JUC包(三) 锁与锁实现(AQS)

浅浅的花香味﹌ 2022-02-28 16:20 253阅读 0赞

### 前言 ###

在[原 JUC包 (一) 原子类 与 CAS操作][JUC_ _ _ _ CAS]中我们主要了解了什么是原子操作,以及原子操作的实现`CAS(Compare ans Swap)`操作. 本章,我们继续接着介绍使用`CAS原理`实现的锁操作. 锁在`JUC包`中使用非常广泛, 其取代了传统的`synchronized`关键字被广泛使用. 下文中,我们将一起了解锁的实现原理与基本使用场景.

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

### 队列同步器(AQS) ###

AQS,是(Abstract Queued Synchronizer)队列同步器, 简称同步器.它是用于构建锁和其他同步操作的基础框架.使用一个`int`变量来标示同步操作. 就个人理解而言, 它就是一个模板类, 其中实现了一个同步队列的基本操作.

其实现与队列基本一致. 主要维护了3个线程状态的方法,分别为`getState() / setState(int newState) 和 compareAndSetState()`.

同步器中其他可重写方法:

*  `protected boolean tryAcquire(int args)` 独占式获取同步状态
 *  `protected boolean tryRelease(int args)` 独占式释放同步状态
 *  `protected int tryAcquireShared(int args)` 共享式获取同步状态
 *  `protected boolean tryReleasedShared(int args)`共享式释放同步状态
 *  `protected boolean isHeldExclusively()` 当前同步器是否被独占  
    其中的模板方法主要有:
 *  `void acquire(int arg)`
 *  `void acuqireInterruptibly(int arg)`
 *  `void tryAcquireNanos(int arg, long naos)`
 *  `void acquireShared(int arg)`
 *  `void acquireSharedInterruptily(int arg)`
 *  `boolean tryAcquireSharedNanos(int arg, long naos)`
 *  `boolean release(int arg)`
 *  `boolean releaseShared(int arg)`
 *  `Collection<Thread> getQueuedThreads()`  
    上述的模板方法主要包括2类, `try/release`同步锁与非同步锁 及 获取队列所以线程的`getQueuedThreads()`方法.

我们在自定义锁的构造时,通常是使用同步器来进行实现.使用同步器,我们可以构建自定义锁.(类似ReentrantLock  
)

import java.util.ArrayList;
    import java.util.Collections;
    import java.util.concurrent.TimeUnit;
    import java.util.concurrent.locks.AbstractQueuedSynchronizer;
    import java.util.concurrent.locks.Condition;
    import java.util.concurrent.locks.Lock;
    
    public class Mutex implements Lock{
    	private static class Sync extends AbstractQueuedSynchronizer{
    		// 是否处于占用状态
    		protected boolean isHeldExclusively(){
    			// 判断状态是否为1
    			return getState()==1;
    		}
    		
    		// CAS 尝试获取 成功返回true 失败返回false
    		public boolean tryAcquire(int acquires){
    			if(compareAndSetState(0, 1)){		
    				setExclusiveOwnerThread(Thread.currentThread());
    				return true;
    			}
    			return false;
    		}
    		
    		public boolean tryRelease(){
    			// 如果 状态为0 说明没有获取锁.(所以无法释放)
    			if(getState()==0)throw new IllegalStateException();
    			// 如果存在空 设置同步器的所有线程为空
    			setExclusiveOwnerThread(null);
    			// 设置当前同步器状态为0
    			setState(0);
    			//释放成功
    			return true;
    		}
    		
    		Condition newCondition(){return new ConditionObject();}
    	}
    
    	private final Sync sync = new Sync();
    	@Override
    	public void lock() {
    		sync.acquire(1);
    	}
    
    	@Override
    	public boolean tryLock() {
    		return sync.tryAcquire(1);
    	}
    	
    
    	@Override
    	public void lockInterruptibly() throws InterruptedException {
    		sync.acquireInterruptibly(1);
    	}
    
    
    	@Override
    	public boolean tryLock(long time, TimeUnit unit)
    			throws InterruptedException {
    		return sync.tryAcquireNanos(1, unit.toNanos(time));
    	}
    
    
    	@Override
    	public void unlock() {
    		sync.release(1);
    	}
    
    
    	@Override
    	public Condition newCondition() {
    		return sync.newCondition();
    	}
    
    	public static void main(String[] args) {
    		Collections.synchronizedList(new ArrayList<>());
    	}
    }

我们经常使用的`ReentrantLock`就是靠类似上方的数据进行实现的.(感兴趣的可以读读文尾的附注部分.)

