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Java AQS(AbstractQueuedSynchronizer)源码解析

作者:花醉霜寒

AbstractQueuedSynchronizer被称为队列同步器,简称为大家熟知的AQS,这个类可以称作concurrent包的基础。本文将通过剖析源码来看看AQS是如何工作的,感兴趣的可以了解一下

AbstractQueuedSynchronizer被称为队列同步器,简称为大家熟知的AQS,这个类可以称作concurrent包的基础,该类提供了同步的基本功能。该类包括如下几个核心要素:

下面通过剖析源码来看看AQS是如何工作的。

AQS概要

AQS通过内部类Node记录当前是哪个线程持有锁,Node中有一个前驱节点和一个后继节点,形成一个双向链表,这个链表是一种CLH队列,其中waitStatus表示当前线程的状态,其可能的取值包括以下几种:

Node对象中还有一个nextWaiter变量,指向下一个条件等待节点,相当于在CLH队列的基础上维护了一个简单的单链表来关联条件等待的节点。

	static final class Node {

        static final Node SHARED = new Node();

        static final Node EXCLUSIVE = null;

        static final int CANCELLED =  1;

        static final int SIGNAL    = -1;

        static final int CONDITION = -2;
  
        static final int PROPAGATE = -3;

        volatile int waitStatus;

        volatile Node prev;

        volatile Node next;

        volatile Thread thread;

        Node nextWaiter;

        final boolean isShared() {
            return nextWaiter == SHARED;
        }

        final Node predecessor() throws NullPointerException {
            Node p = prev;
            if (p == null)
                throw new NullPointerException();
            else
                return p;
        }
        ...
        构造方法
        ...
    }

Node提供了两种入队列的方法,即enq和addWaiter,enq方法如下所示,当尾节点tail为null时,表明阻塞队列还没有被初始化,通过CAS操作来设置头结点,头结点为new Node(),实际上头结点中没有阻塞的线程,算得上是一个空的节点(注意空节点和null是不一样的),然后进行tail=head操作,这也说明当head=tail的时候,队列中实际上是不存在阻塞线程的,然后将需要入队列的node放入队列尾部,将tail指向node。

    private Node enq(final Node node) {
        for (;;) {
            Node t = tail;
        	//如果tail为空,说明CLH队列没有被初始化,
            if (t == null) {
            	//初始化CLH队列,将head和tail指向一个new Node(),
            	//此时虽然CLH有一个节点,但是并没有真正意义的阻塞线程
                if (compareAndSetHead(new Node()))
                    tail = head;
            } else {
            	//将node放入队列尾部,并通过cas将tail指向node
                node.prev = t;
                if (compareAndSetTail(t, node)) {
                    t.next = node;
                    return t;
                }
            }
        }
    }

addWaiter通常表示添加一个条件等待的节点入队列,该方法首先尝试通过CAS操作快速入队列,如果失败则通过调用enq来入队列。

    private Node addWaiter(Node mode) {
        Node node = new Node(Thread.currentThread(), mode);
        //尝试快速入队列
        Node pred = tail;
        if (pred != null) {
            node.prev = pred;
            if (compareAndSetTail(pred, node)) {
                pred.next = node;
                return node;
            }
        }
        //快速入队列失败则采用enq方入队列
        enq(node);
        return node;
    }

Node还提供了唤醒后继节点线程的功能,主要是通过LockSupport来实现的,源码如下所示,

    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);
    }

排他获取锁

不支持中断的获取锁\color{green}{不支持中断的获取锁}不支持中断的获取锁

	//不可中断的获取锁
    public final void acquire(int arg) {
        if (!tryAcquire(arg) &&
            acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
            //对中断做补偿,中断当前线程
            selfInterrupt();
    }

acquire方法首先会调用tryAcquire方法尝试获取锁,如果获取锁失败,首先通过addWaiter将当前线程放入CLH队列中,然后通过acquireQueued方法获取锁,acquireQueued方法源码如下所示:

    final boolean acquireQueued(final Node node, int arg) {
        boolean failed = true;
        try {
        	//记录中断状态
            boolean interrupted = false;
            //自旋式的获取锁
            for (;;) {
                final Node p = node.predecessor();
                //当前线程为CLH中的第一个阻塞线程才会尝试去获取锁
                if (p == head && tryAcquire(arg)) {
                	//获取成功则更新head
                    setHead(node);
                    p.next = null; // help GC
                    failed = false;
                    //返回中断状态
                    return interrupted;
                }
                //判断中断信息
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    interrupted = true;
            }
        } finally {
        	//如果获取锁失败,则取消获取锁的操作
            if (failed)
                cancelAcquire(node);
        }
    }

acquireQueued方法是无中断的获取锁,该方法有一个布尔类型的返回值,该值不是表示是否成功获取锁,而是标示当前线程的中断状态,因为acquireQueued方法是无法响应中断的,需要对中断进行补偿,这个补偿体现在acquire方法中。

    //模板方法tryAcquire需要子类进行具体实现
    protected boolean tryAcquire(int arg) {
        throw new UnsupportedOperationException();
    }

支持中断的获取锁

    //可中断的获取锁
    public final void acquireInterruptibly(int arg)
            throws InterruptedException {
        if (Thread.interrupted())
            throw new InterruptedException();
        if (!tryAcquire(arg))
            doAcquireInterruptibly(arg);
    }

acquireInterruptibly方法获取锁的过程中能够响应中断,主要体现在获取锁之前会判断一下当前线程的中断中断状态,若中断则抛出InterruptedException,然后通过tryAcquire获取锁,获取成功直接返回,获取失败则通过doAcquireInterruptibly获取锁,该方法和acquireQueued最大的区别就是在判断parkAndCheckInterrupt后,acquireQueued仅仅记录中断状态,parkAndCheckInterrupt则会抛出异常。

    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())
                    //抛出异常,响应中断
                    throw new InterruptedException();
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }

支持超时时间的获取锁功能

    public final boolean tryAcquireNanos(int arg, long nanosTimeout)
            throws InterruptedException {
        //响应中断
        if (Thread.interrupted())
            throw new InterruptedException();
        //首先通过tryAcquire快速获取锁,若失败则调用doAcquireNanos方法
        return tryAcquire(arg) ||
            doAcquireNanos(arg, nanosTimeout);
    }

从方法tryAcquireNanos的源码可以看出,该方法也是响应中断的,该方法首先调用模板方法tryAcquire快速的获取锁,如果失败则通过doAcquireNanos获取锁,doAcquireNanos中支持超时机制,其源码如下所示:

    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();
                //判断如果超时则直接返回false,代表获取锁失败
                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);
        }
    }

doAcquireNanos方法与acquireQueued方法的区别是每次循环获取锁过程中都会计算deadline和当前时间的差值,如果这个差值小于0,则表示获取锁的操作已经超时,则直接返回false表示获取锁失败。

共享锁获取

AQS中不仅支持排他锁的获取,即acquire、acquireInterruptibly和tryAcquireNanos,还提供了共享锁的获取操作方法,包括acquireShared、acquireSharedInterruptibly和tryAcquireSharedNanos,这三个方法源码如下所示:

    public final void acquireShared(int arg) {
        if (tryAcquireShared(arg) < 0)
            doAcquireShared(arg);
    }

    public final void acquireSharedInterruptibly(int arg)
            throws InterruptedException {
        if (Thread.interrupted())
            throw new InterruptedException();
        if (tryAcquireShared(arg) < 0)
            doAcquireSharedInterruptibly(arg);
    }

    public final boolean tryAcquireSharedNanos(int arg, long nanosTimeout)
            throws InterruptedException {
        if (Thread.interrupted())
            throw new InterruptedException();
        return tryAcquireShared(arg) >= 0 ||
            doAcquireSharedNanos(arg, nanosTimeout);
    }

