深入AQS底层源码

AQS概述

AQS是juc包下的一个抽象类，很多juc包下的工具类都是根据AQS是实现的，比如ThreadPoolExecutor、CountDownLatch、ReetrantLock、ReetrantWriteLock、Semaphore

AQS的核心内容

核心属性state

    /**
     * The synchronization state.
     */
    private volatile int state;

不同的实现，state的意义不同

以ReetrantLock为例：

state为0，没有线程持有这个lock锁

state > 0，当前lock锁被某个线程持有

不存在state小于0的情况

同步队列

    /**
     * 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;

Node：

    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;  //用于Condition，因此Condition是单向链表（不用prev和next）

        /**
         * 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;
        }
    }

以ReetrantLock为例，如果A线程想获取锁，但是发现锁资源被其他线程占用，A线程需要被封装成一个Node对象，进入到同步队列尾部排队，并挂起线程等待锁资源

Condition的单向链表

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;
        }

当持有lock锁的线程，执行了await方法，会将当前线程封装成Node，插入到Condition单向链表。等到其他线程执行singal唤醒，进入同步队列等到竞争锁资源。

lock锁和AQS的继承关系

基本关系

非公平锁的逻辑

基本逻辑：

1. 基于CAS操作，尝试将state从0变成1（在lock方法，非公平锁才有步骤1）

        1.1 成功拿到锁，执行

        1.2 竞争锁资源失败，进行后续的竞争 （进入2）

2. 执行tryAcquire的逻辑

        2.1 查看state是否为0，如果为0就再次执行cas尝试拿锁

        2.2 查看是否是锁重入的逻辑，直接对state+1，锁重入成功

        2.3 再次尝试拿锁的操作失败 （进入3） 

3. 执行addWaiter，准备进入同步队列排队

        3.1 将当前线程封装为Node对象

        3.2 将当前Node添加到同步队列的末尾

4. 执行accquireQueued的逻辑，一方面是再次抢锁，另一方面是让排队的线程挂起/阻塞

        4.1 抢锁只针对head.next的Node线程，抢锁时执行tyrAcquire的逻辑（步骤2）

        4.2 如果head.next的抢锁失败，以及没有资格抢锁的线程就要准备挂起

                4.2.1 找到prev节点，告知将要挂起

                4.2.2 基于park方法挂起当前线程

非公平锁和公平锁的直观体现

从源码上来看，公平和非公平的直观体现是lock方法和tryAcquire方法

lock方法一般是获取锁资源的入口方法，非公平锁会直接抢一次锁资源，而公平锁不会

acquire的底层逻辑

    /**
     * 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.
     */

    // arg = 1
    public final void acquire(int arg) {
        if (!tryAcquire(arg) &&
            acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
            selfInterrupt();
    }

就是之前提到的逻辑：

1. 先走tryAcquire，再次抢锁，抢到了就结束

2. 没抢到执行addWaiter方法准备排队，被封装成Node，进入同步队列

3. 再走acquireQueued方法，获取锁还是挂起线程，基于内部细粒度的逻辑

tryAcquire的底层逻辑

有公平锁和非公平锁两种实现

非公平锁实现

        /**
         * Performs non-fair tryLock.  tryAcquire is implemented in
         * subclasses, but both need nonfair try for trylock method.
         */

        // 
        final boolean nonfairTryAcquire(int acquires) {
            final Thread current = Thread.currentThread();
            int c = getState();
            
            // 步骤2.1
            if (c == 0) {
                // 执行CAS，尝试将state从0修改成1
                if (compareAndSetState(0, acquires)) {
                    //修改成功
                    setExclusiveOwnerThread(current);
                    return true;
                }
            }
            // 步骤2.2
            // 判断是不是锁重入
            else if (current == getExclusiveOwnerThread()) {
                int nextc = c + acquires;
                if (nextc < 0) // overflow
                    throw new Error("Maximum lock count exceeded");
                setState(nextc);
                // 锁重入成功
                return true;
            }
            return false;
        }

公平锁实现

        protected final boolean tryAcquire(int acquires) {
            final Thread current = Thread.currentThread();
            int c = getState();
            if (c == 0) {
                // 如果当前锁资源没被占用，需要满足一定的条件才能通过CAS抢锁
                // 1. 如果AQS的同步队列没有排队的Node，可以抢锁
                // 2. 如果AQS的同步队列有排队的Node，并且排在“第一名”的是当前线程，可以抢锁
                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;
        }

hasQueuedPredecessors方法：

​
    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());
    }

​

addWaiter的底层逻辑

    /**
     * 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;
    }

通过CAS保证将tali指向自己，从而保证了原子性

如果CAS失败，会不断死循环，不断指向1，2操作

accquireQueued的底层逻辑

补充

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

    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;
    }

    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);
        }
    }

    private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
        int ws = pred.waitStatus;
        if (ws == Node.SIGNAL) //当前线程挂起
            /*
             * 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;
    }


    private final boolean parkAndCheckInterrupt() {
        LockSupport.park(this);
        return Thread.interrupted();
    }


    static void selfInterrupt() {
        Thread.currentThread().interrupt();
    }

    /*
        在park中：
        只要permit为1或者中断状态为true，那么执行park就不能够阻塞线程。park只可能消耗掉permit，        
        但不会去消耗掉中断状态。
        因此需要interrupt()去消耗掉，并将这个中断状态暂时保存到一个局部变量interrupted中

        在selfInterrupt中：
        当parkAndCheckInterrupt()方法返回true后又调用了 selfInterrupt()方法重        
        新设置中断标记，这样做的目的是为了让用户代码（同步代码块）能够通过 
        Thread.isInterrupted()等方法 感知到线程在获取同步状态的过程中被中断过。
    */