> `tryAcquire`与`acquire()`的区别?  
> `tryAcquire()`是尝试一次获取锁, 获取成功返回`true`. 否则, 返回`false`. 而`acquire()`是写成了一个死循环进行获取锁. 获取失败,则进入等待队列. 这是`Java`内部一个用于提高运行效率的设计.

从本章开始, 我们具体讲解下同步器是如何具体实现这些需求的.(独占锁与共享锁)

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly95YW54bWwuYmxvZy5jc2RuLm5ldA_size_16_color_FFFFFF_t_70]  
在同步器内部维护了诸如上图的同步队列.同步器维护了两个分别指向`head`与`tail`结点的两个指针.各个结点分别记录了`prev/next/状态`等多个信息.  
Node内的是属性如下所示:

*  int waitStatus(CANCELLED-(1)-中断/SIGNAL-(-1)-后继等待/CONDITION-(-2)-Condition等待/PROPAGATE-(-3)-共享式等待/INITAL-(0)-初始状态 )
 *  Node prev
 *  Node next
 *  Node nextWaiter(等待队列中的后继结点)
 *  Thread thread

在同步队列中,主要有两个主要操作: 设置头结点/设置尾结点.

*  设置头结点: 将当前`head`的下一个结点置为`head`结点.(`head = head->next`) 无多线程问题.
 *  设置尾结点: 使用`compareAndSetTail(Node expect, Noode update)`进行更新.由于是`CAS`操作, 所以在尝试失败后会不断的使用重试机制进行重复操作. 会遇到多线程问题, 通过CAS进行解决.

OK, 我们下面看下源码:

> compareAndSetHead() & compareAndSetTail()

/**
         * 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);
        }
       // 使用Unsafe类,调用系统底层进行CAS操作.

> acquire() -> tryAcquire/acquireQueued -> (addWaiter-> enq)

public final void acquire(int arg) {
            if (!tryAcquire(arg) &&
                acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
                selfInterrupt();
        }
        // AQS内并没有太多关于tryAcquire()的操作(后续给出一个ReentrantLock的例子)
        protected boolean tryAcquire(int arg) {
            throw new UnsupportedOperationException();
        }
        // 获取队列内的值
        final boolean acquireQueued(final Node node, int arg) {
        	// 失败标示标为true
            boolean failed = true;
            try {
            	// 中断标示标为false
                boolean interrupted = false;
                // 自旋操作 阻塞当前线程
                for (;;) {
                	// 获取前置结点
                    final Node p = node.predecessor();
                    // 如果前置结点为头结点(当前锁持有者) 并且获取锁成功
                    if (p == head && tryAcquire(arg)) {
                    	// 如果完成上述条件 则将当前结点设置为头结点
                        setHead(node);
                        p.next = null; // help GC
                        failed = false;
                        return interrupted;
                    }
                    if (shouldParkAfterFailedAcquire(p, node) &&
                        parkAndCheckInterrupt())
                        interrupted = true;
                }
            } finally {
                if (failed)
                	// 放弃获取
                    cancelAcquire(node);
            }
        }
     // 增加等待结点
    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;
                // 使用CAS进行将新结点设置到末尾
                if (compareAndSetTail(pred, node)) {
                    pred.next = node;
                    return node;
                }
            }
            // 如果CAS失败 则进入enq()方法
            enq(node);
            return node;
        }
        // 如果第一次CAS失败 那么不断循环自旋,直到将结点添加到末尾为止
     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;
                    }
                }
            }
        }

在上述到过程中, 当线程获取锁时. 线程会先通过`tryAcquire()`获取线程当锁. 当尝试失败时, 线程会将当前线程通过`addWaiter()`方法将当前线程放入同步队列中.(如果放入失败,则通过`enq()`方法不断循环添加, 直到成功结束.) 对于放入成功的结点,则不断进行自旋`for循环`, 直到当前线程获取到锁(成为同步队列的头结点为止).

