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Java中的AQS（AbstractQueuedSynchronizer，抽象队列同步器）是用来实现锁及其他同步功能组件的Java底层技术基础，java.util.concurrent包下大部分类的实现都离不开它。
通过继承AQS：
1、ReentrantLock的内部类实现了公平锁和非公平锁；
2、CountDownLatch的内部类实现了发令枪；
3、ReentrantReadWriteLock的内部类实现了独享锁和共享锁；
4、Semaphore的内部类实现了公平锁和非公平锁。
AQS主要实现两大功能：独占（Exclusive，有时也叫排他）和共享（Share）。
AQS在内部维护一个FIFO（First In First Out，先进先出）的CLH（Craig，Landin，and Hagersten）线程阻塞队列和一个资源同步状态的变量volatile int state。
CLH是一个虚拟的双向队列，也就是不存在队列实例，仅存在节点之间的关联关系的队列。AQS是将每一条请求共享资源的线程，封装成一个CLH线程队列节点（Node），从而实现锁的分配。因此，说了一大堆，用一句简单的话来形容AQS就是：基于CLH线程阻塞队列，通过volatile变量 + CAS + 自旋方式来改变线程状态，成功则获取锁，失败则进入CLH队列。
AQS已经实现了CLH线程阻塞队列的维护，所以一般子类自定义实现AQS，要么是独占，要么是共享，也就是要么实现tryAcquire()和tryRelease()等系列方法，要么实现tryAcquireShared()和tryReleaseShared()等系列方法。
CLH队列由多个node节点组成，而且大量使用“CAS自旋volatile变量”这种经典代码：
CLH队列的结构为：
给CLH设置首节点：
给CLH设置尾节点：
整个AQS的流程如图：
AQS特别复杂，如果想把多线程搞透的，就需要深入研究每个方法的流程，拿acquire(int)方法的执行流程为例：
我把AQS的源码做了较为详细的注释，可以结合注释看看。例如：
/** * Provides a framework for implementing blocking locks and related * synchronizers (semaphores, events, etc) that rely on first-in-first-out * (FIFO) wait queues. 提供了一个实现阻塞锁和依赖FIFO的等待队列的相关的同步器（信号灯、事件等）框架 * * This class is designed to be a useful basis for most kinds of synchronizers * that rely on a single atomic {@code int} value to represent state. * 这个类对于大多数使用一个单独原子类的int值来表示状态的同步器很有用 * * Subclasses must define the protected methods that change this state, and * which define what that state means in terms of this object being acquired or * released. 子类必须定义protected方法来改变这个状态值，并且定义状态值是获取还是释放对象 * * Given these, the other methods in this class carry out all queuing and * blocking mechanics. 鉴于此，这个类中的其他方法实现了所有排队和阻塞的机制 * * Subclasses can maintain other state fields, but only the atomically updated * {@code int} value manipulated using methods {@link #getState}, * {@link #setState} and {@link #compareAndSetState} is tracked with respect to * synchronization. 子类可以维护其他的状态值字段，但只有getState、setState和compareAndSetState * 方法是通过原子更新来实现同步的 * ** Subclasses should be defined as non-public internal helper classes that are * used to implement the synchronization properties of their enclosing class. * 子类应该定义成非public的内部helper工具类，用于实现其封闭类的同步属性 * * Class {
@code AbstractQueuedSynchronizer} does not implement any * synchronization interface. AbstractQueuedSynchronizer类没有实现任何同步接口 * * Instead it defines methods such as {@link #acquireInterruptibly} that can be * invoked as appropriate by concrete locks and related synchronizers to * implement their public methods. * 取而代之的是，它定义了像acquireInterruptibly这样的方法，通过调用恰当的具体 锁和相关同步器方法，以便实现他们自己的公共方法 * ** This class supports either or both a default exclusive mode and a * shared mode. 这个类既支持默认的独占模式，也支持共享模式，也支持两种模式一起实现 * * When acquired in exclusive mode, attempted acquires by other threads cannot * succeed. 当在独占模式获取到锁时，其他线程再尝试获取锁会失败 * * Shared mode acquires by multiple threads may (but need not) succeed. * 共享模式，多个线程都能成功获取到锁 * * This class does not understand these differences except in the mechanical * sense that when a shared mode acquire succeeds, the next waiting thread (if * one exists) must also determine whether it can acquire as well. * 这个类不会理解机制的不同，共享模式中的一个线程获取锁成功了，下一个线程 （如果存在）仍然会去确定它自己是否也可以获取 * * Threads waiting in the different modes share the same FIFO queue. * 线程虽在不同的模式中，却都在等待共享相同的FIFO队列 * * Usually, implementation subclasses support only one of these modes, but both * can come into play for example in a {
@link ReadWriteLock}. * 通常，子类只需要实现这两种模式中的一种，但也能两种都实现，例如ReadWriteLock * * Subclasses that support only exclusive or only shared modes need not define * the methods supporting the unused mode. 仅支持一种模式的子类，不必定义另一种模式下的方法 * ** This class defines a nested {
@link ConditionObject} class that can be used as * a {@link Condition} implementation by subclasses supporting exclusive mode * for which method {@link#isHeldExclusively} reports whether synchronization is * exclusively held with respect to the current thread, method {@link #release} * invoked with the current {@link #getState} value fully releases this object, * and {@link #acquire}, given this saved state value, eventually restores this * object to its previous acquired state. * 这个类定义了一个嵌套的ConditionObject类，该类可以被支持独占模式的子类用作 * Condition实现，为此，isHeldExclusively()报告当前线程是否持续保持同步， * release方法通过调用getState来完全释放当前对象，并且将当前的资源状态 再保存到state中，最后会将此对象恢复为先前的获取状态 * * No {@code AbstractQueuedSynchronizer} method otherwise creates such a * condition, so if this constraint cannot be met, do not use it. * 没有AbstractQueuedSynchronizer方法去创建condition，因此如果不能满足 这个约束，就不要使用它 * * The behavior of {@link ConditionObject} depends of course on the semantics of * its synchronizer implementation. ConditionObject的行为依赖于其同步器实现的语义 * ** This class provides inspection, instrumentation, and monitoring methods for * the internal queue, as well as similar methods for condition objects. * 这个类提供检查、追踪和监控内部队列的方法，类似于condition对象的方法 * * These can be exported as desired into classes using an * {
@code AbstractQueuedSynchronizer} for their synchronization mechanics. * 可以根据需要使用AbstractQueuedSynchronizer，将它们导入到类中以实现其同步机制 * ** Serialization of this class stores only the underlying atomic integer * maintaining state, so deserialized objects have empty thread queues. * 这个类仅序列化state的原子值，因此反序列化出来的对象中的线程队列是空的 * * Typical subclasses requiring serializability will define a {
@code readObject} * method that restores this to a known initial state upon deserialization. * 需要序列化的子类可以在反序列化的时候定义一个readObject方法来恢复已知的初始状态 * * *Usage
使用 * ** To use this class as the basis of a synchronizer, redefine the following * methods, as applicable, by inspecting and/or modifying the synchronization * state using {
@link #getState}, {@link #setState} and/or * {@link #compareAndSetState}: 使用这个类作为同步器锁，需要重新定义以下方法： * **
@link #tryAcquire} *- {
{ @link #tryRelease} *{ @link #tryAcquireShared} *{ @link #tryReleaseShared} *{ @link #isHeldExclusively} * * * Each of these methods by default throws * {@link UnsupportedOperationException}. * 这些方法默认抛出UnsupportedOperationException异常 * * Implementations of these methods must be internally thread-safe, and should * in general be short and not block. 这些方法的实现必须在内部是线程安全的，而且通常都很简短，没有阻塞 * * Defining these methods is the only supported means of using this * class. 定义这些方法是使用这个类唯一可行的方式 * * All other methods are declared {@code final} because they cannot be * independently varied. 所有其他的方法都被声明为final，因为他们无法独自变化 * ** You may also find the inherited methods from * {
@link AbstractOwnableSynchronizer} useful to keep track of the thread owning * an exclusive synchronizer. * 你可能也发现了继承自AbstractOwnableSynchronizer的方法对于跟踪拥有独占同步器的线程很有用 * * You are encouraged to use them -- this enables monitoring and diagnostic * tools to assist users in determining which threads hold locks. * 鼓励你使用它们——这使得监控和诊断工具能够帮助用户确定那些线程持有锁 * ** Even though this class is based on an internal FIFO queue, it does not * automatically enforce FIFO acquisition policies. * 即使这个类是基于一个内部的FIFO队列，它也不会自动执行FIFO获得策略 * * The core of exclusive synchronization takes the form: 独占锁的核心采用以下形式： * *
* Acquire方法: * while (!tryAcquire(arg)) { * enqueue thread if it is not already queued; * 使线程入队，如果它还没有在队列中的话 * possibly block current thread; * 可能会阻塞当前线程 * } * * Release方法: * if (tryRelease(arg)) * unblock the first queued thread; * 解锁队列中的第一个线程 ** * (Shared mode is similar but may involve cascading signals.) 共享模式类似，但可能涉及级联信号 * *
* Because checks in acquire are invoked before enqueuing, a newly acquiring * thread may barge ahead of others that are blocked and queued. * 因为进入队列之前检查锁的获取，因此一个新的线程可能会插入其他阻塞或排队的线程之前 * * However, you can, if desired, define {
@code tryAcquire} and/or * {@code tryAcquireShared} to disable barging by internally invoking one or * more of the inspection methods, thereby providing a fair FIFO * acquisition order. 但如果你愿意的话，可以定义tryAcquire和/或tryAcquireShared方法禁止插队，从而提供 * 一个公平的获取顺序 * * In particular, most fair synchronizers can define {@code tryAcquire} to * return {@code false} if {@link #hasQueuedPredecessors} (a method specifically * designed to be used by fair synchronizers) returns {@code true}. * 尤其是，如果hasQueuedPredecessors（专用于公平锁的方法）返回true，大多数公平锁 可以定义tryAcquire方法返回false * * Other variations are possible. 其他变化也是可能的 * ** Throughput and scalability are generally highest for the default barging * (also known as greedy, renouncement, and * convoy-avoidance) strategy. * 对于默认插入（也称为greedy，renouncement和convoy-avoidance）策略， 吞吐量和可扩展性通常是最高的 * * While this is not guaranteed to be fair or starvation-free, earlier queued * threads are allowed to recontend before later queued threads, and each * recontention has an unbiased chance to succeed against incoming threads. * 尽管这不能保证公平，也不能保证没有饥饿，但是可以让较早排队的线程在较 晚排队的线程之前进行重新竞争 * * Also, while acquires do not spin in the usual sense, they may perform * multiple invocations of {
