ReentrantLock源码分析AQS原理

向右看齐 2023-09-26 17:18 179阅读 0赞

**目录**

(一)AQS简介

(二)AQS原理剖析

(三)源码思想-加锁：

(1)构造方法

(2)公平锁lock方法(核心)

3、acquire获取资源方法(核心)

--------------------

## **(一)AQS简介** ##

**AQS是AbstractQueuedSynchronizer的简称,是一种实现阻塞锁和依赖FIFO队列的同步器框架。其依赖于一个原子变量state,子类要想使用这个同步器,必须要实现这个AQS，对state(私有可见)变量的操作都需要是原子操作主要是依赖于CAS,同时更改state只能是调用AQS本身提供的原子方法。  
AQS主要使用情况：ReentrantLock、ReentrantReadWriteLock、CountdownLatch、Semaphore等。**

## **(二)AQS原理剖析** ##

**利用ReentrantLock作为切入点对AQS进行源码分析==>ReentrantLock有一个核心成员变量sync(private final Sync sync),Sync是ReentrantLock类的内部类,其继承于AbstractQueuedSynchronizer。ReentrantLock是依赖于sync变量来调用AQS中的方法和重写的方法。**

## **(三)源码思想-****加锁**： ##

###    (1)构造方法 ###

使用**构造方法**设置该锁为公平锁FairSync或非公平锁NonfairSync;(本处选定非公平锁)

![295ef4d59f4a451d9ea805ad2810bc4e.png][]

//构造方法是使用公平锁类(FairSync)和非公平锁类(NonfairSync)继承Sync类来初始化sync变量,实现AQS关联
     //无参构造使用非公平锁
      public ReentrantLock() {
          sync = new NonfairSync();
      }
      //指定设置为公平锁和非公平锁
      public ReentrantLock(boolean fair) {
          sync = fair ? new FairSync() : new NonfairSync();
      }

### (2)公平锁lock方法(核心) ###

调用**lock**方法进行获取锁,**第一次(getState() == 0)**在**NonfairSync**中使用**CAS(compareAndSwapInt)**直接获取(此处默认未获取成功),第一次获取**失败**后,会执行**AQS中的acquire()**方法;![126706c2e8af49e0afc50e730bb523a2.png][]

NonfairSync类(非公平锁类)：
      final void lock() {
          if (compareAndSetState(0, 1))//调用AQS--AbstractQueuedSynchronizer中封装的CAS方法,对state变量进行设置为1
              setExclusiveOwnerThread(Thread.currentThread());//CAS操作成功,调用AQS继承的AOS中的方法,将当前线程设置为占用
          else
              //核心方法acquire,包含三个大的部分+线程是否中断：tryAcquire、addWaiter、acquireQueued和selfInterrupt
              acquire(1);//CAS争抢线程资源失败,则再次进行资源获取或者入队等操作(核心)
      }
      AbstractOwnableSynchronizer类：
      /**
      * 此抽象类用于将当前线程设置到变量private transient Thread exclusiveOwnerThread;利用模板模式抽取
      * AQS类继承AbstractOwnableSynchronizer类
      */
      protected final void setExclusiveOwnerThread(Thread thread) {
          exclusiveOwnerThread = thread;
      }
      AbstractQueuedSynchronizer类：
      //封装CAS操作,内部基于Unsafe类的原子操作CAS
      protected final boolean compareAndSetState(int expect, int update) {
          return unsafe.compareAndSwapInt(this, stateOffset, expect, update);
      }

### (3)acquire获取资源方法(核心) ###

当执行到**AQS**中**acquire**方法时,首先会再次尝试获取锁**tryAcquire**被子类NonfairSync重写,实际上是执行**Sync中的****nonfairTryAcquire**方法(如上图), 再次执行CAS进行锁获取(此处再次默认未获取成功),然后再判断**当前锁对象是否被同一线程获取**,可重入锁的由来(默认不是);