下面我们来看下`release()`释放锁的方法:

> release() -> tryRelease() / unparkSuccessor() -> LockSupport.unpark(s.thread)

public final boolean release(int arg) {
            if (tryRelease(arg)) {
                Node h = head;
                if (h != null && h.waitStatus != 0)
                    unparkSuccessor(h);
                return true;
            }
            return false;
        }
        // 模板方法
        protected boolean tryRelease(int arg) {
            throw new UnsupportedOperationException();
        }
        // 使用`LockSupport`的`unpark`方法唤醒后续结点
        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);
        }

本章中, 讲述了`AQS`获取独占锁的获取和释放的过程:

*  获取: 在获取同步状态时, 同步器构建了一个同步队列, 当获取同步队列失败时, 会将当前线程放入队列的结尾, 并进行阻塞自旋操作.
 *  释放: 移出队列的条件是当前结点的前结点是头结点. 将当前结点的后一个结点作为新的队首结点.

##### 共享式同步状态 #####

上文分析了独占式同步队列的基本实现. 本节讲述下共享式同步状态的获取和释放.  
共享式同步状态的获取和释放和独占式不太一样. 值得一提的是, 独占式的同步状态与共享式同步状态不能同时存在. 两种模式式互斥的. 基本源码如下所示:

> acquireShared() -> tryAcquireShared()/doAcquireShared() ->addWaiter()/

public final void acquireShared(int arg) {
            if (tryAcquireShared(arg) < 0)
                doAcquireShared(arg);
        }
        
    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) {
                    // 如果当前结点为头结点 尝试获取
                        int r = tryAcquireShared(arg);
                        if (r >= 0) {
                        	// r>=0 说明已经获取到锁
                            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);
            }
        }
        //两个入参，一个是当前成功获取共享锁的节点，一个就是tryAcquireShared方法的返回值，注意上面说的，它可能大于0也可能等于0
        private void setHeadAndPropagate(Node node, int propagate) {
            Node h = head; //记录当前头节点
            //设置新的头节点，即把当前获取到锁的节点设置为头节点
            //注：这里是获取到锁之后的操作，不需要并发控制
            setHead(node);
            //这里意思有两种情况是需要执行唤醒操作
            //1.propagate > 0 表示调用方指明了后继节点需要被唤醒
            //2.头节点后面的节点需要被唤醒（waitStatus<0），不论是老的头结点还是新的头结点
            if (propagate > 0 || h == null || h.waitStatus < 0 ||
                (h = head) == null || h.waitStatus < 0) {
                Node s = node.next;
                //如果当前节点的后继节点是共享类型或者没有后继节点，则进行唤醒
                //这里可以理解为除非明确指明不需要唤醒（后继等待节点是独占类型），否则都要唤醒
                if (s == null || s.isShared())
                    //后面详细说
                    doReleaseShared();
            }
        }
    
        private void setHead(Node node) {
            head = node;
            node.thread = null;
            node.prev = null;
        }