@code tryAcquire} interspersed with other * computations before blocking. * 同样，尽管获得锁通常不会自旋，但它们在阻塞之前，可以执行多个对tryAcquire的调用与其他阻塞前的计算 * * This gives most of the benefits of spins when exclusive synchronization is * only briefly held, without most of the liabilities when it isn't. * 这提供了自旋的大部分好处，而在不进行排他同步时，也不会带来很多负担 * * If so desired, you can augment this by preceding calls to acquire methods * with "fast-path" checks, possibly prechecking {@link #hasContended} and/or * {@link #hasQueuedThreads} to only do so if the synchronizer is likely not to * be contended. 如果需要，你可以通过在调用之前对获取方法进行“快速路径”检查来增强此功能， * 可能会预先检查hasContended和/或hasQueuedThreads * ** This class provides an efficient and scalable basis for synchronization in * part by specializing its range of use to synchronizers that can rely on * {
@code int} state, acquire, and release parameters, and an internal FIFO wait * queue. 此类为同步提供了有效且可扩展的基础，部分原因是依赖于使用state，获取和释放参数 以及内部FIFO等待队列的同步器 * * When this does not suffice, you can build synchronizers from a lower level * using {@link java.util.concurrent.atomic atomic} classes, your own custom * {@link java.util.Queue} classes, and {@link LockSupport} blocking support. * 如果这不够，你可以使用原子类、实现Queue接口和LockSupport提供低级别的阻塞支持 * *Usage Examples
使用示例 * *
* Here is a non-reentrant mutual exclusion lock class that uses the value zero * to represent the unlocked state, and one to represent the locked state. * 这是一个非重入互斥独占锁类，使用0表示非锁定状态，1表示锁定状态 * * While a non-reentrant lock does not strictly require recording of the current * owner thread, this class does so anyway to make usage easier to monitor. * 而非重入锁并不严格要求记录当前所有者线程，无论如何，这样做是为了更易于使用 * * It also supports conditions and exposes one of the instrumentation methods: * 它也支持conditions并公开了一种检测方法： * *
* {@code * class Mutex implements Lock, java.io.Serializable { * * // Our internal helper class * // 内部helper类 * private static class Sync extends AbstractQueuedSynchronizer { * // Reports whether in locked state * // 是否持有锁 * protected boolean isHeldExclusively() { * return getState() == 1; * } * * // Acquires the lock if state is zero * // 如果state是0就获得锁 * public boolean tryAcquire(int acquires) { * assert acquires == 1; // Otherwise unused 断言acquires=1，否则退出 * if (compareAndSetState(0, 1)) { * setExclusiveOwnerThread(Thread.currentThread()); * return true; * } * return false; * } * * // Releases the lock by setting state to zero * // 通过设置state=0来释放锁 * protected boolean tryRelease(int releases) { * assert releases == 1; // Otherwise unused 断言acquires=1，否则退出 * if (getState() == 0) throw new IllegalMonitorStateException(); * setExclusiveOwnerThread(null); * setState(0); * return true; * } * * // Provides a Condition * Condition newCondition() { * return new ConditionObject(); * } * * // Deserializes properly * // 反序列化 * private void readObject(ObjectInputStream s) * throws IOException, ClassNotFoundException { * s.defaultReadObject(); * setState(0); // reset to unlocked state * } * } * * // The sync object does all the hard work. We just forward to it. * // 同步对象完成了所有困难的工作，我们只需要利用它实现下面的方法 * * private final Sync sync = new Sync(); * * public void lock() { sync.acquire(1); } * public boolean tryLock() { return sync.tryAcquire(1); } * public void unlock() { sync.release(1); } * public Condition newCondition() { return sync.newCondition(); } * public boolean isLocked() { return sync.isHeldExclusively(); } * public boolean hasQueuedThreads() { return sync.hasQueuedThreads(); } * public void lockInterruptibly() throws InterruptedException { * sync.acquireInterruptibly(1); * } * public boolean tryLock(long timeout, TimeUnit unit) throws InterruptedException { * return sync.tryAcquireNanos(1, unit.toNanos(timeout)); * } * }} *
* *
* Here is a latch class that is like a * {@link java.util.concurrent.CountDownLatch CountDownLatch} except that it * only requires a single {@code signal} to fire. * 这是一个和CountDownLatch类很像的latch类，除了它仅仅需要一个获取信号启动外 * * Because a latch is non-exclusive, it uses the {@code shared} acquire and * release methods. 因为latch类是一个非独占锁，它使用共享的获取和释放方法 * *
* { * @code * class BooleanLatch { * * private static class Sync extends AbstractQueuedSynchronizer { * boolean isSignalled() { * return getState() != 0; * } * * protected int tryAcquireShared(int ignore) { * return isSignalled() ? 1 : -1; * } * * protected boolean tryReleaseShared(int ignore) { * setState(1); * return true; * } * } * * private final Sync sync = new Sync(); * * public boolean isSignalled() { * return sync.isSignalled(); * } * * public void signal() { * sync.releaseShared(1); * } * * public void await() throws InterruptedException { * sync.acquireSharedInterruptibly(1); * } * } * } * * * @since 1.5 * @author Doug Lea */public abstract class AbstractQueuedSynchronizer extends AbstractOwnableSynchronizer implements java.io.Serializable { private static final long serialVersionUID = 7373984972572414691L; /** * Creates a new {@code AbstractQueuedSynchronizer} instance with initial * synchronization state of zero. */ /** * 用0初始化state同步状态，创建一个新的AbstractQueuedSynchronizer实例 */ protected AbstractQueuedSynchronizer() { } /** * Wait queue node class. 等待队列的Node类 * *
* The wait queue is a variant of a "CLH" (Craig, Landin, and Hagersten) lock * queue. 等待队列是CLH锁队列的变体 * * CLH locks are normally used for spinlocks. CLH锁通常用于自旋锁 * * We instead use them for blocking synchronizers, but use the same basic tactic * of holding some of the control information about a thread in the predecessor * of its node. 我们将用他们用于阻塞同步器，但使用相同的基本策略， 将有关线程的某些控制信息保存在其节点的前继节点中 * * A "status" field in each node keeps track of whether a thread should block. * 每个节点中的“status”字段都保持线程是否应该阻塞的状态 * * A node is signalled when its predecessor releases. 当节点的前继释放时，会给当前节点发信号 * * Each node of the queue otherwise serves as a specific-notification-style * monitor holding a single waiting thread. The status field does NOT control * whether threads are granted locks etc though. A thread may try to acquire if * it is first in the queue. But being first does not guarantee success; it only * gives the right to contend. So the currently released contender thread may * need to rewait. * *
* To enqueue into a CLH lock, you atomically splice it in as new tail. To * dequeue, you just set the head field. * *
* +------+ prev +-----+ +-----+ * head | | <---- | | <---- | | tail * +------+ +-----+ +-----+ *
* *
* Insertion into a CLH queue requires only a single atomic operation on "tail", * so there is a simple atomic point of demarcation from unqueued to queued. * Similarly, dequeuing involves only updating the "head". However, it takes a * bit more work for nodes to determine who their successors are, in part to * deal with possible cancellation due to timeouts and interrupts. * 插入到CLH队列中只需要对tail执行一次原子操作，因此存在一个简单的原子分界点，即从未排队到排队 * 同样，出队仅涉及更新head。但是，节点需要花费更多的精力来确定其后继者是谁， * 部分原因是要处理由于超时和中断而可能导致的取消 * *
* The "prev" links (not used in original CLH locks), are mainly needed to * handle cancellation. If a node is cancelled, its successor is (normally) * relinked to a non-cancelled predecessor. For explanation of similar mechanics * in the case of spin locks, see the papers by Scott and Scherer at * http://www.cs.rochester.edu/u/scott/synchronization/ * *
* We also use "next" links to implement blocking mechanics. The thread id for * each node is kept in its own node, so a predecessor signals the next node to * wake up by traversing next link to determine which thread it is. * Determination of successor must avoid races with newly queued nodes to set * the "next" fields of their predecessors. This is solved when necessary by * checking backwards from the atomically updated "tail" when a node's successor * appears to be null. (Or, said differently, the next-links are an optimization * so that we don't usually need a backward scan.) * *
* Cancellation introduces some conservatism to the basic algorithms. Since we * must poll for cancellation of other nodes, we can miss noticing whether a * cancelled node is ahead or behind us. This is dealt with by always unparking * successors upon cancellation, allowing them to stabilize on a new * predecessor, unless we can identify an uncancelled predecessor who will carry * this responsibility. * *
* CLH queues need a dummy header node to get started. But we don't create them * on construction, because it would be wasted effort if there is never * contention. Instead, the node is constructed and head and tail pointers are * set upon first contention. * *
* Threads waiting on Conditions use the same nodes, but use an additional link. * Conditions only need to link nodes in simple (non-concurrent) linked queues * because they are only accessed when exclusively held. Upon await, a node is * inserted into a condition queue. Upon signal, the node is transferred to the * main queue. A special value of status field is used to mark which queue a * node is on. * *
