除了内置锁（synchronized）外，java AQS（AbstractQueuedSynchronizer）同步器几乎是所有同步容器，同步工具类的基础。ReentrantLock、ReentrantReadWriteLock就是通过内部类继承并实现AQS的接口来实现相关功能的。
查看AQS的源码，包含以下几个内容：
- Node数据结构定义，用于存放被阻塞（自旋或者挂起）的线程信息，或者存放等待condition的线程，通过标记区别，共用统一数据结构
- node队列通用操作，包括队列操作和release操作（释放资源），node队列扩展于CLH队列，
- 各种acquire操作（获取资源）的通用方法
- 暴露的方法，包括tryAcquire/tryRelease等非阻塞接口方法，以及acquire/release等阻塞方法
- 队列检查方法，包括队列中是否还有node、线程是否在队列中等
- Condition相关数据结构以及方法
- node节点字段以及同步器字段field的CAS辅助方法，用于实现原子CAS操作
Node节点的数据结构如下所示:
static final class Node { /** Marker to indicate a node is waiting in shared mode */ // 非独占 static final Node SHARED = new Node(); /** Marker to indicate a node is waiting in exclusive mode */ // 独占 static final Node EXCLUSIVE = null; /** waitStatus value to indicate thread has cancelled */ // 线程已取消 static final int CANCELLED = 1; /** waitStatus value to indicate successor's thread needs unparking */ // 后续节点线程需要调度 static final int SIGNAL = -1; /** waitStatus value to indicate thread is waiting on condition */ // 线程等待某一条件 static final int CONDITION = -2; /** * waitStatus value to indicate the next acquireShared should * unconditionally propagate */ // 下次acquireShared无条件传递 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 enqueing, 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; } /** * 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; } }