> doReleaseShared() -> unparkSuccessor()  
> // 这段没怎么看懂

private void doReleaseShared() {
            for (;;) {
                //唤醒操作由头结点开始，注意这里的头节点已经是上面新设置的头结点了
                //其实就是唤醒上面新获取到共享锁的节点的后继节点
                Node h = head;
                if (h != null && h != tail) {
                    int ws = h.waitStatus;
                    //表示后继节点需要被唤醒
                    if (ws == Node.SIGNAL) {
                        //这里需要控制并发，因为入口有setHeadAndPropagate跟release两个，避免两次unpark
                        if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0))
                            continue;      
                        //执行唤醒操作      
                        unparkSuccessor(h);
                    }
                    //如果后继节点暂时不需要唤醒，则把当前节点状态设置为PROPAGATE确保以后可以传递下去
                    else if (ws == 0 &&
                             !compareAndSetWaitStatus(h, 0, Node.PROPAGATE))
                        continue;                
                }
                //如果头结点没有发生变化，表示设置完成，退出循环
                //如果头结点发生变化，比如说其他线程获取到了锁，为了使自己的唤醒动作可以传递，必须进行重试
                if (h == head)                   
                    break;
            }
        }
    
    
      private void unparkSuccessor(Node node) {
            int ws = node.waitStatus;
            if (ws < 0)
                compareAndSetWaitStatus(node, ws, 0);
            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);
        }

共享式同步状态的获取和释放真的比较绕. 就个人理解而言,

*  共享的获取.在于将`Head`头结点的值进行了改变,导致了后续的结点也能够获取到了锁. (Head结点为 `volatile`类型变量)
 *  资源的释放,在于,释放自身资源的同时, 还要释放相同获取到资源的其他线程.  
    [深入浅出AQS之共享锁模式][AQS]

##### 独占式超时获取同步状态 #####

> tryAcquireNanos -> doAcquireNanos

public final boolean tryAcquireNanos(int arg, long nanosTimeout)
                throws InterruptedException {
            if (Thread.interrupted())
                throw new InterruptedException();
            return tryAcquire(arg) ||
                doAcquireNanos(arg, nanosTimeout);
        }
     private boolean doAcquireNanos(int arg, long nanosTimeout)
            throws InterruptedException {
            long lastTime = System.nanoTime();
            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;
                    }
                    if (nanosTimeout <= 0)
                        return false;
                    if (shouldParkAfterFailedAcquire(p, node) &&
                        nanosTimeout > spinForTimeoutThreshold)
                        LockSupport.parkNanos(this, nanosTimeout);
                    long now = System.nanoTime();
                    nanosTimeout -= now - lastTime;
                    lastTime = now;
                    if (Thread.interrupted())
                        throw new InterruptedException();
                }
            } finally {
                if (failed)
                    cancelAcquire(node);
            }
        }

通过`if (nanosTimeout <= 0)return false;`可以看出, 当超过时间界限, 会退出等待循环.

##### 自定义等待组件 TwinsLock #####

> TwinsLock

package com.yanxml.multithreading.art.lock;
    
    import java.util.concurrent.TimeUnit;
    import java.util.concurrent.locks.AbstractQueuedSynchronizer;
    import java.util.concurrent.locks.Condition;
    import java.util.concurrent.locks.Lock;
    
    /**
     * 同步式锁 允许2个线程进行访问
     * */
    public class TwinsLock implements Lock{
    	private final Sync sync = new Sync(2);
    	public static final class Sync extends AbstractQueuedSynchronizer{
    		public Sync(int count){
    			if(count<0){
    				throw new IllegalArgumentException();
    			}
    			setState(count);
    		}
    		
    //		public int tryAcquireShared(int reduceCount){
    		public int tryAcquireShared(int reduceCount){
    			for(;;){
    				int current = getState();
    				int newCount = current - reduceCount;
    				if(newCount < 0 || compareAndSetState(current, newCount)){
    //				if(compareAndSetState(current, newCount)){
    
    					return newCount;
    				}
    			}
    		}
    		
    //		public boolean tryReleaseShared(int returnCount){
    		public boolean tryReleaseShared(int returnCount){
    			for(;;){
    				int current = getState();
    				int newCount = current+returnCount;
    				if(compareAndSetState(current, newCount)){
    					return true;
    				}
    			}
    		}
    		