![cb68733530884bde82cef81e7802f8cc.png][]

/**
 * AQS方法acquire包含如下：
 * 再次请求获取资源tryAcquire
 * 创建等待节点addWaiter
 * 入队方法acquireQueued
 * 该争抢资源的线程是否设置了中断selfInterrupt
 */
 public final void acquire(int arg) {
 if (!tryAcquire(arg) &&//tryAcquire是抽象方法且设置为throw new UnsupportedOperationException();强制子类重写该方法
 acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
 selfInterrupt();//自我进行中断,acquireQueued中将已被标记的中断记号清楚掉,需要重新进行中断
 }
 NonfairSync类：
 //该方法为重写父类AQS中的抽象方法tryAcquire(抛出异常),子类强制重写,不然就会抛异常throw new UnsupportedOperationException()
 protected final boolean tryAcquire(int acquires) {
 return nonfairTryAcquire(acquires);//调用直系父类Sync中的方法
 }
 Sync类：
 //Sync类定义了非公平锁尝试获取资源方法acquires值为1
 final boolean nonfairTryAcquire(int acquires) {
 final Thread current = Thread.currentThread();//获取当前线程
 //调用AQS中的方法,主要是返回state值,因为state值是私有,必须通过public或者protected修饰方法获取
 int c = getState();
 if (c == 0) {//根据state值进行当前锁是否被占用 0代表未被占用 其他值代表已被占用
 if (compareAndSetState(0, acquires)) {//调用AQS封装的CAS方法进行设置state值
 //调用AQS继承的AOS中定义的方法,将当前线程值到exclusiveOwnerThread变量中
 //主要是用于当前线程如果在释放前再次获取锁,防止死锁即可重入锁
 setExclusiveOwnerThread(current);
 return true;
 }
 }else if (current == getExclusiveOwnerThread()) {//判断是否再次获取锁,当前线程与exclusiveOwnerThread变量对比即可重入锁
 int nextc = c + acquires;//锁的次数+1
 if (nextc < 0) //防止超过定义的最大值即Integer.Max
 throw new Error("Maximum lock count exceeded");
 setState(nextc);//调用AQS中定义的setState方法,对state设值,不需要原子操作,当前没有线程在竞争
 return true;
 }
 return false;//不满足上述两种条件,代表是在竞争锁资源且当前线程竞争失败
 }

### (4)addWaiter创建等待者方法(核心) ###

当**tryAcquire**方法获取失败后,会再次执行addWaiter方法,以独占模式进行创建等待着包含了哨兵节点的创建和当前线程创建的node节点连接;

AQS类：
     /**
     * 增加一个等待者==》当前线程node且与队列连接
     * addWaiter方法是基于前面tryAcquire方法获取锁资源失败的情况下进行执行
     * mode = Node.EXCLUSIVE(互斥模式),该值默认值是独占锁节点为null
     */
     private Node addWaiter(Node mode) {
         Node node = new Node(Thread.currentThread(), mode);//基于当前线程创建Node节点
         Node pred = tail;//获取当前队列的尾节点
         if (pred != null) {//判断是否有被初始化
             node.prev = pred;//将当前新生Node节点的前节点为原队列中的尾节点
             if (compareAndSetTail(pred, node)) {//利用CAS对尾节点tail进行设置新的节点值
                 pred.next = node;//队列中的前后节点连接,原节点的下一节点为当前节点
                 return node;
             }
         }
         enq(node);//tail节点为null时,代表未被初始化,进行头尾节点初始化即队列初始化操作
         return node;
     }
     /**
      * 基于当前线程的node,利用自旋和CAS对队列进行初始化即创建哨兵节点和新节点连接
      */
     private Node enq(final Node node) {
         for (;;) {//自旋操作
             Node t = tail;//获取尾节点
             if (t == null) { //尾节点是空则进行初始化
                 if (compareAndSetHead(new Node()))//新建一个空的哨兵节点设置为头节点
                     tail = head;//尾节点也是指向哨兵节点
             } else {//已被初始化
                 node.prev = t;//当前线程的前节点为当前尾节点
                 if (compareAndSetTail(t, node)) {//利用CAS尾节点重新赋值
                     t.next = node;//原尾节点的下一节点为当前node节点
                     return t;
                 }
             }
         }
     }