* Thanks go to Dave Dice, Mark Moir, Victor Luchangco, Bill Scherer and Michael * Scott, along with members of JSR-166 expert group, for helpful ideas, * discussions, and critiques on the design of this class. */ 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. * 值为-1，表示当前节点的的后继节点将要或者已经被阻塞，在当前节点释放的时候需要unpark（唤醒）后继节点 * * 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. * 值为1，表示当前节点被取消 * * 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.) * 值为-2，表示当前节点在等待condition，即在condition队列中 * * 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 * 值为-3，表示releaseShared需要被传播给后续节点（仅在共享模式下使用） * * 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). * 无状态，表示当前节点在队列中等待获取锁 * */ 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; } /** * 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. * 返回前继节点，如果为空则抛出异常 * * @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; } } /** * 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. 等待队列头部节点，懒加载，它仅仅通过setHead方法修改 * 注意：如果头部节点存在，它的等待状态不保证会是CANCELLED */ private transient volatile Node head; /** * Tail of the wait queue, lazily initialized. Modified only via method enq to * add new wait node. 等待队列的队尾节点，懒加载，只能通过enq方法加载新节点到队尾 */ private transient volatile Node tail; /** * The synchronization state. 同步状态 * 该变量对不同的子类实现具有不同的意义 * 对ReentrantLock来说，它表示加锁的状态： * 无锁时state=0，有锁时state>0 * 第一次加锁时，将state+1 * 而对于CountDownLatch来说，它是初始化时子线程的数量 * */ private volatile int state; /** * Returns the current value of synchronization state. This operation has memory * semantics of a {@code volatile} read. * * @return current state value */ 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 */ 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. 以原子方式设置同步状态为指定的值 * * @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. */ protected final boolean compareAndSetState(int expect, int update) { // See below for intrinsics setup to support this return unsafe.compareAndSwapInt(this, stateOffset, expect, update); } // Queuing utilities /** * The number of nanoseconds for which it is faster to spin rather than to use * timed park. A rough estimate suffices to improve responsiveness with very * short timeouts. 自旋超时时间，使用比park更快的纳秒，足以在非常短的时间内提高响应能力，默认值1000纳秒 */ static final long spinForTimeoutThreshold = 1000L; /** * Inserts node into queue, initializing if necessary. See picture above. * 插入节点到队尾，如果有必要的话初始化 * * @param node the node to insert * @return node's predecessor */ private Node enq(final Node node) { // 自旋 for (;;) { // 将队尾指针给当前节点 Node t = tail; if (t == null) { // Must initialize 必须初始化 // 如果尾节点为null，说明队列还没有任何节点，那么头节点也就是尾节点 if (compareAndSetHead(new Node())) { tail = head; } } else { // 否则尾节点成为当前待加入节点的前继节点 node.prev = t; // 将当前节点设置为尾节点 if (compareAndSetTail(t, node)) { // 尾节点的后续节点为当前节点 t.next = node; return t; } } } } /** * Creates and enqueues node for current thread and given mode. * 按给定模式将当前线程包装成一个入队的节点 * * @param mode Node.EXCLUSIVE for exclusive, Node.SHARED for shared * @return the new node */ private Node addWaiter(Node mode) { // 将当前线程包装成节点 Node node = new Node(Thread.currentThread(), mode); // Try the fast path of enq; backup to full enq on failure // 尝试快速入队 Node pred = tail; // 尾节点是否为null if (pred != null) { // 将尾节点设置为当前节点的前继节点 node.prev = pred; // 将当前节点设置为尾节点 if (compareAndSetTail(pred, node)) { // 尾节点的后续节点为当前节点 pred.next = node; return node; } } // 尾节点为null，则执行enq enq(node); // 返回当前节点 return node; } /** * Sets head of queue to be node, thus dequeuing. Called only by acquire * methods. Also nulls out unused fields for sake of GC and to suppress * unnecessary signals and traversals. * 将节点设置为队列头，从而让持有锁的节点出列，仅由acquire调用 * 为了GC和抑制不必要的信号和遍历，也会清空未使用的字段 * * @param node the node */ private void setHead(Node node) { // 将节点设置为队列头 head = node; // 头节点没有线程 node.thread = null; // 头节点没有前继节点 node.prev = null; } /** * Wakes up node's successor, if one exists. * 唤醒节点的后续节点 * * @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. * 如果状态值为负，就尝试清除预期信号值 * 如果失败或状态由等待线程更改，则OK */ 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); } } /** * Release action for shared mode -- signals successor and ensures propagation. * (Note: For exclusive mode, release just amounts to calling unparkSuccessor of * head if it needs signal.) * 共享模式下的释放行为——发出后续信号并确保传播 * （注意：对于独占模式，释放只是在需要信号时调用head的unparkSuccessor方法） * */ private void doReleaseShared() { /* * Ensure that a release propagates, even if there are other in-progress * acquires/releases. This proceeds in the usual way of trying to * unparkSuccessor of head if it needs signal. But if it does not, status is set * to PROPAGATE to ensure that upon release, propagation continues. * Additionally, we must loop in case a new node is added while we are doing * this. Also, unlike other uses of unparkSuccessor, we need to know if CAS to * reset status fails, if so rechecking. */ for (;;) { Node h = head; if (h != null && h != tail) { int ws = h.waitStatus; if (ws == Node.SIGNAL) { if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0)) { continue; // loop to recheck cases } unparkSuccessor(h); } else if (ws == 0 && !compareAndSetWaitStatus(h, 0, Node.PROPAGATE)) { continue; // loop on failed CAS } } if (h == head) { // loop if head changed break; } } } /** * Sets head of queue, and checks if successor may be waiting in shared mode, if * so propagating if either propagate > 0 or PROPAGATE status was set. * * @param node the node * @param propagate the return value from a tryAcquireShared */ private void setHeadAndPropagate(Node node, int propagate) { Node h = head; // Record old head for check below setHead(node); /* * Try to signal next queued node if: Propagation was indicated by caller, or * was recorded (as h.waitStatus either before or after setHead) by a previous * operation (note: this uses sign-check of waitStatus because PROPAGATE status * may transition to SIGNAL.) and The next node is waiting in shared mode, or we * don't know, because it appears null * * The conservatism in both of these checks may cause unnecessary wake-ups, but * only when there are multiple racing acquires/releases, so most need signals * now or soon anyway. */ if (propagate > 0 || h == null || h.waitStatus < 0 || (h = head) == null || h.waitStatus < 0) { Node s = node.next; if (s == null || s.isShared()) { doReleaseShared(); } } } // Utilities for various versions of acquire /** * Cancels an ongoing attempt to acquire. * 取消一个不断尝试获取锁的线程节点 * * @param node the node */ private void cancelAcquire(Node node) { // Ignore if node doesn't exist if (node == null) { return; } node.thread = null; // Skip cancelled predecessors Node pred = node.prev; while (pred.waitStatus > 0) { node.prev = pred = pred.prev; } // predNext is the apparent node to unsplice. CASes below will // fail if not, in which case, we lost race vs another cancel // or signal, so no further action is necessary. Node predNext = pred.next; // Can use unconditional write instead of CAS here. // After this atomic step, other Nodes can skip past us. // Before, we are free of interference from other threads. node.waitStatus = Node.CANCELLED; // If we are the tail, remove ourselves. if (node == tail && compareAndSetTail(node, pred)) { compareAndSetNext(pred, predNext, null); } else { // If successor needs signal, try to set pred's next-link // so it will get one. Otherwise wake it up to propagate. int ws; if (pred != head && ((ws = pred.waitStatus) == Node.SIGNAL || (ws <= 0 && compareAndSetWaitStatus(pred, ws, Node.SIGNAL))) && pred.thread != null) { Node next = node.next; if (next != null && next.waitStatus <= 0) { compareAndSetNext(pred, predNext, next); } } else { unparkSuccessor(node); } node.next = node; // help GC } } /** * 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. * 节点获取锁失败时检查并且更新状态值，如果线程应该阻塞返回true * 在所有获取锁的循环中这是主要的信号控制 * * @param pred node's predecessor holding status * @param node the node * @return {@code true} if thread should block */ private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) { int ws = pred.waitStatus; 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. */ 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. */ compareAndSetWaitStatus(pred, ws, Node.SIGNAL); } return false; } /** * Convenience method to interrupt current thread. * 中断当前线程的快捷方法 */ static void selfInterrupt() { Thread.currentThread().interrupt(); } /** * Convenience method to park and then check if interrupted * * @return {@code true} if interrupted */ private final boolean parkAndCheckInterrupt() { LockSupport.park(this); return Thread.interrupted(); } /* * Various flavors of acquire, varying in exclusive/shared and control modes. * Each is mostly the same, but annoyingly different. Only a little bit of * factoring is possible due to interactions of exception mechanics (including * ensuring that we cancel if tryAcquire throws exception) and other control, at * least not without hurting performance too much. * 在独占和共享模式中，获取锁有多种方式，大多数都相同 * 由于异常机制（包括确保在tryAcquire抛出异常时取消）和其他控件的交互， * 性能可能会受一点影响，但至少不会造成太大的损害 */ /** * Acquires in exclusive uninterruptible mode for thread already in queue. Used * by condition wait methods as well as acquire. * 以独占不中断模式获取队列中已存在的线程。