加锁过程(源码)

源码

从lock方法开始

1.lock

    static final class NonfairSync extends Sync {
        private static final long serialVersionUID = 7316153563782823691L;
        final void lock() {
            if (compareAndSetState(0, 1))
                setExclusiveOwnerThread(Thread.currentThread());
            else
                acquire(1);
        }
        protected final boolean tryAcquire(int acquires) {
            return nonfairTryAcquire(acquires);
        }
    }

    static final class FairSync extends Sync {
        private static final long serialVersionUID = -3000897897090466540L;

        final void lock() {
            acquire(1);
        }

        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;
        }
    }

2.AbstractQueuedSynchronizer.acquire()

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

3.AbstractQueuedSynchronizer.tryAcquire() => nonfairTryAcquire()

        final boolean nonfairTryAcquire(int acquires) {
            final Thread current = Thread.currentThread();
            int c = getState();
            if (c == 0) {
                if (compareAndSetState(0, acquires)) {
                    setExclusiveOwnerThread(current);
                    return true;
                }
            }
            else if (current == getExclusiveOwnerThread()) {
                int nextc = c + acquires;
                if (nextc < 0) // overflow
                    throw new Error("Maximum lock count exceeded");
                setState(nextc);
                return true;
            }
            return false;
        }

3.1 如果t2加锁成功 setExclusiveOwnerThread()

  protected final void setExclusiveOwnerThread(Thread thread) {
        exclusiveOwnerThread = thread;
    }

4. t2加锁失败 先addWaiter()

    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;
    }

    private Node enq(final Node node) {
        for (;;) {
            Node t = tail;
            if (t == null) { // Must initialize
                // 初始化 如果头结点为空，创建一个空的Node
                if (compareAndSetHead(new Node()))
                    tail = head;
            } else {
                // 将node加入队尾(双向链表)
                node.prev = t;                
                if (compareAndSetTail(t, node)) {
                    t.next = node;
                    return t;
                }
            }
        }
    }

5.再acquireQueued()

    final boolean acquireQueued(final Node node, int arg) {
        boolean failed = true;
        try {
            //interrupted 这个值用来防止打断
            boolean interrupted = false;
            for (;;) {
                final Node p = node.predecessor();
                //判断前一个节点是不是头结点，也就是初始化的那个空节点，如果是，就表示队列前面没有人排队，自己可以抢锁。
                if (p == head && tryAcquire(arg)) {
                    setHead(node);
                    p.next = null; // help GC
                    failed = false;
                    return interrupted;
                }
                //前面有人，就需要排队了
                // shouldParkAfterFailedAcquire  判断加锁失败之后是否需要去睡眠
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    interrupted = true;
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }

5.1 shouldParkAfterFailedAcquire 告诉前一个节点解锁的时候把自己叫醒

    private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
        int ws = pred.waitStatus;
        // 判断上一个节点是不是需要唤醒别人的节点
        if (ws == Node.SIGNAL)
            /*
             * This node has already set status asking a release
             * to signal it, so it can safely park.
             */
            //如果上一个节点已经设置为-1，自己就直接去睡眠
            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.
             */
            // 如果上一个节点还没有设置-1，就把它改成-1
            compareAndSetWaitStatus(pred, ws, Node.SIGNAL);
        }
        return false;
    }

加锁流程图

加锁之前

第一次加锁之后

第二个线程加锁失败的场景

第二个线程加锁成功的场景

1. t1先释放锁

1. t2加锁成功

第三个线程来加锁的场景