### (5)acquireQueued入队等待方法(核心) ###

当等待者创建完成后,执行acquireQueued入队操作,会判断当前节点的前节点是否为头节点,如果是则再次进行tryAcquire获取锁资源,若不是,则进行信号量设置和真正意义上的线程阻塞LockSupport.park(this)进行阻塞;  
说明：每个等待者(线程)的初始waitStatus都是0,在真正进入park之前,会将其前节点waitStatus设置为-1即shouldParkAfterFailedAcquire实现,这是为了线程在运行释放锁时,唤醒最近的后继节点,也即表明其后面是存在等待线程的。

AQS类：
     /**
     * 再次验证是否需要进入真正的等待即park
     * node为新创建的等待着节点 arg为1代表争抢线程资源
     */
     final boolean acquireQueued(final Node node, int arg) {
         boolean failed = true;//当前node线程默认取消获取资源
         try {
             boolean interrupted = false;//默认中断标记为false
             for (;;) {
                 final Node p = node.predecessor();//获取当前节点的前节点
                 if (p == head && tryAcquire(arg)) {//当前节点的前节点为头节点后,尝试获取下锁资源
                     setHead(node);//设置当前线程为头结点
                     p.next = null; // help GC
                     failed = false;//表示当前线程不需要取消获取锁资源
                     return interrupted;//跳出循环并返回,根据线程是否需要中断,返回true或false
                 }
                 //当前线程的前节点设置信号量和当前线程真正意义上的阻塞
                 if (shouldParkAfterFailedAcquire(p, node) &&
                     parkAndCheckInterrupt())
                     interrupted = true;//满足上述if条件则表明中断标记即放弃获取资源,此处是可中断的真正用处
             }
         } finally {
         //只有当try中出现异常时会执行finally代码即在doAcquireInterruptibly中会显式跑出异常,
         // doAcquireInterruptibly和acquireQueued代码基本相同,此处也是为了防止try突然出现一些其他原因导致程序异常的最后保障
             if (failed)//经过上述try逻辑,判断是否需要取消当前线程的锁资源
                 cancelAcquire(node);//取消获取锁资源,方式如队列中跳过该节点或者将当前节点设置为超时状态
         }
     }
    
     //该方法为将当前节点的前节点线程等待值由0->-1,代表可唤醒状态
     private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
         int ws = pred.waitStatus;
         if (ws == Node.SIGNAL)//pred作为当前线程的前节点,判断是否等于-1即Unpark信号量
             return true;
         if (ws > 0) {//线程被取消时跳过该线程获取锁
             do {
                 node.prev = pred = pred.prev;
             } while (pred.waitStatus > 0);
             pred.next = node;//跳过该线程获取锁资源
         } else {
              //对于前节点的线程是初始值0、-2(某种条件)、-3(共享模式)进行设置为-1,代表信号量
             compareAndSetWaitStatus(pred, ws, Node.SIGNAL);
         }
         return false;//除了上述第一种情况代表true以外,其他情况代表本次执行仅仅是调整节点或者设置信号量值
     }
    
     //基于前面的shouldParkAfterFailedAcquire返回true的情况下,该方法是真正意义上的park,线程阻塞
     private final boolean parkAndCheckInterrupt() {
         LockSupport.park(this);//线程阻塞
         return Thread.interrupted();//线程如果没有中断标记,则返回false,反之则返回true即该方法是清楚线程中断标记
     }