用于condition等待方法以及获取锁 * * @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(); if (p == head && tryAcquire(arg)) { setHead(node); p.next = null; // help GC failed = false; return interrupted; } if (shouldParkAfterFailedAcquire(p, node) && parkAndCheckInterrupt()) { interrupted = true; } } } finally { if (failed) { cancelAcquire(node); } } } /** * Acquires in exclusive interruptible mode. * 独占中断模式获取锁 * * @param arg the acquire argument */ private void doAcquireInterruptibly(int arg) throws InterruptedException { final Node node = addWaiter(Node.EXCLUSIVE); boolean failed = true; try { for (;;) { final Node p = node.predecessor(); if (p == head && tryAcquire(arg)) { setHead(node); p.next = null; // help GC failed = false; return; } if (shouldParkAfterFailedAcquire(p, node) && parkAndCheckInterrupt()) throw new InterruptedException(); } } finally { if (failed) cancelAcquire(node); } } /** * Acquires in exclusive timed mode. * 独占超时模式获取锁 * * @param arg the acquire argument * @param nanosTimeout max wait time * @return {@code true} if acquired */ private boolean doAcquireNanos(int arg, long nanosTimeout) throws InterruptedException { if (nanosTimeout <= 0L) { return false; } final long deadline = System.nanoTime() + nanosTimeout; final Node node = addWaiter(Node.EXCLUSIVE); boolean failed = true; try { for (;;) { final Node p = node.predecessor(); if (p == head && tryAcquire(arg)) { setHead(node); p.next = null; // help GC failed = false; return true; } nanosTimeout = deadline - System.nanoTime(); if (nanosTimeout <= 0L) { return false; } if (shouldParkAfterFailedAcquire(p, node) && nanosTimeout > spinForTimeoutThreshold) { LockSupport.parkNanos(this, nanosTimeout); } if (Thread.interrupted()) { throw new InterruptedException(); } } } finally { if (failed) { cancelAcquire(node); } } } /** * Acquires in shared uninterruptible mode. * 共享非中断模式获取锁 * * @param arg the acquire argument */ private void doAcquireShared(int arg) { final Node node = addWaiter(Node.SHARED); boolean failed = true; try { boolean interrupted = false; for (;;) { final Node p = node.predecessor(); if (p == head) { int r = tryAcquireShared(arg); if (r >= 0) { setHeadAndPropagate(node, r); p.next = null; // help GC if (interrupted) { selfInterrupt(); } failed = false; return; } } if (shouldParkAfterFailedAcquire(p, node) && parkAndCheckInterrupt()) { interrupted = true; } } } finally { if (failed) { cancelAcquire(node); } } } /** * Acquires in shared interruptible mode. * 共享中断模式获取锁 * * @param arg the acquire argument */ private void doAcquireSharedInterruptibly(int arg) throws InterruptedException { final Node node = addWaiter(Node.SHARED); boolean failed = true; try { for (;;) { final Node p = node.predecessor(); if (p == head) { int r = tryAcquireShared(arg); if (r >= 0) { setHeadAndPropagate(node, r); p.next = null; // help GC failed = false; return; } } if (shouldParkAfterFailedAcquire(p, node) && parkAndCheckInterrupt()) { throw new InterruptedException(); } } } finally { if (failed) { cancelAcquire(node); } } } /** * Acquires in shared timed mode. * 共享超时模式获取锁 * * @param arg the acquire argument * @param nanosTimeout max wait time * @return {@code true} if acquired */ private boolean doAcquireSharedNanos(int arg, long nanosTimeout) throws InterruptedException { if (nanosTimeout <= 0L) { return false; } final long deadline = System.nanoTime() + nanosTimeout; final Node node = addWaiter(Node.SHARED); boolean failed = true; try { for (;;) { final Node p = node.predecessor(); if (p == head) { int r = tryAcquireShared(arg); if (r >= 0) { setHeadAndPropagate(node, r); p.next = null; // help GC failed = false; return true; } } nanosTimeout = deadline - System.nanoTime(); if (nanosTimeout <= 0L) { return false; } if (shouldParkAfterFailedAcquire(p, node) && nanosTimeout > spinForTimeoutThreshold) { LockSupport.parkNanos(this, nanosTimeout); } if (Thread.interrupted()) { throw new InterruptedException(); } } } finally { if (failed) { cancelAcquire(node); } } } // Main exported methods // 主要的自定义方法 /** * Attempts to acquire in exclusive mode. This method should query if the state * of the object permits it to be acquired in the exclusive mode, and if so to * acquire it. * 尝试以独占模式获取锁，此方法应该查询对象的状态state是否允许以独占模式获取锁，如果允许则获取锁 * *
* This method is always invoked by the thread performing acquire. If this * method reports failure, the acquire method may queue the thread, if it is not * already queued, until it is signalled by a release from some other thread. * This can be used to implement method {@link Lock#tryLock()}. * 此方法始终由执行获取锁的线程调用，如果获取失败，则会将线程放到CLH队列队尾（如果尚未排队）， * 直到某个其他线程发出释放信号，这可用于实现接口方法tryLock * *
* The default implementation throws {@link UnsupportedOperationException}. * 缺省实现是抛出UnsupportedOperationException异常 * * @param arg the acquire argument. This value is always the one passed to an * acquire method, or is the value saved on entry to a condition * wait. The value is otherwise uninterpreted and can represent * anything you like. * 获取锁的参数，表示需要获取锁的数量 * * @return {@code true} if successful. Upon success, this object has been acquired. * @throws IllegalMonitorStateException * if acquiring would place this synchronizer in an illegal state. * This exception must be thrown in a consistent fashion for * synchronization to work correctly. * @throws UnsupportedOperationException * if exclusive mode is not supported */ protected boolean tryAcquire(int arg) { throw new UnsupportedOperationException(); } /** * Attempts to set the state to reflect a release in exclusive mode. * 尝试将状态state设置为以独占模式释放锁 * *
* This method is always invoked by the thread performing release. * 此方法始终由执行释放的线程调用 * *
* The default implementation throws {@link UnsupportedOperationException}. * 缺省实现是抛出UnsupportedOperationException异常 * * @param arg the release argument. This value is always the one passed to a * release method, or the current state value upon entry to a * condition wait. The value is otherwise uninterpreted and can * represent anything you like. * 释放锁的参数，表示需要释放锁的数量，与tryAcquire中需要获取的数量一一对应 * * @return {@code true} if this object is now in a fully released state, so that * any waiting threads may attempt to acquire; and {@code false} * otherwise. * @throws IllegalMonitorStateException * if releasing would place this synchronizer in an illegal state. * This exception must be thrown in a consistent fashion for * synchronization to work correctly. * @throws UnsupportedOperationException * if exclusive mode is not supported */ protected boolean tryRelease(int arg) { throw new UnsupportedOperationException(); } /** * Attempts to acquire in shared mode. This method should query if the state of * the object permits it to be acquired in the shared mode, and if so to acquire * it. * 共享模式尝试获取锁 * *
* This method is always invoked by the thread performing acquire. If this * method reports failure, the acquire method may queue the thread, if it is not * already queued, until it is signalled by a release from some other thread. * 此方法始终由执行获取的线程调用，如果调用失败，则会将线程放到CLH队列队尾（如果尚未排队）， * 直到某个其他线程发出释放信号 * *
* The default implementation throws {@link UnsupportedOperationException}. * 缺省实现是抛出UnsupportedOperationException异常 * * @param arg the acquire argument. This value is always the one passed to an * acquire method, or is the value saved on entry to a condition * wait. The value is otherwise uninterpreted and can represent * anything you like. * @return a negative value on failure; zero if acquisition in shared mode * succeeded but no subsequent shared-mode acquire can succeed; and a * positive value if acquisition in shared mode succeeded and subsequent * shared-mode acquires might also succeed, in which case a subsequent * waiting thread must check availability. (Support for three different * return values enables this method to be used in contexts where * acquires only sometimes act exclusively.) Upon success, this object * has been acquired. * @throws IllegalMonitorStateException * if acquiring would place this synchronizer in an illegal state. * This exception must be thrown in a consistent fashion for * synchronization to work correctly. * @throws UnsupportedOperationException * if shared mode is not supported */ protected int tryAcquireShared(int arg) { throw new UnsupportedOperationException(); } /** * Attempts to set the state to reflect a release in shared mode. * 尝试将状态state设置为以共享模式释放锁 * *
* This method is always invoked by the thread performing release. * 此方法始终由执行获取的线程调用 * *
