在于 tryAcquire 等抽象函数如何实现。从这个角度来看，AQS(AbstractQueuedSynchronizer) 这个基类设计的真的很不错，能够包容各种同步控制方案，并提供了必须的下层依赖：比如阻塞，队列等。接下来我们就来揭开它神秘的面纱。

内部数据

AQS 顾名思义，就是通过队列来组织修改互斥资源的请求。当这个资源空闲时间，那么修改请求可以直接进行，而当这个资源处于锁定状态时，就需要等待，AQS 会将所有等待的请求维护在一个类似于 CLH 的队列中。CLH：Craig、Landin and Hagersten队列，是单向链表，AQS中的队列是CLH变体的虚拟双向队列（FIFO），AQS是通过将每条请求共享资源的线程封装成一个节点来实现锁的分配。主要原理图如下：

图中的 state 是一个用 volatile 修饰的 int 变量，它的使用都是通过 CAS 来进行的，而 FIFO 队列完成请求排队的工作，队列的操作也是通过 CAS 来进行的，正因如此该队列的操作才能达到理想的性能要求。

通过 CAS 修改 state 比较容易，大家应该都能理解，但是如果要通过 CAS 维护一个双向队列要怎么做呢？这里我们看一下 AQS 中 CLH 队列的实现。在 AQS 中有两个指针一个指针指向了队列头，一个指向了队列尾。它们都是懒初始化的，也就是说最初都为null。

/**
* Head of the wait queue, lazily initialized. Except for
* initialization, it is modified only via method setHead. Note:
* If head exists, its waitStatus is guaranteed not to be
* CANCELLED.
*/
private transient volatile Node head;
/**
* Tail of the wait queue, lazily initialized. Modified only via
* method enq to add new wait node.
*/
private transient volatile Node tail;

队列中的每个节点，都是一个 Node 实例，该实例的第一个关键字段是 waitState，它表述了当前节点所处的状态，通过 CAS 进行修改：

SIGNAL：表示当前节点承担唤醒后继节点的责任
CANCELLED：表示当前节点已经超时或者被打断
CONDITION：表示当前节点正在 Condition 上等待（通过锁可以创建 Condition 对象）
PROPAGATE：只会设置在 head 节点上，用于表明释放共享锁时，需要将这个行为传播到其他节点上，这个我们稍后详细介绍。

static final class Node {
/** Marker to indicate a node is waiting in shared mode */
static final Node SHARED = new Node();
/** Marker to indicate a node is waiting in exclusive mode */
static final Node EXCLUSIVE = null;
/** waitStatus value to indicate thread has cancelled */
static final int CANCELLED = 1;
/** waitStatus value to indicate successor's thread needs unparking */
static final int SIGNAL = –1;
/** waitStatus value to indicate thread is waiting on condition */
static final int CONDITION = –2;
/**
* waitStatus value to indicate the next acquireShared should
* unconditionally propagate
*/
static final int PROPAGATE = –3;
/**
* 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).
*/
volatile int waitStatus;
/**
* Link to predecessor node that current node/thread relies on
* for checking waitStatus. Assigned during enqueuing, and nulled
* out (for sake of GC) only upon dequeuing. Also, upon
* cancellation of a predecessor, we short-circuit while
* finding a non-cancelled one, which will always exist
* because the head node is never cancelled: A node becomes
* head only as a result of successful acquire. A
* cancelled thread never succeeds in acquiring, and a thread only
* cancels itself, not any other node.
*/
volatile Node prev;
/**
* Link to the successor node that the current node/thread
* unparks upon release. Assigned during enqueuing, adjusted
* when bypassing cancelled predecessors, and nulled out (for
* sake of GC) when dequeued. The enq operation does not
* assign next field of a predecessor until after attachment,
* so seeing a null next field does not necessarily mean that
* node is at end of queue. However, if a next field appears
* to be null, we can scan prev's from the tail to
* double-check. The next field of cancelled nodes is set to
* point to the node itself instead of null, to make life
* easier for isOnSyncQueue.
*/
volatile Node next;
/**
* The thread that enqueued this node. Initialized on
* construction and nulled out after use.
*/
volatile Thread thread;
/**
* Link to next node waiting on condition, or the special
* value SHARED. Because condition queues are accessed only
* when holding in exclusive mode, we just need a simple
* linked queue to hold nodes while they are waiting on
* conditions. They are then transferred to the queue to
* re-acquire. And because conditions can only be exclusive,
* we save a field by using special value to indicate shared
* mode.
*/
Node nextWaiter;
/**
* Returns true if node is waiting in shared mode.
*/
final boolean isShared() {
return nextWaiter == SHARED;
}
//…
}

因为是双向队列，所以 Node 实例中势必有 prev 和 next 指针，此外 Node 中还会保存与其对应的线程。最后是 nextWaiter，当一个节点对应了共享请求时，nextWaiter 指向了 Node. SHARED 而当一个节点是排他请求时，nextWaiter 默认指向了 Node. EXCLUSIVE 也就是 null。我们知道 AQS 也提供了 Condition 功能，该功能就是通过 nextWaiter 来维护在 Condition 上等待的线程。也就是说这里的 nextWaiter 在锁的实现部分中，扮演者共享锁和排它锁的标志位，而在条件等待队列中，充当链表的 next 指针。

同步队列

接下来，我们由最常见的入队操作出发，介绍 AQS 框架的实现与使用。从下面的代码中可以看到入队操作支持两种模式，一种是排他模式，一种是共享模式，分别对应了排它锁场景和共享锁场景。

当任意一种请求，要入队时，先会构建一个 Node 实例，然后获取当前 AQS 队列的尾结点，如果尾结点为空，就是说队列还没初始化，初始化过程在后面 enq 函数中实现
这里我们先看初始化之后的情况，即 tail != null，先将当前 Node 的前向指针 prev 更新，然后通过 CAS 将尾结点修改为当前 Node，修改成功时，再更新前一个节点的后向指针 next，因为只有修改尾指针过程是原子的，所以这里会出现新插入一个节点时，之前的尾节点 previousTail 的 next 指针为null的情况，也就是说会存在短暂的正向指针和反向指针不同步的情况，不过在后面的介绍中，你会发现 AQS 很完备地避开了这种不同步带来的风险（通过从后往前遍历）
如果上述操作成功，则当前线程已经进入同步队列，否则，可能存在多个线程的竞争，其他线程设置尾结点成功了，而当前线程失败了，这时候会和尾结点未初始化一样进入 enq 函数中。

/**
* Creates and enqueues node for current thread and given mode.
*
* @param mode Node.EXCLUSIVE for exclusive, Node.SHARED for shared
* @return the new node
*/
private Node addWaiter(Node mode) {
Node node = new Node(Thread.currentThread(), mode);
// Try the fast path of enq; backup to full enq on failure
Node pred = tail;
if (pred != null) {
// 已经进行了初始化
node.prev = pred;
// CAS 修改尾节点
if (compareAndSetTail(pred, node)) {
// 成功之后再修改后向指针
pred.next = node;
return node;
}
}
// 循环 CAS 过程和初始化过程
enq(node);
return node;
}

正常通过 CAS 修改数据都会在一个循环中进行，而这里的 addWaiter 只是在一个 if 中进行，这是为什么呢？实际上，大家看到的 addWaiter 的这部分 CAS 过程是一个快速执行线，在没有竞争时，这种方式能省略不少判断过程。当发生竞争时，会进入 enq 函数中，那里才是循环 CAS 的地方。