### 6、cancelAcquire取消线程获取方法(非核心) ###

//基于当前节点去处理节点中未取消状态的节点,不是指中断原理唷
 private void cancelAcquire(Node node) {
 if (node == null)
 return;
 	// 将当前节点封装的线程设置为NULL
 node.thread = null;
 // 通过循环获取当前节点不为CANCELLED状态的前驱节点
 Node pred = node.prev;
 while (pred.waitStatus > 0)
 node.prev = pred = pred.prev;
 // 获取前驱节点的后继节点（如果节点的前驱节点不是CANCELLED状态，那么前驱节点的后继节点就是它自己）
 Node predNext = pred.next;
 // 将节点的等待状态设置为CANCELLED
 node.waitStatus = Node.CANCELLED;
 //如果当前节点是尾节点,则直接通过CAS将tail指针指向当前节点不为CANCELLED状态的前驱节点,
 //同时通过CAS将前驱节点的后继指针设置为NULL
 if (node == tail && compareAndSetTail(node, pred)) {
 compareAndSetNext(pred, predNext, null);
 } else {
 int ws;
 			//如果当前节点的前驱节点不是头节点且前驱节点的等待状态为SIGNAL(如果不是SIGNAL那就
 			//设置为SIGNAL)且前驱节点封装的线程不为NULL
 if (pred != head &&
 ((ws = pred.waitStatus) == Node.SIGNAL ||
 (ws <= 0 && compareAndSetWaitStatus(pred, ws, Node.SIGNAL))) &&
 pred.thread != null) {
 // 获取节点的后继节点
 Node next = node.next;
 // 如果后继节点的等待状态不为CANCELLED，则通过CAS将前驱节点的后继指针指向当前节点的后继节点
 if (next != null && next.waitStatus <= 0)
 // 这里并没有将当前节点的后继节点的前驱指针指向前驱节点（不用设置，unparkSuccessor()方法会自动跳过）
 compareAndSetNext(pred, predNext, next);
 } else {
 // 如果当前节点的前驱节点是头节点，则直接唤醒当前节点的后继节点，让它来剔除当前节点
 unparkSuccessor(node);
 }
 node.next = node;
 }
 }

## 解锁： ##

(2)在AQS释放方法release中首先会进行释放锁的预备工作tryRelease方法,主要是在正常加解锁的前提下对state还原为0; 
(3)在tryRelease预备工作满足下,开始真正唤醒头节点的后继节点且该后继节点满足等待状态不是0的调用唤醒方法unparkSuccessor; 
(4)在唤醒方法unparkSuccessor中,对于状态为超时状态的线程进行过滤且从尾节点往前找到最靠近头结点的后继节点(<=0,正常一般是信号量-1)进行唤醒。源码逻辑

### (1)unlock解锁 ###

调用ReentrantLock中的unlock方法,会使用sync核心变量调用AQS中的release方法;

ReentrantLock类：
    public void unlock() {
        sync.release(1);//核心成员变量sync调用AQS中的release方法
    }

###    (2)release或tryRelease方法 ###

在AQS释放方法release中首先会进行释放锁的预备工作tryRelease方法,主要是在正常加解锁的前提下对state还原为0;

AQS类：
    public final boolean release(int arg) {
        if (tryRelease(arg)) {//尝试调用释放锁方法即为真正释放锁做准备
            Node h = head;//头结点获取
            //头结点被初始化和头结点的等待状态是被改变过,主要是在acquireQueued方法中会在当前线程对前节点进行改变：0=>-1
            if (h != null && h.waitStatus != 0)
                unparkSuccessor(h);//满足唤醒被阻塞的等待线程
            return true;
        }
        return false;
    }
    Sync类：Sync继承于AQS类,父类定义了tryRelease抽象方法并抛出异常,强制子类重写
    //此方法主要是为真正释放锁做预备工作,是否正常能将state值还原为0
    protected final boolean tryRelease(int releases) {
        int c = getState() - releases;//使用state值-1的值给到更新state值
        if (Thread.currentThread() != getExclusiveOwnerThread())//保证当前释放的线程是锁的拥有线程
            throw new IllegalMonitorStateException();
        boolean free = false;//默认释放标志失败
        if (c == 0) {//正常释放后,state值是为0
            free = true;
            setExclusiveOwnerThread(null);//清空锁的占有者
        }
        setState(c);//更新state值,将1变为初始状态0,作用是为后面真正释放做准备
        return free;//返回释放结果即其结果会影响是否真正进行释放
    }