* The default implementation throws {@link UnsupportedOperationException}. * 缺省实现是抛出UnsupportedOperationException异常 * * @param arg the release argument. This value is always the one passed to a * release method, or the current state value upon entry to a * condition wait. The value is otherwise uninterpreted and can * represent anything you like. * @return {@code true} if this release of shared mode may permit a waiting * acquire (shared or exclusive) to succeed; and {@code false} otherwise * @throws IllegalMonitorStateException * if releasing would place this synchronizer in an illegal state. * This exception must be thrown in a consistent fashion for * synchronization to work correctly. * @throws UnsupportedOperationException * if shared mode is not supported */ protected boolean tryReleaseShared(int arg) { throw new UnsupportedOperationException(); } /** * Returns {@code true} if synchronization is held exclusively with respect to * the current (calling) thread. This method is invoked upon each call to a * non-waiting {@link ConditionObject} method. (Waiting methods instead invoke * {@link #release}.) * 如果以独占方式保持与当前（调用）线程的同步，则返回true * 每次调用非等待的ConditionObject方法时都会调用此方法（等待方法改为调用release） * *
* The default implementation throws {@link UnsupportedOperationException}. This * method is invoked internally only within {@link ConditionObject} methods, so * need not be defined if conditions are not used. * 缺省实现是抛出UnsupportedOperationException异常 * 此方法仅在ConditionObject内部调用，因此如果不使用Condition，则无需定义 * * @return {@code true} if synchronization is held exclusively; {@code false} * otherwise * @throws UnsupportedOperationException if conditions are not supported */ protected boolean isHeldExclusively() { throw new UnsupportedOperationException(); } /** * 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}. * 以独占模式获取锁，忽略中断，通过调用至少一次tryAcquire来实现，成功时返回，否则线程将排队 * 可能会反复阻塞和解除阻塞，调用tryAcquire直到成功获取锁，此方法可用于实现接口方法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) { /** * 该方法主要做了如下工作： * 先看tryAcquire尝试获取独占锁是否成功，获取成功则返回 * 否则用addWaiter方法将当前线程封装成Node对象，并添加到队列尾部 * 自旋获取锁，并判断中断标志位 * 如果中断标志位为true，则设置中断线程，否则返回 */ if (!tryAcquire(arg) && acquireQueued(addWaiter(Node.EXCLUSIVE), arg)) { selfInterrupt(); } } /** * Acquires in exclusive mode, aborting if interrupted. Implemented by first * checking interrupt status, then invoking at least once {@link #tryAcquire}, * returning on success. Otherwise the thread is queued, possibly repeatedly * blocking and unblocking, invoking {@link #tryAcquire} until success or the * thread is interrupted. This method can be used to implement method * {@link Lock#lockInterruptibly}. * 独占模式获取锁，如果中断则取消 * 首先检查中断状态，然后至少调用一次tryAcquire来实现方法，在成功时返回，否则线程将进入队尾 * 可能会反复阻塞和解除阻塞，调用tryAcquire，直到成功或线程被中断 * 此方法可用于实现接口方法lockInterruptibly * * @param arg the acquire argument. This value is conveyed to * {@link #tryAcquire} but is otherwise uninterpreted and can * represent anything you like. * @throws InterruptedException if the current thread is interrupted */ public final void acquireInterruptibly(int arg) throws InterruptedException { if (Thread.interrupted()) { throw new InterruptedException(); } if (!tryAcquire(arg)) { doAcquireInterruptibly(arg); } } /** * Attempts to acquire in exclusive mode, aborting if interrupted, and failing * if the given timeout elapses. Implemented by first checking interrupt status, * then invoking at least once {@link #tryAcquire}, returning on success. * Otherwise, the thread is queued, possibly repeatedly blocking and unblocking, * invoking {@link #tryAcquire} until success or the thread is interrupted or * the timeout elapses. This method can be used to implement method * {@link Lock#tryLock(long, TimeUnit)}. * 尝试以独占模式获取锁，如果中断则中止，如果超时则失败 * 通过首先检查中断状态，然后至少调用一次tryAcquire来实现，在成功时返回，否则线程将进入队尾 * 可能会反复阻塞和解除阻塞，调用tryAcquire，直到成功或线程中断或超时结束 * 此方法可用于实现接口方法tryLock(long, TimeUnit) * * @param arg the acquire argument. This value is conveyed to * {@link #tryAcquire} but is otherwise uninterpreted and can * represent anything you like. * @param nanosTimeout the maximum number of nanoseconds to wait * @return {@code true} if acquired; {@code false} if timed out * @throws InterruptedException if the current thread is interrupted */ public final boolean tryAcquireNanos(int arg, long nanosTimeout) throws InterruptedException { if (Thread.interrupted()) { throw new InterruptedException(); } return tryAcquire(arg) || doAcquireNanos(arg, nanosTimeout); } /** * 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}. * 独占模式时释放锁，通过解除一个或多个阻塞线程来实现，如果tryRelease返回true * 此方法可用于实现接口方法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; } /** * Acquires in shared mode, ignoring interrupts. Implemented by first invoking * at least once {@link #tryAcquireShared}, returning on success. Otherwise the * thread is queued, possibly repeatedly blocking and unblocking, invoking * {@link #tryAcquireShared} until success. * * @param arg the acquire argument. This value is conveyed to * {@link #tryAcquireShared} but is otherwise uninterpreted and can * represent anything you like. */ public final void acquireShared(int arg) { if (tryAcquireShared(arg) < 0) { doAcquireShared(arg); } } /** * Acquires in shared mode, aborting if interrupted. Implemented by first * checking interrupt status, then invoking at least once * {@link #tryAcquireShared}, returning on success. Otherwise the thread is * queued, possibly repeatedly blocking and unblocking, invoking * {@link #tryAcquireShared} until success or the thread is interrupted. * * @param arg the acquire argument. This value is conveyed to * {@link #tryAcquireShared} but is otherwise uninterpreted and can * represent anything you like. * @throws InterruptedException * if the current thread is interrupted */ public final void acquireSharedInterruptibly(int arg) throws InterruptedException { if (Thread.interrupted()) { throw new InterruptedException(); } if (tryAcquireShared(arg) < 0) { doAcquireSharedInterruptibly(arg); } } /** * Attempts to acquire in shared mode, aborting if interrupted, and failing if * the given timeout elapses. Implemented by first checking interrupt status, * then invoking at least once {@link #tryAcquireShared}, returning on success. * Otherwise, the thread is queued, possibly repeatedly blocking and unblocking, * invoking {@link #tryAcquireShared} until success or the thread is interrupted * or the timeout elapses. * * @param arg the acquire argument. This value is conveyed to * {@link #tryAcquireShared} but is otherwise uninterpreted and can * represent anything you like. * @param nanosTimeout * the maximum number of nanoseconds to wait * @return {@code true} if acquired; {@code false} if timed out * @throws InterruptedException * if the current thread is interrupted */ public final boolean tryAcquireSharedNanos(int arg, long nanosTimeout) throws InterruptedException { if (Thread.interrupted()) { throw new InterruptedException(); } return tryAcquireShared(arg) >= 0 || doAcquireSharedNanos(arg, nanosTimeout); } /** * Releases in shared mode. Implemented by unblocking one or more threads if * {@link #tryReleaseShared} returns true. * * @param arg the release argument. This value is conveyed to * {@link #tryReleaseShared} but is otherwise uninterpreted and can * represent anything you like. * @return the value returned from {@link #tryReleaseShared} */ public final boolean releaseShared(int arg) { if (tryReleaseShared(arg)) { doReleaseShared(); return true; } return false; } // Queue inspection methods /** * Queries whether any threads are waiting to acquire. Note that because * cancellations due to interrupts and timeouts may occur at any time, a * {@code true} return does not guarantee that any other thread will ever * acquire. * 查询是否有线程正在等待获取锁 * 请注意，由于中断和超时导致的取消可能随时发生，因此返回true不能保证任何其他线程将获得锁 * *
* In this implementation, this operation returns in constant time. * 在该实现中，操作以指定的时间返回 * * @return {@code true} if there may be other threads waiting to acquire */ public final boolean hasQueuedThreads() { return head != tail; } /** * Queries whether any threads have ever contended to acquire this synchronizer; * that is if an acquire method has ever blocked. * 查询是否有任何线程曾争用获取此同步器，也就是说，是否某个获取锁方法曾被阻塞 * *
* In this implementation, this operation returns in constant time. * 在该实现中，操作以指定的时间返回 * * @return {@code true} if there has ever been contention */ public final boolean hasContended() { return head != null; } /** * Returns the first (longest-waiting) thread in the queue, or {@code null} if * no threads are currently queued. * *