    	}
    	
    
    	@Override
    	public void lock() {
    		sync.acquireShared(1);
    	}
    
    	@Override
    	public void unlock() {
    		sync.releaseShared(1);
    	}
    
    	@Override
    	public void lockInterruptibly() throws InterruptedException {
    		
    	}
    
    	@Override
    	public boolean tryLock() {
    		return false;
    	}
    
    	@Override
    	public boolean tryLock(long time, TimeUnit unit)
    			throws InterruptedException {
    		return false;
    	}
    
    	@Override
    	public Condition newCondition() {
    		return null;
    	}
    
    }

> TwinsLockTest 测试类

package com.yanxml.multithreading.art.lock;
    
    import java.util.concurrent.locks.Lock;
    
    import com.yanxml.multithreading.art.lock.TwinsLock;
    
    /**
     * 用于测试TwinsLock的功能.
     * 
     * */
    class Worker extends Thread{
    	Lock lock;
    	public Worker(Lock lock){
    		this.lock = lock;
    	}
    	public void run(){
    		while(true){
    			lock.lock();
    			try{
    				Thread.sleep(3000);
    				System.out.println(Thread.currentThread().getName());
    				Thread.sleep(3000);
    			}catch(Exception e){
    				e.printStackTrace();
    			}finally{
    				lock.unlock();
    			}
    		}
    	}
    }
    public class TwinsLockTest {
    	public static void main(String[] args) throws InterruptedException {
    		Lock lock = new TwinsLock();
    		for(int i=0;i<10;i++){
    			Worker w = new Worker(lock);
    			w.setDaemon(true);
    			w.start();
    		}
    		
    		
    		for(int i=0;i<10;i++){
    			Thread.sleep(3000);
    			System.out.println();
    		}
    	}
    }

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

### Condition及其实现 ###

##### Condition常见API #####

*  `await()` 线程等待
 *  `awaitUninterruptibly()` 等待状态直到被通知
 *  `awaitNanos(long nanosTimeout)` 等待(时间段内未收到信息,唤醒)
 *  `boolean awaitUntil(Date deadline)` 等待(死亡时间)
 *  `void signal()` 唤醒一个等待`Condition`的线程
 *  `void signalAll()` 唤醒所有等待`Condition`的线程

##### Condition实现(阻塞队列) #####

##### Condition原理 #####

AQS的同步队列中, 同时维护一个等待队列.

*  当调用`Condition.await()`方法后,将其从AQS队列中取出,放入Condition的队列中.
 *  当调用`Condition.singnal()`方法后,将其从Condition的队列中拿出,放入AQS队列.  
    ![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly95YW54bWwuYmxvZy5jc2RuLm5ldA_size_16_color_FFFFFF_t_70 1]

> await /

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);
            }
    public final void signal() {
                if (!isHeldExclusively())
                    throw new IllegalMonitorStateException();
                Node first = firstWaiter;
                if (first != null)
                    doSignal(first);
            }

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

### Tips ###

##### LockSupport工具 #####

*  `void park()` 阻塞当前线程
 *  `void parkNanos(long nanos)` 阻塞当前线程(时间间隔nanos)
 *  `void parkUtil(long deadline)` 阻塞当前线程(直到deadline)
 *  `void unpark(Thread thread)` 释放某个线程的锁定

##### 双锁样例 #####

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

### Others ###

*  重入锁  
    获取时, 如果当前线程为锁的持有者,自增.  
    释放时, 需要进行多次释放.

final boolean nonfairTryAcquire(int acquires) {
                final Thread current = Thread.currentThread();
                int c = getState();
                if (c == 0) {
                    if (compareAndSetState(0, acquires)) {
                    // CAS 获取锁 成功即为所有者
                        setExclusiveOwnerThread(current);
                        return true;
                    }
                }
                else if (current == getExclusiveOwnerThread()) {
                // 如果当前线程为线程所有者 nextc=c+1
                    int nextc = c + acquires;
                    if (nextc < 0) // overflow
                        throw new Error("Maximum lock count exceeded");
                    setState(nextc);
                    return true;
                }
                return false;
            }
     protected final boolean tryRelease(int releases) {
                int c = getState() - releases;
                if (Thread.currentThread() != getExclusiveOwnerThread())
                    throw new IllegalMonitorStateException();
                boolean free = false;
                if (c == 0) {
                	// 如果c==0 说明全部释放成功了
                    free = true;
                    setExclusiveOwnerThread(null);
                }
                setState(c);
                return free;
            }