###    (3)unparkSuccessor唤醒方法 ###

在tryRelease预备工作满足下,开始真正唤醒头节点的后继节点且该后继节点满足等待状态不是0的调用唤醒方法unparkSuccessor;

在唤醒方法unparkSuccessor中,对于状态为超时状态的线程进行过滤且从尾节点往前找到最靠近头结点的后继节点(<=0,正常一般是信号量-1)进行唤醒。

//唤醒头结点的后继节点
 private void unparkSuccessor(Node node) {
 int ws = node.waitStatus;//获取传入的头结点的等待状态
 if (ws < 0)//头结点线程等待状态正常情况下一定是-1
 //因为拥有锁的线程执行释放逻辑时,等待队列中的头结点要么是哨兵空节点,要么是当前线程即执行锁释放的线程
 //等待队列头结点其实是一个占位节点,其身份为：初始化头结点(空节点)或者是拥有锁执行权的线程
 compareAndSetWaitStatus(node, ws, 0);//将头节点等待状态设置为0
 
 Node s = node.next;//当前线程的直接后继节点或空节点的直接后继节点
 if (s == null || s.waitStatus > 0) {//判断是否存在后继节点或者是超时节点
 s = null;
 for (Node t = tail; t != null && t != node; t = t.prev)
 if (t.waitStatus <= 0)//从队列末尾开始往前寻找最靠前等待状态为<=0的线程
 s = t;
 }
 if (s != null)
 LockSupport.unpark(s.thread);//唤醒最靠前且等待状态为<=0的等待线程
 }

### **总结**： ###

> 1.在多线程竞争条件下，CAS操作失败，会进行自旋，如果竞争激烈，就会给CPU带来非常大的开销  
> 2.CAS每次只能保证一个共享变量的原子操作  
> 3.CAS会产生ABA问题

### 小知识： ###

Java中的CAS方法都是来自Unsafe类。  
Java中Unsafe提供了三种CAS方法：

// 第一个参数:对象的实例  第二个参数：内存偏移量  第三个参数：对比期望的值  第四个参数：字段的新值
    public final native boolean compareAndSwapObject(Object var1, long var2, Object var4, Object var5);
    
    public final native boolean compareAndSwapInt(Object var1, long var2, int var4, int var5);
    
    public final native boolean compareAndSwapLong(Object var1, long var2, long var4, long var6);

//CAS示例
    public static void main(String[] args) {
        AtomicInteger atomicInteger = new AtomicInteger(1);
        //这个方法底层就是使用compareAndSwapInt()
        boolean b = atomicInteger.compareAndSet(1, 3);
        System.out.println("第一次修改的结果： "+b+ "修改后的值： " +atomicInteger.get());
        boolean b1 = atomicInteger.compareAndSet(1, 6);
        System.out.println("第二次修改的结果： "+b1+ "修改后的值： "+atomicInteger.get());
    //第一次修改的结果:true修改后的值:  3
    //第二次修改的结果:false修改后的值: 3
    }

最后上一份**ReentrantLock**的源码：在rt.jar中的java.util.concurrent.locks中

AbstractQueuedSynchronizer 源码  java.util.concurrent.locks

/*
 * ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 *
 */
 
 /*
 *
 *
 *
 *
 *
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/publicdomain/zero/1.0/
 */
 
 package java.util.concurrent.locks;
 import java.util.concurrent.TimeUnit;
 import java.util.ArrayList;
 import java.util.Collection;
 import java.util.Date;
 import sun.misc.Unsafe;
 