* In this implementation, this operation normally returns in constant time, but * may iterate upon contention if other threads are concurrently modifying the * queue. * * @return the first (longest-waiting) thread in the queue, or {@code null} if * no threads are currently queued */ public final Thread getFirstQueuedThread() { // handle only fast path, else relay return (head == tail) ? null : fullGetFirstQueuedThread(); } /** * Version of getFirstQueuedThread called when fastpath fails */ private Thread fullGetFirstQueuedThread() { /* * The first node is normally head.next. Try to get its thread field, ensuring * consistent reads: If thread field is nulled out or s.prev is no longer head, * then some other thread(s) concurrently performed setHead in between some of * our reads. We try this twice before resorting to traversal. */ Node h, s; Thread st; if (((h = head) != null && (s = h.next) != null && s.prev == head && (st = s.thread) != null) || ((h = head) != null && (s = h.next) != null && s.prev == head && (st = s.thread) != null)) { return st; } /* * Head's next field might not have been set yet, or may have been unset after * setHead. So we must check to see if tail is actually first node. If not, we * continue on, safely traversing from tail back to head to find first, * guaranteeing termination. */ Node t = tail; Thread firstThread = null; while (t != null && t != head) { Thread tt = t.thread; if (tt != null) { firstThread = tt; } t = t.prev; } return firstThread; } /** * Returns true if the given thread is currently queued. * *
* This implementation traverses the queue to determine presence of the given thread. * * @param thread the thread * @return {@code true} if the given thread is on the queue * @throws NullPointerException if the thread is null */ public final boolean isQueued(Thread thread) { if (thread == null) { throw new NullPointerException(); } for (Node p = tail; p != null; p = p.prev) { if (p.thread == thread) { return true; } } return false; } /** * Returns {@code true} if the apparent first queued thread, if one exists, is * waiting in exclusive mode. If this method returns {@code true}, and the * current thread is attempting to acquire in shared mode (that is, this method * is invoked from {@link #tryAcquireShared}) then it is guaranteed that the * current thread is not the first queued thread. Used only as a heuristic in * ReentrantReadWriteLock. */ final boolean apparentlyFirstQueuedIsExclusive() { Node h, s; return (h = head) != null && (s = h.next) != null && !s.isShared() && s.thread != null; } /** * Queries whether any threads have been waiting to acquire longer than the * current thread. * 查询是否有任何线程等待获取锁的时间超过当前线程 * *
* An invocation of this method is equivalent to (but may be more efficient * than): * 调用此方法相当于调用：getFirstQueuedThread() != Thread.currentThread() && hasQueuedThreads() * *
* {@code getFirstQueuedThread() != Thread.currentThread() && hasQueuedThreads()} * * *
* Note that because cancellations due to interrupts and timeouts may occur at * any time, a {@code true} return does not guarantee that some other thread * will acquire before the current thread. Likewise, it is possible for another * thread to win a race to enqueue after this method has returned {@code false}, * due to the queue being empty. * *
* This method is designed to be used by a fair synchronizer to avoid * barging. Such a * synchronizer's {@link #tryAcquire} method should return {@code false}, and * its {@link #tryAcquireShared} method should return a negative value, if this * method returns {@code true} (unless this is a reentrant acquire). For * example, the {@codetryAcquire} method for a fair, reentrant, exclusive mode * synchronizer might look like this: * *
* {@code * protected boolean tryAcquire(int arg) { * if (isHeldExclusively()) { * // A reentrant acquire; increment hold count * return true; * } else if (hasQueuedPredecessors()) { * return false; * } else { * // try to acquire normally * } * }} * * * @return {@code true} if there is a queued thread preceding the current * thread, and {@code false} if the current thread is at the head of the * queue or the queue is empty * @since 1.7 */ public final boolean hasQueuedPredecessors() { // The correctness of this depends on head being initialized // before tail and on head.next being accurate if the current // thread is first in queue. Node t = tail; // Read fields in reverse initialization order Node h = head; Node s; return h != t && ((s = h.next) == null || s.thread != Thread.currentThread()); } // Instrumentation and monitoring methods /** * Returns an estimate of the number of threads waiting to acquire. The value is * only an estimate because the number of threads may change dynamically while * this method traverses internal data structures. This method is designed for * use in monitoring system state, not for synchronization control. * * @return the estimated number of threads waiting to acquire */ public final int getQueueLength() { int n = 0; for (Node p = tail; p != null; p = p.prev) { if (p.thread != null) { ++n; } } return n; } /** * Returns a collection containing threads that may be waiting to acquire. * Because the actual set of threads may change dynamically while constructing * this result, the returned collection is only a best-effort estimate. The * elements of the returned collection are in no particular order. This method * is designed to facilitate construction of subclasses that provide more * extensive monitoring facilities. * * @return the collection of threads */ public final Collection getQueuedThreads() { ArrayList list = new ArrayList(); for (Node p = tail; p != null; p = p.prev) { Thread t = p.thread; if (t != null) { list.add(t); } } return list; } /** * Returns a collection containing threads that may be waiting to acquire in * exclusive mode. This has the same properties as {@link #getQueuedThreads} * except that it only returns those threads waiting due to an exclusive * acquire. * * @return the collection of threads */ public final Collection getExclusiveQueuedThreads() { ArrayList list = new ArrayList(); for (Node p = tail; p != null; p = p.prev) { if (!p.isShared()) { Thread t = p.thread; if (t != null) { list.add(t); } } } return list; } /** * Returns a collection containing threads that may be waiting to acquire in * shared mode. This has the same properties as {@link #getQueuedThreads} except * that it only returns those threads waiting due to a shared acquire. * * @return the collection of threads */ public final Collection getSharedQueuedThreads() { ArrayList list = new ArrayList(); for (Node p = tail; p != null; p = p.prev) { if (p.isShared()) { Thread t = p.thread; if (t != null) { list.add(t); } } } return list; } /** * Returns a string identifying this synchronizer, as well as its state. The * state, in brackets, includes the String {@code "State ="} followed by the * current value of {@link #getState}, and either {@code "nonempty"} or * {@code "empty"} depending on whether the queue is empty. * * @return a string identifying this synchronizer, as well as its state */ public String toString() { int s = getState(); String q = hasQueuedThreads() ? "non" : ""; return super.toString() + "[State = " + s + ", " + q + "empty queue]"; } // Internal support methods for Conditions /** * Returns true if a node, always one that was initially placed on a condition * queue, is now waiting to reacquire on sync queue. * * @param node the node * @return true if is reacquiring */ 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); } /** * Returns true if node is on sync queue by searching backwards from tail. * Called only when needed by isOnSyncQueue. * * @return true if present */ private boolean findNodeFromTail(Node node) { Node t = tail; for (;;) { if (t == node) { return true; } if (t == null) { return false; } t = t.prev; } } /** * Transfers a node from a condition queue onto sync queue. Returns true if * successful. * * @param node the node * @return true if successfully transferred (else the node was cancelled before * signal) */ final boolean transferForSignal(Node node) { /* * If cannot change waitStatus, the node has been cancelled. */ if (!compareAndSetWaitStatus(node, Node.CONDITION, 0)) { return false; } /* * Splice onto queue and try to set waitStatus of predecessor to indicate that * thread is (probably) waiting. If cancelled or attempt to set waitStatus * fails, wake up to resync (in which case the waitStatus can be transiently and * harmlessly wrong). */ Node p = enq(node); int ws = p.waitStatus; if (ws > 0 || !compareAndSetWaitStatus(p, ws, Node.SIGNAL)) { LockSupport.unpark(node.thread); } return true; } /** * Transfers node, if necessary, to sync queue after a cancelled wait. Returns * true if thread was cancelled before being signalled. * * @param node the node * @return true if cancelled before the node was signalled */ final boolean transferAfterCancelledWait(Node node) { if (compareAndSetWaitStatus(node, Node.CONDITION, 0)) { enq(node); return true; } /* * If we lost out to a signal(), then we can't proceed