1、简介

在高并发下，CAS会恶性空自旋造成大量CPU资源浪费，解决CAS恶性空自旋的方式之一为空间换时间，较为常见的方法有两种，分散热点和队列削峰。
JUC并发包是通过队列削峰的方案解决CAS性能问题，并提供了一个基于双向队列削峰的抽象基类AbstractQueuedSynchronizer（抽象同步器类，简称AQS）

AQS是构建锁或者其他同步组件的基础框架（如ReentrantLock、ReentrantReadWriteLock、Semaphore等），JUC并发包的作者（Doug Lea）期望它能够成为实现大部分同步需求的基础。它是JUC并发包中的核心基础组件。

有了AQS，构建线程协作类就容易多了。AQS的主要使用方式是继承，子类通过继承同步器并实现它的抽象方法来管理同步状态

AQS实现类

2、AQS三要素

2.1、同步状态

AQS使用一个int类型的成员变量state来表示同步状态。State的具体含义会根据具体实现类的不同而不同，比如在Semaphore里，它表示“剩余许可证的数量”，而CountDownLatch中，它表示“还需要倒数的数量”，ReentrantLcok中它表示锁的占有情况，包括可重入计数

2.2、控制线程抢锁和配合的FIFO队列

队列用来存放等待的线程，AQS是排队管理器，当多个线程争用同一把锁的时候，必须有排队机制将没有拿到锁的线程管理起来。当锁释放时，锁管理器就会挑选一个合适的线程来占用这个刚释放的锁。

2.3、期望协作类去实现的获取/释放等重要方法

这里的获取和释放方法是利用AQS的协作工具累里最重要的方法，是由协作类自己去实现的，并且含义各不相同。

获取方法

获取操作经常会依赖state变量，经常会阻塞

在semaphore中，获取就是acquire方法，作用是获取一个许可证
在CountDownLatch里面，获取就是await方法，作用就是等待，直到倒数结束

释放方法

释放操作不会被阻塞

在Semaphore中，释放就是release方法，作用是释放一个许可证
在CountDownLatch里面，获取就是CountDown方法，作用是倒数一个数

3、AQS源码解析

3.1、AQS用法

3.2、AQS在COuntDownLatch中的应用

构造函数

 public CountDownLatch(int count) {
        if (count < 0) throw new IllegalArgumentException("count < 0");
        this.sync = new Sync(count);
    }
   Sync(int count) {
            setState(count);
        }
  protected final void setState(int newState) {
        state = newState;
    }

getCount()

 public long getCount() {
        return sync.getCount();
    }
  int getCount() {
            return getState();
        }
  protected final int getState() {
        return state;
    }
 protected boolean tryReleaseShared(int releases) {
            // Decrement count; signal when transition to zero
            for (;;) {
                int c = getState();
                if (c == 0)
                    return false;
                int nextc = c-1;
                if (compareAndSetState(c, nextc))
                    return nextc == 0;
            }
        }

countDown()

    public void countDown() {
        sync.releaseShared(1);
    }
public final boolean releaseShared(int arg) {
        if (tryReleaseShared(arg)) {
            doReleaseShared();
            return true;
        }
        return false;
    }
//自旋和CAS倒数,倒数到0时返回true
 protected boolean tryReleaseShared(int releases) {
            // Decrement count; signal when transition to zero
            for (;;) {
                int c = getState();
                if (c == 0)
                    return false;
                int nextc = c-1;
                if (compareAndSetState(c, nextc))
                    return nextc == 0;
            }
        }
//唤醒阻塞线程
 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;
        }
    }

await()

  public void await() throws InterruptedException {
        sync.acquireSharedInterruptibly(1);
    }
   public final void acquireSharedInterruptibly(int arg)
            throws InterruptedException {
        if (Thread.interrupted())
            throw new InterruptedException();
        if (tryAcquireShared(arg) < 0)
            doAcquireSharedInterruptibly(arg);
    }
  protected int tryAcquireShared(int acquires) {
            return (getState() == 0) ? 1 : -1;
        }
    //把当前线程放入阻塞队列，并使线程陷入阻塞状态
    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);
        }
    }
    private final boolean parkAndCheckInterrupt() {
        LockSupport.park(this);
        return Thread.interrupted();
    }
   public static void park(Object blocker) {
        Thread t = Thread.currentThread();
        setBlocker(t, blocker);
        UNSAFE.park(false, 0L);
        setBlocker(t, null);
    }

3.3、AQS在Semaphore中的应用

acquire()

 public void acquire() throws InterruptedException {
        sync.acquireSharedInterruptibly(1);
    }
   public final void acquireSharedInterruptibly(int arg)
            throws InterruptedException {
        if (Thread.interrupted())
            throw new InterruptedException();
        if (tryAcquireShared(arg) < 0)
            doAcquireSharedInterruptibly(arg);
    }
 protected int tryAcquireShared(int acquires) {
            for (;;) {
                if (hasQueuedPredecessors())
                    return -1;
                int available = getState();
                int remaining = available - acquires;
                if (remaining < 0 ||
                    compareAndSetState(available, remaining))
                    return remaining;
            }
        }
 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);
        }
    }

3.4、AQS在ReentrantLock中的应用

unlock()

 public void unlock() {
        sync.release(1);
    }
  public final boolean release(int arg) {
        if (tryRelease(arg)) {
            Node h = head;
            if (h != null && h.waitStatus != 0)
                unparkSuccessor(h);
            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) {
                free = true;
                setExclusiveOwnerThread(null);
            }
            setState(c);
            return free;
        }
//唤醒线程
    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);
    }

lock()

 public void lock() {
        sync.lock();
    }
FairSync：
 final void lock() {
            acquire(1);
        }
  public final void acquire(int arg) {
        if (!tryAcquire(arg) &&
            acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
            selfInterrupt();
    }
 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;
        }
    }
  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);
        }
    }

4、利用AQS实现自己的latch门栓

public class OneSHotLatch {
    private Sync sync = new Sync();
    public void await(){
        sync.acquireShared(0);
    }
    public void signal(){
        sync.releaseShared(0);
    }
    private class Sync extends AbstractQueuedSynchronizer{
        @Override
        protected int tryAcquireShared(int arg) {
            return (getState() == 1) ? 1 : -1;
        }
        @Override
        protected boolean tryReleaseShared(int arg) {
            setState(1);
            return true;
        }
    }
    public static void main(String[] args) throws InterruptedException {
        OneSHotLatch oneSHotLatch = new OneSHotLatch();
        ExecutorService executorService = Executors.newFixedThreadPool(10);
        Runnable runnable = () -> {
            System.out.println(Thread.currentThread().getName() + "尝试获取锁，获取失败则等待");
            oneSHotLatch.await();
            System.out.println(Thread.currentThread().getName() +  "获取锁成功，继续运行");
        };
        IntStream.range(0,10).forEach(e -> executorService.submit(runnable));
        Thread.sleep(1000);
        oneSHotLatch.signal();
        new Thread(runnable).start();
    }
}

5、AQS资料

11-10 AQS补充材料（选修）.pdf

并发编程

9、AQS