 /**
 * Provides a framework for implementing blocking locks and related
 * synchronizers (semaphores, events, etc) that rely on
 * first-in-first-out (FIFO) wait queues. 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. 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. Given these, the other methods in this class carry
 * out all queuing and blocking mechanics. 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.
 *
 * Subclasses should be defined as non-public internal helper
 * classes that are used to implement the synchronization properties
 * of their enclosing class. Class
 * {@code AbstractQueuedSynchronizer} does not implement any
 * synchronization interface. 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.
 *
 * This class supports either or both a default exclusive
 * mode and a shared mode. When acquired in exclusive mode,
 * attempted acquires by other threads cannot succeed. Shared mode
 * acquires by multiple threads may (but need not) succeed. 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. Threads waiting in the different modes share the
 * same FIFO queue. Usually, implementation subclasses support only
 * one of these modes, but both can come into play for example in a
 * {@link ReadWriteLock}. Subclasses that support only exclusive or
 * only shared modes need not define the methods supporting the unused mode.
 *
 * 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. No
 * {@code AbstractQueuedSynchronizer} method otherwise creates such a
 * condition, so if this constraint cannot be met, do not use it. The
 * behavior of {@link ConditionObject} depends of course on the
 * semantics of its synchronizer implementation.
 *
 * This class provides inspection, instrumentation, and monitoring
 * methods for the internal queue, as well as similar methods for
 * condition objects. These can be exported as desired into classes
 * using an {@code AbstractQueuedSynchronizer} for their
 * synchronization mechanics.
 *
 * Serialization of this class stores only the underlying atomic
 * integer maintaining state, so deserialized objects have empty
 * thread queues. Typical subclasses requiring serializability will
 * define a {@code readObject} method that restores this to a known
 * initial state upon deserialization.
 *
 * <h3>Usage</h3>
 *
 * 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}:
 *
 * <ul>
 * <li> {@link #tryAcquire}
 * <li> {@link #tryRelease}
 * <li> {@link #tryAcquireShared}
 * <li> {@link #tryReleaseShared}
 * <li> {@link #isHeldExclusively}
 * </ul>
 *
 * Each of these methods by default throws {@link
 * UnsupportedOperationException}. Implementations of these methods
 * must be internally thread-safe, and should in general be short and
 * not block. Defining these methods is the only supported
 * means of using this class. All other methods are declared
 * {@code final} because they cannot be independently varied.
 *
 * You may also find the inherited methods from {@link
 * AbstractOwnableSynchronizer} useful to keep track of the thread
 * owning an exclusive synchronizer. You are encouraged to use them
 * -- this enables monitoring and diagnostic tools to assist users in
 * determining which threads hold locks.
 *
 * Even though this class is based on an internal FIFO queue, it
 * does not automatically enforce FIFO acquisition policies. The core
 * of exclusive synchronization takes the form:
 *
 * <pre>
 * Acquire:
 * while (!tryAcquire(arg)) {
 * enqueue thread if it is not already queued;
 * possibly block current thread;
 * }
 *
 * Release:
 * if (tryRelease(arg))
 * unblock the first queued thread;
 * </pre>
 *
 * (Shared mode is similar but may involve cascading signals.)
 *
 * Because checks in acquire are invoked before
 * enqueuing, a newly acquiring thread may barge ahead of
 * others that are blocked and queued. 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.
 * 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}. Other variations are possible.
 *
 * Throughput and scalability are generally highest for the
 * default barging (also known as greedy,
 * renouncement, and convoy-avoidance) strategy.
 * 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. Also, while acquires do not
 * "spin" in the usual sense, they may perform multiple
 * invocations of {@code tryAcquire} interspersed with other
 * computations before blocking. This gives most of the benefits of
 * spins when exclusive synchronization is only briefly held, without
 * most of the liabilities when it isn't. 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.
 *
 * 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. 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.
 *
 * <h3>Usage Examples</h3>
 *
 * 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. 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.
 * It also supports conditions and exposes
 * one of the instrumentation methods:
 *
 * <pre> {@code
 * class Mutex implements Lock, java.io.Serializable {
 *
 * // Our internal helper class
 * private static class Sync extends AbstractQueuedSynchronizer {
 * // Reports whether in locked state
 * protected boolean isHeldExclusively() {
 * return getState() == 1;
 * }
 *
 * // Acquires the lock if state is zero
 * public boolean tryAcquire(int acquires) {
 * assert acquires == 1; // Otherwise unused
 * if (compareAndSetState(0, 1)) {
 * setExclusiveOwnerThread(Thread.currentThread());
 * return true;
 * }
 * return false;
 * }
 *
 * // Releases the lock by setting state to zero
 * protected boolean tryRelease(int releases) {
 * assert releases == 1; // Otherwise unused
 * if (getState() == 0) throw new IllegalMonitorStateException();
 * setExclusiveOwnerThread(null);
 * setState(0);
 * return true;
 * }
 *
 * // Provides a Condition
 * Condition newCondition() { return new ConditionObject(); }
 *
 * // Deserializes properly
 * 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.
 * 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));
 * }
 * }}</pre>
 *
 * 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. Because a latch is non-exclusive, it uses the {@code shared}
 * acquire and release methods.
 *
 * <pre> {@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);
 * }
 * }}</pre>
 *
 * @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.
 */
 protected AbstractQueuedSynchronizer() { }
 
