AQS 通过内部实现的 FIFO 等待队列来完成资源获取线程的等待工作，如果当前线程获取资源事变，AQS 则会将当前线程以及等待状态等信息构造成一个 Node 节点，并将其加入等待队列中，同时会阻塞当前线程；当其它获取到资源的线程释放持有的资源时，则会把等待队列中的线程唤醒，使其再次尝试获取对应的资源。
AbstractQueuedSynchronizer 同步器
队列
首先从源码看一下同步队列的结构
/**
* 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.
*/
// 等待队列头结点
private transient volatile Node head;
/**
* Tail of the wait queue, lazily initialized. Modified only via
* method enq to add new wait node.
*/
// 等待队列的尾结点
private transient volatile Node tail;
/**
* The synchronization state.
*/
// 同步状态
private volatile int state;同步队列的结构如下图：
AQS 同步器类中的其他方法主要是用于插入节点、释放节点：
同步状态
AQS 类使用单个 int（32 位）来保存同步状态，并暴露出 getState、setState 以及 compareAndSet 操作来读取和更新这个同步状态。state 被声明为 volatile，并且通过 CAS 操作来实现 compareAndSetState，使得当且仅当同步状态拥有一个一致的期望值时，才会被原子的设置成新值，这样就达到了同步状态的原子性管理，确保了同步状态的原子性、可见性和有序性。
基于 AQS 实现的类必须根据暴露出的状态相关方法定义 tryAcquire 和 tryRelease 方法，以控制 acquire 和 release 操作。当同步状态满足时，tryAcquire 方法必须返回 true，而当新的同步状态允许后续 acquire 时，tryRelease 方法也必须返回 true。
/**
* Returns the current value of synchronization state.
* This operation has memory semantics of a {@code volatile} read.
* @return current state value
*/
// 获取变量的读取状态
protected final int getState() {return state;}
/**
* Sets the value of synchronization state.
* This operation has memory semantics of a {@code volatile} write.
* @param newState the new state value
*/
// 写入变量的同步状态
protected final void setState(int newState) {state = newState;}
/**
* Atomically sets synchronization state to the given updated
* value if the current state value equals the expected value.
* This operation has memory semantics of a {@code volatile} read
* and write.
*
* @param expect the expected value
* @param update the new value
* @return {@code true} if successful. False return indicates that the actual
* value was not equal to the expected value.
*/
// 使用 CAS 设置同步状态，确保线程安全
protected final boolean compareAndSetState(int expect, int update) {
// See below for intrinsics setup to support this
return unsafe.compareAndSwapInt(this, stateOffset, expect, update);
}Node 节点
static final class Node {
/** Marker to indicate a node is waiting in shared mode */
// 声明共享模式下的等待节点
static final Node SHARED = new Node();
/** Marker to indicate a node is waiting in exclusive mode */
// 声明独占模式下的等待节点
static final Node EXCLUSIVE = null;
/** waitStatus value to indicate thread has cancelled */
// waitStatus 的常量值 -- 表示线程已取消
static final int CANCELLED = 1;
/** waitStatus value to indicate successor's thread needs unparking */
// waitStatus 的常量值 -- 表示后继线程需要取消挂起
static final int SIGNAL = -1;
/** waitStatus value to indicate thread is waiting on condition */
// waitStatus 的常量值 -- 表示线程正在等待条件
static final int CONDITION = -2;
/**
* waitStatus value to indicate the next acquireShared should
* unconditionally propagate
*/
// waitStatus 的常量值 -- 表示下一个 acquireShared 应无条件传播
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).
*/
// 其值只能为 CANCELLED、SIGNAL、CONDITION、PROPAGATE 或 0
// 初始值为 0
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.
*/
// 前驱节点
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.
*/
// 后继节点
volatile Node next;
/**
* The thread that enqueued this node. Initialized on
* construction and nulled out after use.
*/
// 当前节点的线程，在节点初始化时赋值，使用后为 null
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.
*/
// 下一个等待节点
Node nextWaiter;
/**
* Returns true if node is waiting in shared mode.
*/
final boolean isShared() {return nextWaiter == SHARED;}
/**
* Returns previous node, or throws NullPointerException if null.
* Use when predecessor cannot be null. The null check could
* be elided, but is present to help the VM.
*
* @return the predecessor of this node
*/
final Node predecessor() throws NullPointerException {
Node p = prev;
if (p == null)
throw new NullPointerException();
else
return p;
}
Node() { // Used to establish initial head or SHARED marker}
Node(Thread thread, Node mode) { // Used by addWaiter
this.nextWaiter = mode;
this.thread = thread;
}
Node(Thread thread, int waitStatus) { // Used by Condition
this.waitStatus = waitStatus;
this.thread = thread;
}
}Node 节点的结构如下图：