AbstractQueuedSynchronizer，它是阻塞式锁和相关同步器的框架。
AbstractQueuedSynchronizer 的结构和 Monitor 对象的结构有些类似，都有只有所得线程、阻塞队列等。
1. 属性与结构
1.1 几个重要的属性
public abstract class AbstractQueuedSynchronizer
extends AbstractOwnableSynchronizer
implements java.io.Serializable {
/**
* 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;
/**
* The synchronization state.
*/
private volatile int state;
public abstract class AbstractOwnableSynchronizer
implements java.io.Serializable {
/**
* The current owner of exclusive mode synchronization.
*/
private transient Thread exclusiveOwnerThread;
加上从 AbstractOwnableSynchronizer 继承来的属性，这里重点关注的是以下四个属性：
state: 正整数，表示锁的状态，0 表示没有被占用，1 表示被占用，大于 1 则表示重入的次数。
head: Node 类型的对象，阻塞队列的头结点，头结点是无意义的，只是用来连接，也被称为哑元或哨兵
tail: Node 类型的对象，阻塞队列的尾结点
exclusiveOwnerThread: 当前持有锁的线程
1.2 内部类
- 双向阻塞队列的节点类
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;
volatile Node prev;
volatile Node next;
volatile Thread thread;
Node nextWaiter;
Node 是链表（类似于 monitor 的 entryList）的节点类，上面提到的 head、tail 都是这个类型对象。每个阻塞的线程都用 Node 封装起来构成一个链表节点加在队尾。
除了前驱节点、后继节点，还有以下几个属性：
waitStatus：线程的等待状态
thread：用于存储阻塞的线程
SHARED：new Node()，静态常量，代表共享锁
EXCLUSIVE：值为 null，是静态常量，代表排它锁
nextWaiter：下一个节点，ConditionObject 专用。prev、next 是 AQS 阻塞队列专用。
AQS 阻塞队列是双向链表（prev、next），ConditionObject 中的队列是单向链表（nextWaiter）
其中 waitStatus 的几种状态：
CANCELLED：值为 1，代表取消等待，除了这个状态，其他几个都是有效状态
SIGNAL： -1，等待被唤醒
CONDITION：-2，等待被条件变量唤醒
PROPAGATE：-3，没看懂....
只有 ConditionObject 中 waitStatus 才可能是 CONDITION
- 条件变量类
public class ConditionObject implements Condition, java.io.Serializable {
/** First node of condition queue. */
private transient Node firstWaiter;
/** Last node of condition queue. */
private transient Node lastWaiter;
条件变量实际也是一个 Node 组成的链表，这里没有哑元，都是有效节点，而且这个链表是单向的，因为他用的是nextWaiter属性。
1.3 加锁、解锁方法没有具体实现
对于加锁、解锁方法，AQS 并没有具体实现，而是留给子类去实现。
2. 加锁、就锁过程
因为 AQS 是抽象类，加锁、解锁的方法没有具体实现，而是留给子类去实现，所以这里只是说一下大概思想和流程。
加锁成功
通过对 state 属性 CAS 尝试将其从 0 改为 1，如果修改成功，就进一步把 exclusiveOwnerThread 设置为当前线程。这样就加锁成功了。
锁重入
CAS 修改 state 的时候失败的话，会先判断 exclusiveOwnerThread == Thread.currentThread() 是否为真，如果是真，就说明是锁重入，state++ 即可。
加锁失败
和 monitor 类似，如果加锁失败而且不是锁重入的情况，就需要让线程进入阻塞队列，将线程封装在 Node 对象中，添加到队尾。
解锁
解锁，就是每释放一次锁，就 state--，减到 0，说明已经是最初加锁的地方了，将 exclusiveOwnerThread 设置为 null。
AQS 只是给出抽象的框架，具体是公平还是非公平，共享还是排他等等一些细节仍需要子类去具体实现。
AQS 对属性的 CAS 操作都有实现，基于 Unsafe，阻塞、唤醒线程用的是 LockSupport.park(t), LockSupport.unPark(),实际上也是 基于Unsafe 类
3. 条件变量
3.1 await 阻塞
public final void await() throws InterruptedException {
if (Thread.interrupted())
throw new InterruptedException();
Node node = addConditionWaiter();
int savedState = fullyRelease(node);
int interruptMode = 0;
while (!isOnSyncQueue(node)) {
LockSupport.park(this);
if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
break;
}
if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
interruptMode = REINTERRUPT;
if (node.nextWaiter != null) // clean up if cancelled
unlinkCancelledWaiters();
if (interruptMode != 0)
reportInterruptAfterWait(interruptMode);
}
addConditionWaiter 方法就是将阻塞线程用 Node 封装一下，加到队尾。
private Node addConditionWaiter() {
Node t = lastWaiter;
// If lastWaiter is cancelled, clean out.