 /**
 * Wait queue node class.
 *
 * The wait queue is a variant of a "CLH" (Craig, Landin, and
 * Hagersten) lock queue. CLH locks are normally used for
 * spinlocks. 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. A
 * "status" field in each node keeps track of whether a thread
 * should block. A node is signalled when its predecessor
 * releases. 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.
 * <pre>
 * +------+ prev +-----+ +-----+
 * head | | <---- | | <---- | | tail
 * +------+ +-----+ +-----+
 * </pre>
 *
 * 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.
 *
 * 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.
 * 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 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.
 *
 * @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.
 */
 private transient volatile Node head;
 
 /**
 * Tail of the wait queue, lazily initialized. Modified only via
 * method enq to add new wait node.
 */
 private transient volatile Node tail;
 
 /**
 * The synchronization state.
 */
 private volatile int state;
 
 /**
 * 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.
 *
 * @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.
 */
 static final long spinForTimeoutThreshold = 1000L;
 
 /**
 * 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;
 }
 }
 }
 }
 
 /**
 * 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;
 }
 
 /**
 * 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.
 *
 * @param node the node
 */
 private void setHead(Node node) {
 head = node;
 node.thread = null;
 node.prev = null;
 }
 
 /**
 * 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);
 }
 
 /**
 * 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.)
 */
 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.
 *
 * @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.
 *
 * @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.
 */
 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.
 */
 
 /**
 * 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();
 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.
 * @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.
 *
 * @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.
 * @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.
 * @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.
 *
 * @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
 
 /**
 * 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.
 *
 * 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()}.
 *
 * The default
 * implementation throws {@link UnsupportedOperationException}.
 *
 * @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 {@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.
 *
 * This method is always invoked by the thread performing release.
 *
 * The default implementation throws
 * {@link UnsupportedOperationException}.
 *
 * @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 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.
 *
 * 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.
 *
 * The default implementation throws {@link
 * UnsupportedOperationException}.
 *
 * @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.
 *
 * This method is always invoked by the thread performing release.
 *
 * The default implementation throws
 * {@link UnsupportedOperationException}.
 *
 * @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}.)
 *
 * 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.
 *
 * @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}.
 *
 * @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();
 }
 