共享锁的获取和排他锁的获取方法类似,共享锁调用了不同的模板方法tryAcquireShared,这里介绍一下doAcquireShared方法,其他方法变化的套路和共享锁的使用套路一样,doAcquireShared方法源码如下所示:

    private void doAcquireShared(int arg) {
    	//当前线程入队列
        final Node node = addWaiter(Node.SHARED);
        boolean failed = true;
        try {
            boolean interrupted = false;
            //自旋式的获取锁
            for (;;) {
                final Node p = node.predecessor();
                //只有队列中的第一个阻塞线程才能获取锁	
                if (p == head) {
                    int r = tryAcquireShared(arg);
                    if (r >= 0) {
                        setHeadAndPropagate(node, r);
                        p.next = null; // help GC
                        //获取锁成功,补偿中断
                        if (interrupted)
                            selfInterrupt();
                        failed = false;
                        return;
                    }
                }
                //通过interrupted记录中断信息
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    interrupted = true;
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }

doAcquireShared方法没有返回值,与acquireQueued不同的是:

    private void setHeadAndPropagate(Node node, int propagate) {
        Node h = head; 
        //设置新的头结点
        setHead(node);
        if (propagate > 0 || h == null || h.waitStatus < 0 ||
            (h = head) == null || h.waitStatus < 0) {
            Node s = node.next;
            //如果后继节点为空或者为SHARED类型的节点,执行doReleaseShared方法
            if (s == null || s.isShared())
                doReleaseShared();
        }
    }

    private void doReleaseShared() {
        for (;;) {
            Node h = head;
            if (h != null && h != tail) {
                int ws = h.waitStatus;
                if (ws == Node.SIGNAL) {
                	//状态为SIGNAL,则唤醒后继节点中的线程
                    if (!compareAndSetWaitStatus(h, Node.SIGNAL, 0))
                        continue;            
                    unparkSuccessor(h);
                }
                //若状态为0,则设置状态为PROPAGATE
                else if (ws == 0 &&
                         !compareAndSetWaitStatus(h, 0, Node.PROPAGATE))
                    continue;                
            }
            if (h == head)                  
                break;
        }
    }

锁的释放

锁的释放也分为释放排他锁和释放共享锁,分别为release方法和releaseShared方法,源码如下所示,

	//释放排他锁
    public final boolean release(int arg) {
    	//释放锁,然后唤醒后继节点的线程
        if (tryRelease(arg)) {
            Node h = head;
            if (h != null && h.waitStatus != 0)
                unparkSuccessor(h);
            return true;
        }
        return false;
    }


    //释放共享锁
    public final boolean releaseShared(int arg) {
    	//释放锁,然后调用doReleaseShared方法
        if (tryReleaseShared(arg)) {
            doReleaseShared();
            return true;
        }
        return false;
    }

release方法和releaseShared方法分别调用模板方法tryRelease和tryReleaseShared来释放锁,release方法中直接通过调用unparkSuccessor唤醒后继线程,而releaseShared的唤醒操作在doReleaseShared方法中进行。

取消获取锁

当获取锁失败时,需要进行一些状态清理和变化,cancelAcquire方法就是用来实现这些功能的,其源码如下所示,

    private void cancelAcquire(Node node) {
       
        if (node == null)
            return;
        //节点线程置为null
        node.thread = null;

        //从CLH队列中清除已经取消的节点(CANCELLED)
        Node pred = node.prev;
        while (pred.waitStatus > 0)
            node.prev = pred = pred.prev;

        Node predNext = pred.next;

        node.waitStatus = Node.CANCELLED;

        //判断如果node是尾部节点,则设置尾部节点
        if (node == tail && compareAndSetTail(node, pred)) {
            compareAndSetNext(pred, predNext, null);
        } else {
            int ws;
           	//若不是头节点则直接从CLH队列中清除当前节点
            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);
            //若为头结点,则唤醒后继节点中的线程
            } else {
                unparkSuccessor(node);
            }

            node.next = node; // help GC
        }
    }

取消获取锁的操作首先将队列中处于CANCELLED状态的节点剔除,然后根据当前节点在CLH队列中的位置进行不同的操作:

总结

AQS是整个concurrent包的基础,可重入锁、线程池、信号量(Semaphore)等同步工具类都需要借助AQS来完成,了解AQS是深入学习concurrent包的前提。

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