if (t != null && t.waitStatus != Node.CONDITION) {
unlinkCancelledWaiters();
t = lastWaiter;
}
Node node = new Node(Thread.currentThread(), Node.CONDITION);
if (t == null)
firstWaiter = node;
else
t.nextWaiter = node;
lastWaiter = node;
return node;
}
添加到条件变量队尾之后，因为线程还未阻塞，有可能在此过程中获取到了锁，然而开发者调用 await 是为了阻塞线程，并等待 signal 唤醒，并不希望它此时得到锁，所以调用 fullyRelease 将锁释放（如果得到了锁）。
final int fullyRelease(Node node) {
boolean failed = true;
try {
int savedState = getState();
if (release(savedState)) {
failed = false;
return savedState;
} else {
throw new IllegalMonitorStateException();
}
} finally {
if (failed)
node.waitStatus = Node.CANCELLED;
}
}
当前节点已经加入到 ConditionObject 的单向队列中，但是是否加到了 AQS 阻塞队列需要用 isOnSyncQueue 方法来判断。
final boolean isOnSyncQueue(Node node) {
if (node.waitStatus == Node.CONDITION || node.prev == null)
return false;
if (node.next != null) // If has successor, it must be on queue
return true;
/*
* node.prev can be non-null, but not yet on queue because
* the CAS to place it on queue can fail. So we have to
* traverse from tail to make sure it actually made it. It
* will always be near the tail in calls to this method, and
* unless the CAS failed (which is unlikely), it will be
* there, so we hardly ever traverse much.
*/
return findNodeFromTail(node);
}
如果还没有被添加到 AQS 队列中，就将线程 park。如果能从 while (!isOnSyncQueue(node)) 循环中出来，说明被加载到了 AQS 阻塞队列中了，或者是 park 被唤醒了。
3.2 signal 唤醒
如果线程持有锁，那 signal 方法会报错，否则就唤醒队列中的第一个节点。
public final void signal() {
if (!isHeldExclusively())
throw new IllegalMonitorStateException();
Node first = firstWaiter;
if (first != null)
doSignal(first);
}
private void doSignal(Node first) {
do {
//将头结点更换为头结点的后继节点
if ( (firstWaiter = first.nextWaiter) == null)
lastWaiter = null;
first.nextWaiter = null;
} while (!transferForSignal(first) &&
(first = firstWaiter) != null);
}
尝试释放
final boolean transferForSignal(Node node) {
/*
* If cannot change waitStatus, the node has been cancelled.
*/
if (!compareAndSetWaitStatus(node, Node.CONDITION, 0))
return false;
/*
* 将从条将变量取出的头结点，添加到 AQS 的尾结点中
*/
Node p = enq(node);
int ws = p.waitStatus;
// 判断 waitStatus，如果大于零或者，修改为 SIGNAL 失败，就 unpark 该节点退出阻塞
if (ws > 0 || !compareAndSetWaitStatus(p, ws, Node.SIGNAL))
LockSupport.unpark(node.thread);
return true;
}
线程是阻塞在上面的 await 方法中，在 transferForSignal 将线程 unpark 之后，就会执行 await 后面的代码，大致就是清除节点之类的。之后就可以正常实行业务逻辑了。
还有一种情况，修改没有进入到 unpark 的逻辑之中，这种的就要排队等着了。。。等它的前驱节点唤醒它。