 /**
 * 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}.
 *
 * @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)}.
 *
 * @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}.
 *
 * @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.
 *
 * In this implementation, this operation returns in
 * constant time.
 *
 * @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.
 *
 * In this implementation, this operation returns in
 * constant time.
 *
 * @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.
 *
 * An invocation of this method is equivalent to (but may be
 * more efficient than):
 * <pre> {@code
 * getFirstQueuedThread() != Thread.currentThread() &&
 * hasQueuedThreads()}</pre>
 *
 * 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 <a href="AbstractQueuedSynchronizer#barging">barging</a>.
 * 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 {@code
 * tryAcquire} method for a fair, reentrant, exclusive mode
 * synchronizer might look like this:
 *
 * <pre> {@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
 * }
 * }}</pre>
 *
 * @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<Thread> getQueuedThreads() {
 ArrayList<Thread> list = new ArrayList<Thread>();
 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<Thread> getExclusiveQueuedThreads() {
 ArrayList<Thread> list = new ArrayList<Thread>();
 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<Thread> getSharedQueuedThreads() {
 ArrayList<Thread> list = new ArrayList<Thread>();
 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<Thread> 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.
 * <ol>
 * <li> Save lock state returned by {@link #getState}.
 * <li> Invoke {@link #release} with saved state as argument,
 * throwing IllegalMonitorStateException if it fails.
 * <li> Block until signalled.
 * <li> Reacquire by invoking specialized version of
 * {@link #acquire} with saved state as argument.
 * </ol>
 */
 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.
 * <ol>
 * <li> If current thread is interrupted, throw InterruptedException.
 * <li> Save lock state returned by {@link #getState}.
 * <li> Invoke {@link #release} with saved state as argument,
 * throwing IllegalMonitorStateException if it fails.
 * <li> Block until signalled or interrupted.
 * <li> Reacquire by invoking specialized version of
 * {@link #acquire} with saved state as argument.
 * <li> If interrupted while blocked in step 4, throw InterruptedException.
 * </ol>
 */
 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.
 * <ol>
 * <li> If current thread is interrupted, throw InterruptedException.
 * <li> Save lock state returned by {@link #getState}.
 * <li> Invoke {@link #release} with saved state as argument,
 * throwing IllegalMonitorStateException if it fails.
 * <li> Block until signalled, interrupted, or timed out.
 * <li> Reacquire by invoking specialized version of
 * {@link #acquire} with saved state as argument.
 * <li> If interrupted while blocked in step 4, throw InterruptedException.
 * </ol>
 */
 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.
 * <ol>
 * <li> If current thread is interrupted, throw InterruptedException.
 * <li> Save lock state returned by {@link #getState}.
 * <li> Invoke {@link #release} with saved state as argument,
 * throwing IllegalMonitorStateException if it fails.
 * <li> Block until signalled, interrupted, or timed out.
 * <li> Reacquire by invoking specialized version of
 * {@link #acquire} with saved state as argument.
 * <li> If interrupted while blocked in step 4, throw InterruptedException.
 * <li> If timed out while blocked in step 4, return false, else true.
 * </ol>
 */
 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.
 * <ol>
 * <li> If current thread is interrupted, throw InterruptedException.
 * <li> Save lock state returned by {@link #getState}.
 * <li> Invoke {@link #release} with saved state as argument,
 * throwing IllegalMonitorStateException if it fails.
 * <li> Block until signalled, interrupted, or timed out.
 * <li> Reacquire by invoking specialized version of
 * {@link #acquire} with saved state as argument.
 * <li> If interrupted while blocked in step 4, throw InterruptedException.
 * <li> If timed out while blocked in step 4, return false, else true.
 * </ol>
 */
 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<Thread> getWaitingThreads() {
 if (!isHeldExclusively())
 throw new IllegalMonitorStateException();
 ArrayList<Thread> list = new ArrayList<Thread>();
 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);
 }
 }

本文参考 https://blog.csdn.net/txk396879586/article/details/123055812

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[cb68733530884bde82cef81e7802f8cc.png]: https://img-blog.csdnimg.cn/cb68733530884bde82cef81e7802f8cc.png