*  公平锁 / 非公平锁  
    `ReentrantLoc`k内`FairLock`与`NoFairLock`的实现各不相同.

public ReentrantLock() {
            sync = new NonfairSync();
    }
    public ReentrantLock(boolean fair) {
            sync = fair ? new FairSync() : new NonfairSync();
    }

protected final boolean tryAcquire(int acquires) {
                final Thread current = Thread.currentThread();
                int c = getState();
                if (c == 0) {
                    if (!hasQueuedPredecessors() &&
                        compareAndSetState(0, acquires)) {
                        setExclusiveOwnerThread(current);
                        return true;
                    }
                }
                else if (current == getExclusiveOwnerThread()) {
                    int nextc = c + acquires;
                    if (nextc < 0)
                        throw new Error("Maximum lock count exceeded");
                    setState(nextc);
                    return true;
                }
                return false;
            }
      // 我还是看不懂 这个判断的幺蛾子.
     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());
        }

*  读写锁  
    读写锁可以参考`ReentrantWriteReadLock`.

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

### Reference ###

\[1\] Java多线程核心  
\[2\] Java多线程编程艺术  
\[3\] [Java并发之AQS详解][Java_AQS]  
\[4\] [一步步透彻理解Lock的Acquire和Release原理源码][Lock_Acquire_Release]  
\[5\] [JDK中多线程之JUC锁的JDK源码解读配合大神的一起看，秒懂。][JDK_JUC_JDK]  
\[6\] [java并发编程的艺术——第五章总结（Lock锁与队列同步器）][java_Lock]  
\[7\] [setState 和 compareAndSetState方法作用分析][setState _ compareAndSetState]  
\[8\] [深入浅出AQS之共享锁模式][AQS]  
\[9\] [锁与CAS介绍][CAS]  
\[10\] [Java并发源码剖析（二）——AbstractQueuedSynchronizer共享模式][Java_AbstractQueuedSynchronizer]  
\[11\] [同步器节点的waitStatus解释][waitStatus]  
\[12\] [AQS——同步队列共享模式][AQS 1]  
\[13\] [Java中的锁——队列同步器][Java]  
\[14\] [Java线程并发中的锁——Lock(下)][Java_Lock]

[JUC_ _ _ _ CAS]: https://yanxml.blog.csdn.net/article/details/88678305
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly95YW54bWwuYmxvZy5jc2RuLm5ldA_size_16_color_FFFFFF_t_70]: /images/20220228/ea37943d39e6417d92e036d25280a981.png
[AQS]: https://www.jianshu.com/p/1161d33fc1d0
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly95YW54bWwuYmxvZy5jc2RuLm5ldA_size_16_color_FFFFFF_t_70 1]: /images/20220228/b42179c3d2d747198555e462535f7a2d.png
[Java_AQS]: https://www.cnblogs.com/waterystone/p/4920797.html
[Lock_Acquire_Release]: https://www.jianshu.com/p/e4301229f59e
[JDK_JUC_JDK]: https://blog.csdn.net/MRA__S__/article/details/59108417
[java_Lock]: https://www.cnblogs.com/lilinzhiyu/p/8125195.html
[setState _ compareAndSetState]: https://blog.csdn.net/walleit/article/details/88542881
[CAS]: https://blog.csdn.net/wen3011/article/details/78548521
[Java_AbstractQueuedSynchronizer]: https://www.jianshu.com/p/6b8579280475
[waitStatus]: https://www.jianshu.com/p/b8865fdf8445
[AQS 1]: https://blog.csdn.net/O11111001100/article/details/81279884
[Java]: https://blog.csdn.net/u010723709/article/details/50357247
[Java_Lock]: https://blog.csdn.net/fuyuwei2015/article/details/72571659