until it finishes its * enq(). Cancelling during an incomplete transfer is both rare and transient, * so just spin. */ while (!isOnSyncQueue(node)) { Thread.yield(); } return false; } /** * Invokes release with current state value; returns saved state. Cancels node * and throws exception on failure. * * @param node the condition node for this wait * @return previous sync state */ 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; } } } // Instrumentation methods for conditions /** * Queries whether the given ConditionObject uses this synchronizer as its lock. * * @param condition the condition * @return {@code true} if owned * @throws NullPointerException if the condition is null */ public final boolean owns(ConditionObject condition) { return condition.isOwnedBy(this); } /** * Queries whether any threads are waiting on the given condition associated * with this synchronizer. Note that because timeouts and interrupts may occur * at any time, a {@code true} return does not guarantee that a future * {@code signal} will awaken any threads. This method is designed primarily for * use in monitoring of the system state. * * @param condition the condition * @return {@code true} if there are any waiting threads * @throws IllegalMonitorStateException if exclusive synchronization is not held * @throws IllegalArgumentException * if the given condition is not associated with this synchronizer * @throws NullPointerException if the condition is null */ public final boolean hasWaiters(ConditionObject condition) { if (!owns(condition)) { throw new IllegalArgumentException("Not owner"); } return condition.hasWaiters(); } /** * Returns an estimate of the number of threads waiting on the given condition * associated with this synchronizer. Note that because timeouts and interrupts * may occur at any time, the estimate serves only as an upper bound on the * actual number of waiters. This method is designed for use in monitoring of * the system state, not for synchronization control. * * @param condition the condition * @return the estimated number of waiting threads * @throws IllegalMonitorStateException if exclusive synchronization is not held * @throws IllegalArgumentException * if the given condition is not associated with this synchronizer * @throws NullPointerException if the condition is null */ public final int getWaitQueueLength(ConditionObject condition) { if (!owns(condition)) { throw new IllegalArgumentException("Not owner"); } return condition.getWaitQueueLength(); } /** * Returns a collection containing those threads that may be waiting on the * given condition associated with this synchronizer. Because the actual set of * threads may change dynamically while constructing this result, the returned * collection is only a best-effort estimate. The elements of the returned * collection are in no particular order. * * @param condition the condition * @return the collection of threads * @throws IllegalMonitorStateException if exclusive synchronization is not held * @throws IllegalArgumentException * if the given condition is not associated with this synchronizer * @throws NullPointerException if the condition is null */ public final Collection getWaitingThreads(ConditionObject condition) { if (!owns(condition)) { throw new IllegalArgumentException("Not owner"); } return condition.getWaitingThreads(); } /** * Condition implementation for a {@link AbstractQueuedSynchronizer} serving as * the basis of a {@link Lock} implementation. * *
* Method documentation for this class describes mechanics, not behavioral * specifications from the point of view of Lock and Condition users. Exported * versions of this class will in general need to be accompanied by * documentation describing condition semantics that rely on those of the * associated {@code AbstractQueuedSynchronizer}. * *
* This class is Serializable, but all fields are transient, so deserialized * conditions have no waiters. */ public class ConditionObject implements Condition, java.io.Serializable { private static final long serialVersionUID = 1173984872572414699L; /** First node of condition queue. */ private transient Node firstWaiter; /** Last node of condition queue. */ private transient Node lastWaiter; /** * Creates a new {@code ConditionObject} instance. */ public ConditionObject() { } // Internal methods /** * Adds a new waiter to wait queue. * * @return its new wait node */ 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; } /** * Removes and transfers nodes until hit non-cancelled one or null. Split out * from signal in part to encourage compilers to inline the case of no waiters. * * @param first (non-null) the first node on condition queue */ private void doSignal(Node first) { do { if ((firstWaiter = first.nextWaiter) == null) { lastWaiter = null; } first.nextWaiter = null; } while (!transferForSignal(first) && (first = firstWaiter) != null); } /** * Removes and transfers all nodes. * * @param first (non-null) the first node on condition queue */ private void doSignalAll(Node first) { lastWaiter = firstWaiter = null; do { Node next = first.nextWaiter; first.nextWaiter = null; transferForSignal(first); first = next; } while (first != null); } /** * Unlinks cancelled waiter nodes from condition queue. Called only while * holding lock. This is called when cancellation occurred during condition * wait, and upon insertion of a new waiter when lastWaiter is seen to have been * cancelled. This method is needed to avoid garbage retention in the absence of * signals. So even though it may require a full traversal, it comes into play * only when timeouts or cancellations occur in the absence of signals. It * traverses all nodes rather than stopping at a particular target to unlink all * pointers to garbage nodes without requiring many re-traversals during * cancellation storms. */ private void unlinkCancelledWaiters() { Node t = firstWaiter; Node trail = null; while (t != null) { Node next = t.nextWaiter; if (t.waitStatus != Node.CONDITION) { t.nextWaiter = null; if (trail == null) { firstWaiter = next; } else { trail.nextWaiter = next; } if (next == null) { lastWaiter = trail; } } else { trail = t; } t = next; } } // public methods /** * Moves the longest-waiting thread, if one exists, from the wait queue for this * condition to the wait queue for the owning lock. * * @throws IllegalMonitorStateException * if {@link #isHeldExclusively} returns {@code false} */ public final void signal() { if (!isHeldExclusively()) { throw new IllegalMonitorStateException(); } Node first = firstWaiter; if (first != null) { doSignal(first); } } /** * Moves all threads from the wait queue for this condition to the wait queue * for the owning lock. * * @throws IllegalMonitorStateException * if {@link #isHeldExclusively} returns {@code false} */ public final void signalAll() { if (!isHeldExclusively()) { throw new IllegalMonitorStateException(); } Node first = firstWaiter; if (first != null) { doSignalAll(first); } } /** * Implements uninterruptible condition wait. * 实现不可中断的condition等待 * *
- *
- Save lock state returned by {@link #getState}. *
- Invoke {
@link #release} with saved state as argument, throwing * IllegalMonitorStateException if it fails. *- Block until signalled. *
- Reacquire by invoking specialized version of {
@link #acquire} with saved * state as argument. *
*/ public final void awaitUninterruptibly() { Node node = addConditionWaiter(); int savedState = fullyRelease(node); boolean interrupted = false; while (!isOnSyncQueue(node)) { LockSupport.park(this); if (Thread.interrupted()) { interrupted = true; } } if (acquireQueued(node, savedState) || interrupted) { selfInterrupt(); } } /* * For interruptible waits, we need to track whether to throw * InterruptedException, if interrupted while blocked on condition, versus * reinterrupt current thread, if interrupted while blocked waiting to * re-acquire. */ /** Mode meaning to reinterrupt on exit from wait */ private static final int REINTERRUPT = 1; /** Mode meaning to throw InterruptedException on exit from wait */ private static final int THROW_IE = -1; /** * Checks for interrupt, returning THROW_IE if interrupted before signalled, * REINTERRUPT if after signalled, or 0 if not interrupted. */ private int checkInterruptWhileWaiting(Node node) { return Thread.interrupted() ? (transferAfterCancelledWait(node) ? THROW_IE : REINTERRUPT) : 0; } /** * Throws InterruptedException, reinterrupts current thread, or does nothing, * depending on mode. */ private void reportInterruptAfterWait(int interruptMode) throws InterruptedException { if (interruptMode == THROW_IE) { throw new InterruptedException(); } else if (interruptMode == REINTERRUPT) { selfInterrupt(); } } /** * Implements interruptible condition wait. * 实现可中断的condition等待 * *
- *
