AbstractQueuedSynchronizer
1 简介
AbstractQueuedSynchronizer简称AQS是一个抽象同步框架，可以用来实现一个依赖状态的同步器。
JDK1.5中提供的java.util.concurrent包中的大多数的同步器(Synchronizer)如Lock, Semaphore, Latch, Barrier等，这些类之间大多可以互相实现，如使用Lock实现一个Semaphore或者反过来，但是它们都是基于java.util.concurrent.locks.AbstractQueuedSynchronizer这个类的框架实现的，理解了这个稍微复杂抽象的类再去理解其他的同步器就很轻松了。
2 原理介绍
AQS的核心是一个线程等待队列，采用的是一个先进先出FIFO队列。用来实现一个非阻塞的同步器队列有主要有两个选择Mellor-Crummey and Scott (MCS) locks和Craig, Landin, and Hagersten (CLH) locks的变种。CLH锁更适合处理取消和超时，所以AQS基于CLH进行修改作为线程等待队列。
CLH队列使用pred引用前节点形成一个队列，入队enqueue和出队dequeue操作都可以通过原子操作完成。
在 AQS 内部，通过维护一个FIFO 队列来管理多线程的排队工作。在公平竞争的情况下，无法获取同步状态的线程将会被封装成一个节点，置于队列尾部。入队的线程将会通过自旋的方式获取同步状态，若在有限次的尝试后，仍未获取成功，线程则会被阻塞住。大致示意图如下：
当头结点释放同步状态后，且后继节点对应的线程被阻塞，此时头结点线程将会去唤醒后继节点线程。后继节点线程恢复运行并获取同步状态后，会将旧的头结点从队列中移除，并将自己设为头结点。大致示意图如下：
其中每个节点包含如下状态：
/** 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).
*/
CANCELED表示线程等待已经取消，是唯一一个大于0的状态。
SINALG表示需要唤醒next节点
CONDITION表明线程正在等待一个条件
PROPAGATE用于acquireShared中向后传播
3 acquire
/**
* Acquires in exclusive mode, ignoring interrupts. Implemented
* by invoking at least once {@link #tryAcquire},
* returning on success. Otherwise the thread is queued, possibly
* repeatedly blocking and unblocking, invoking {@link
* #tryAcquire} until success. This method can be used
* to implement method {@link Lock#lock}.
*
* @param arg the acquire argument. This value is conveyed to
* {@link #tryAcquire} but is otherwise uninterpreted and
* can represent anything you like.
*/
public final void acquire(int arg) {
if (!tryAcquire(arg) &&
acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
selfInterrupt();
}
1、如果尝试获取锁成功整个获取操作就结束，否则转到2. 尝试获取锁是通过方法tryAcquire来实现的，AQS中并没有该方法的具体实现，只是简单地抛出一个不支持操作异常，在AQS简介中谈到tryAcquire有很多实现方法，这里不再细化，只需要知道如果获取锁成功该方法返回true即可；
2、如果获取锁失败，那么就创建一个代表当前线程的结点加入到等待队列的尾部，是通过addWaiter方法实现的，来看该方法的具体实现：
/**
* 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) {
node.prev = pred;
if (compareAndSetTail(pred, node)) {
pred.next = node;
return node;
}
}
enq(node);
return node;
}
该方法创建了一个独占式结点，然后判断队列中是否有元素，如果有（pred!=null）就设置当前结点为队尾结点，即将当前节点插入到尾节点的后面，然后返回；
如果没有元素（pred==null），表示队列为空，走的是入队操作
/**
* Inserts node into queue, initializing if necessary. See picture above.
* @param node the node to insert
* @return node's predecessor
*/
private Node enq(final Node node) {
for (;;) {
Node t = tail;
if (t == null) { // Must initialize
if (compareAndSetHead(new Node()))
tail = head;
} else {
node.prev = t;
if (compareAndSetTail(t, node)) {
t.next = node;
return t;
}
}
}
}
enq方法采用的是变种CLH算法，先看头结点是否为空，如果为空就创建一个傀儡结点，头尾指针都指向这个傀儡结点，这一步只会在队列初始化时会执行；
如果头结点非空，就采用CAS操作将当前结点插入到头结点后面，如果在插入的时候尾结点有变化，就将尾结点向后移动直到移动到最后一个结点为止，然后再把当前结点插入到尾结点后面，尾指针指向当前结点，入队成功。
3、将新加入的结点放入队列之后，这个结点有两种状态，要么获取锁，要么就挂起，如果这个结点不是头结点的后继节点，就看看这个结点是否应该挂起，如果应该挂起，就挂起当前结点，是否应该挂起是通过shouldParkAfterFailedAcquire方法来判断的
/**
* Acquires in exclusive uninterruptible mode for thread already in
* queue. Used by condition wait methods as well as acquire.
*
* @param node the node
* @param arg the acquire argument
* @return {@code true} if interrupted while waiting
*/
final boolean acquireQueued(final Node node, int arg) {
boolean failed = true;
try {
boolean interrupted = false;
for (;;) {
final Node p = node.predecessor();
//判断前驱节点是否是头节点，如果是，则循环获取同步
if (p == head && tryAcquire(arg)) {
setHead(node);
p.next = null; // help GC
failed = false;
return interrupted;
}
//如果获取同步状态失败或者前驱节点不是头节点，则判断是否将当前节点挂起
if (shouldParkAfterFailedAcquire(p, node) &&
parkAndCheckInterrupt())
interrupted = true;
}
} finally {
if (failed)
cancelAcquire(node);
}
}
/**
* Checks and updates status for a node that failed to acquire.