- If current thread is interrupted, throw InterruptedException. *
- Save lock state returned by {@link #getState}. *
- Invoke {
@link #release} with saved state as argument, throwing * IllegalMonitorStateException if it fails. *- Block until signalled or interrupted. *
- Reacquire by invoking specialized version of {
@link #acquire} with saved * state as argument. *- If interrupted while blocked in step 4, throw InterruptedException. *
*/ 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); } } /** * Implements timed condition wait. * 实现超时condition等待 * *
- *
- If current thread is interrupted, throw InterruptedException. *
- Save lock state returned by {@link #getState}. *
- Invoke {
@link #release} with saved state as argument, throwing * IllegalMonitorStateException if it fails. *- Block until signalled, interrupted, or timed out. *
- Reacquire by invoking specialized version of {
@link #acquire} with saved * state as argument. *- If interrupted while blocked in step 4, throw InterruptedException. *
*/ public final long awaitNanos(long nanosTimeout) throws InterruptedException { if (Thread.interrupted()) { throw new InterruptedException(); } Node node = addConditionWaiter(); int savedState = fullyRelease(node); final long deadline = System.nanoTime() + nanosTimeout; int interruptMode = 0; while (!isOnSyncQueue(node)) { if (nanosTimeout <= 0L) { transferAfterCancelledWait(node); break; } if (nanosTimeout >= spinForTimeoutThreshold) { LockSupport.parkNanos(this, nanosTimeout); } if ((interruptMode = checkInterruptWhileWaiting(node)) != 0) { break; } nanosTimeout = deadline - System.nanoTime(); } if (acquireQueued(node, savedState) && interruptMode != THROW_IE) { interruptMode = REINTERRUPT; } if (node.nextWaiter != null) { unlinkCancelledWaiters(); } if (interruptMode != 0) { reportInterruptAfterWait(interruptMode); } return deadline - System.nanoTime(); } /** * Implements absolute timed condition wait. * 实现绝对的超时condition等待 * *
- *
- If current thread is interrupted, throw InterruptedException. *
- Save lock state returned by {@link #getState}. *
- Invoke {
@link #release} with saved state as argument, throwing * IllegalMonitorStateException if it fails. *- Block until signalled, interrupted, or timed out. *
- Reacquire by invoking specialized version of {
@link #acquire} with saved * state as argument. *- If interrupted while blocked in step 4, throw InterruptedException. *
- If timed out while blocked in step 4, return false, else true. *
*/ public final boolean awaitUntil(Date deadline) throws InterruptedException { long abstime = deadline.getTime(); if (Thread.interrupted()) { throw new InterruptedException(); } Node node = addConditionWaiter(); int savedState = fullyRelease(node); boolean timedout = false; int interruptMode = 0; while (!isOnSyncQueue(node)) { if (System.currentTimeMillis() > abstime) { timedout = transferAfterCancelledWait(node); break; } LockSupport.parkUntil(this, abstime); if ((interruptMode = checkInterruptWhileWaiting(node)) != 0) { break; } } if (acquireQueued(node, savedState) && interruptMode != THROW_IE) { interruptMode = REINTERRUPT; } if (node.nextWaiter != null) { unlinkCancelledWaiters(); } if (interruptMode != 0) { reportInterruptAfterWait(interruptMode); } return !timedout; } /** * Implements timed condition wait. * 实现超时condition等待 * *
- *
- If current thread is interrupted, throw InterruptedException. *
- Save lock state returned by {@link #getState}. *
- Invoke {
@link #release} with saved state as argument, throwing * IllegalMonitorStateException if it fails. *- Block until signalled, interrupted, or timed out. *
- Reacquire by invoking specialized version of {
@link #acquire} with saved * state as argument. *- If interrupted while blocked in step 4, throw InterruptedException. *
- If timed out while blocked in step 4, return false, else true. *
*/ public final boolean await(long time, TimeUnit unit) throws InterruptedException { long nanosTimeout = unit.toNanos(time); if (Thread.interrupted()) { throw new InterruptedException(); } Node node = addConditionWaiter(); int savedState = fullyRelease(node); final long deadline = System.nanoTime() + nanosTimeout; boolean timedout = false; int interruptMode = 0; while (!isOnSyncQueue(node)) { if (nanosTimeout <= 0L) { timedout = transferAfterCancelledWait(node); break; } if (nanosTimeout >= spinForTimeoutThreshold) { LockSupport.parkNanos(this, nanosTimeout); } if ((interruptMode = checkInterruptWhileWaiting(node)) != 0) { break; } nanosTimeout = deadline - System.nanoTime(); } if (acquireQueued(node, savedState) && interruptMode != THROW_IE) { interruptMode = REINTERRUPT; } if (node.nextWaiter != null) { unlinkCancelledWaiters(); } if (interruptMode != 0) { reportInterruptAfterWait(interruptMode); } return !timedout; } // support for instrumentation /** * Returns true if this condition was created by the given synchronization * object. * * @return {@code true} if owned */ final boolean isOwnedBy(AbstractQueuedSynchronizer sync) { return sync == AbstractQueuedSynchronizer.this; } /** * Queries whether any threads are waiting on this condition. Implements * {@link AbstractQueuedSynchronizer#hasWaiters(ConditionObject)}. * * @return {@code true} if there are any waiting threads * @throws IllegalMonitorStateException * if {@link #isHeldExclusively} returns {@code false} */ protected final boolean hasWaiters() { if (!isHeldExclusively()) { throw new IllegalMonitorStateException(); } for (Node w = firstWaiter; w != null; w = w.nextWaiter) { if (w.waitStatus == Node.CONDITION) { return true; } } return false; } /** * Returns an estimate of the number of threads waiting on this condition. * Implements {@link AbstractQueuedSynchronizer#getWaitQueueLength(ConditionObject)}. * * @return the estimated number of waiting threads * @throws IllegalMonitorStateException if {@link #isHeldExclusively} returns {@code false} */ protected final int getWaitQueueLength() { if (!isHeldExclusively()) { throw new IllegalMonitorStateException(); } int n = 0; for (Node w = firstWaiter; w != null; w = w.nextWaiter) { if (w.waitStatus == Node.CONDITION) { ++n; } } return n; } /** * Returns a collection containing those threads that may be waiting on this * Condition. Implements {@link AbstractQueuedSynchronizer#getWaitingThreads(ConditionObject)}. * * @return the collection of threads * @throws IllegalMonitorStateException if {@link #isHeldExclusively} returns {@code false} */ protected final Collection getWaitingThreads() { if (!isHeldExclusively()) { throw new IllegalMonitorStateException(); } ArrayList list = new ArrayList(); for (Node w = firstWaiter; w != null; w = w.nextWaiter) { if (w.waitStatus == Node.CONDITION) { Thread t = w.thread; if (t != null) { list.add(t); } } } return list; } } /** * Setup to support compareAndSet. We need to natively implement this here: For * the sake of permitting future enhancements, we cannot explicitly subclass * AtomicInteger, which would be efficient and useful otherwise. So, as the * lesser of evils, we natively implement using hotspot intrinsics API. And * while we are at it, we do the same for other CASable fields (which could * otherwise be done with atomic field updaters). */ private static final Unsafe unsafe = Unsafe.getUnsafe(); private static final long stateOffset; private static final long headOffset; private static final long tailOffset; private static final long waitStatusOffset; private static final long nextOffset; static { try { stateOffset = unsafe.objectFieldOffset(AbstractQueuedSynchronizer.class.getDeclaredField("state")); headOffset = unsafe.objectFieldOffset(AbstractQueuedSynchronizer.class.getDeclaredField("head")); tailOffset = unsafe.objectFieldOffset(AbstractQueuedSynchronizer.class.getDeclaredField("tail")); waitStatusOffset = unsafe.objectFieldOffset(Node.class.getDeclaredField("waitStatus")); nextOffset = unsafe.objectFieldOffset(Node.class.getDeclaredField("next")); } catch (Exception ex) { throw new Error(ex); } } /** * CAS head field. Used only by enq. */ private final boolean compareAndSetHead(Node update) { return unsafe.compareAndSwapObject(this, headOffset, null, update); } /** * CAS tail field. Used only by enq. */ private final boolean compareAndSetTail(Node expect, Node update) { return unsafe.compareAndSwapObject(this, tailOffset, expect, update); } /** * CAS waitStatus field of a node. */ private static final boolean compareAndSetWaitStatus(Node node, int expect, int update) { return unsafe.compareAndSwapInt(node, waitStatusOffset, expect, update); } /** * CAS next field of a node. */ private static final boolean compareAndSetNext(Node node, Node expect, Node update) { return unsafe.compareAndSwapObject(node, nextOffset, expect, update); }}
如果只是了解多线程的简单用法，AQS可以绕过。
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