* Returns true if thread should block. This is the main signal
* control in all acquire loops. Requires that pred == node.prev
*
* @param pred node's predecessor holding status
* @param node the node
* @return {@code true} if thread should block
*/
/**
* 该方法主要用途是，当线程在获取同步状态失败时，根据前驱节点的等待状态，决定后续的动作。比如前驱
* 节点等待状态为 SIGNAL，表明当前节点线程应该被阻塞住了。不能老是尝试，避免 CPU 忙等。
* —————————————————————————————————————————————————————————————————
* | 前驱节点等待状态 | 相应动作 |
* —————————————————————————————————————————————————————————————————
* | SIGNAL | 阻塞 |
* | CANCELLED | 向前遍历, 移除前面所有为该状态的节点 |
* | waitStatus < 0 | 将前驱节点状态设为 SIGNAL, 并再次尝试获取同步状态 |
* —————————————————————————————————————————————————————————————————
*/
private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
int ws = pred.waitStatus;
//首先检查前趋结点的waitStatus位，如果为SIGNAL,表示前趋结点会通知它，那么它可以放心大胆地挂起了
if (ws == Node.SIGNAL)
/*
* This node has already set status asking a release
* to signal it, so it can safely park.
*/
return true;
if (ws > 0) {
/*
* Predecessor was cancelled. Skip over predecessors and
* indicate retry.
*/
//如果前趋结点是一个被取消的结点怎么办呢？那么就向前遍历跳过被取消的结点，直到找到一个没有被取消的结点为止，将找到的这个结点作为它的前趋结点，
//将找到的这个结点的waitStatus位设置为SIGNAL,返回false表示线程不应该被挂起，继续尝试获取锁
do {
node.prev = pred = pred.prev;
} while (pred.waitStatus > 0);
pred.next = node;
} else {
/*
* waitStatus must be 0 or PROPAGATE. Indicate that we
* need a signal, but don't park yet. Caller will need to
* retry to make sure it cannot acquire before parking.
*/
/*
* 等待状态为 0 或 PROPAGATE，设置前驱节点等待状态为 SIGNAL，
* 并再次尝试获取同步状态。
*/
compareAndSetWaitStatus(pred, ws, Node.SIGNAL);
}
return false;
}
private final boolean parkAndCheckInterrupt() {
// 调用 LockSupport.park 阻塞自己
LockSupport.park(this);
return Thread.interrupted();
}
4 release
/**
* Releases in exclusive mode. Implemented by unblocking one or
* more threads if {@link #tryRelease} returns true.
* This method can be used to implement method {@link Lock#unlock}.
*
* @param arg the release argument. This value is conveyed to
* {@link #tryRelease} but is otherwise uninterpreted and
* can represent anything you like.
* @return the value returned from {@link #tryRelease}
*/
public final boolean release(int arg) {
if (tryRelease(arg)) {
Node h = head;
if (h != null && h.waitStatus != 0)
unparkSuccessor(h);
return true;
}
return false;
}
1、release过程比acquire要简单，首先调用tryRelease释放锁，如果释放失败，直接返回；
2、释放锁成功后需要唤醒继任结点，是通过方法unparkSuccessor实现的
/**
* Wakes up node's successor, if one exists.
*
* @param node the node
*/
private void unparkSuccessor(Node node) {
/*
* If status is negative (i.e., possibly needing signal) try
* to clear in anticipation of signalling. It is OK if this
* fails or if status is changed by waiting thread.
*/
int ws = node.waitStatus;
if (ws < 0)
compareAndSetWaitStatus(node, ws, 0);
/*
* Thread to unpark is held in successor, which is normally
* just the next node. But if cancelled or apparently null,
* traverse backwards from tail to find the actual
* non-cancelled successor.
*/
Node s = node.next;
if (s == null || s.waitStatus > 0) {
s = null;
for (Node t = tail; t != null && t != node; t = t.prev)
if (t.waitStatus <= 0)
s = t;
}
if (s != null)
LockSupport.unpark(s.thread);
}
1、node参数传进来的是头结点，首先检查头结点的waitStatus位，如果为负，表示头结点还需要通知后继结点，这里不需要头结点去通知后继，因此将该该标志位清0.
2、然后查看头结点的下一个结点，如果下一个结点不为空且它的waitStatus<=0,表示后继结点没有被取消，是一个可以唤醒的结点，于是唤醒后继结点返回；如果后继结点为空或者被取消了怎么办？寻找下一个可唤醒的结点，然后唤醒它返回。
这里并没有从头向尾寻找，从队列尾部开始向前查找，找到队列最前面没有被取消的节点，然后，将其唤醒。
为什么需要从队列尾部开始向前查找呢？
因为在CLH队列中的结点随时有可能被中断，被中断的结点的waitStatus设置为CANCEL,而且它会被踢出CLH队列，如何个踢出法，就是它的前趋结点的next并不会指向它，而是指向下一个非CANCEL的结点,而它自己的next指针指向它自己。一旦这种情况发生，如何从头向尾方向寻找继任结点会出现问题，因为一个CANCEL结点的next为自己，那么就找不到正确的继任接点。
有的人又会问了，CANCEL结点的next指针为什么要指向它自己，为什么不指向真正的next结点？为什么不为NULL？
第一个问题的答案是这种被CANCEL的结点最终会被GC回收，如果指向next结点，GC无法回收。
