AQS源码分析

AbstractQueueSynchronizer简称AQS（抽象的队列同步器），是重量级基础框架以及JUC体系的基石，主要用于解决锁分配给谁的问题。

AQS入门级理论知识

整体就是一个抽象的FIFO队列来完成线程获取资源排队的工作，并通过一个int类变量（state）表示持有锁的状态。

ReentrantLock、CountDownLatch、ReentrantReadWriteLock、Semaphore底层都是调用的AQS。

AQS中含有静态内部类Node用来存储阻塞线程信息，头尾指针，以及state持有锁状态

ReentrantLock与AQS

ReentrantLock实现了Lock、Serializable接口，ReentrantLock含有静态内部类Sync、FairSync、NofairSync，其中公平锁（FairSync）与非公平锁（NofairSync）都继承了Sync，而Sync继承了AbstractQueueSynchronizer（AQS）类。

创建对象时参数决定是否使用公平锁：

非公平锁在加锁时首先会尝试抢锁，

• 枪锁成功：则会修改锁状态由未被持有0更改为被持有1，然后该锁的持有者为当前线程。

• 枪锁失败：则会和公平锁一样调用AQS中 acquire(1) 方法，然后各自调用自己实现AQS（Sync）中tryAcquire(1)方法。

    static final class NonfairSync extends Sync {
        private static final long serialVersionUID = 7316153563782823691L;

        /**
         * Performs lock.  Try immediate barge, backing up to normal
         * acquire on failure.
         */
        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);
        }

        /**
         * Fair version of tryAcquire.  Don't grant access unless
         * recursive call or no waiters or is first.
         */
        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;
        }
    }

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

非公平锁的tryAcquire()方法：

首先调用NonfairSync的tryAcquire()方法然后进入Sync中的nonfairTryAcquire()方法，

获取当前线程，获取锁持有状态。1被持有，0则未被持有。若锁未被

• state == 1，锁被持有。比较持有锁的线程是否是当前线程，

• state == 0，锁未被持有，比较并交换锁状态，设置当前锁持有线程，返回true。

        protected final boolean tryAcquire(int acquires) {
            return nonfairTryAcquire(acquires);
        }

        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;
        }

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
                if (compareAndSetHead(new Node()))
                    tail = head;
            } else {
                node.prev = t;
                if (compareAndSetTail(t, node)) {
                    t.next = node;
                    return t;
                }
            }
        }
    }

Node.EXCLUSIVE 独占、值为null，创建一个含有当前线程信息的Node节点，

acquireQueued(addWaiter(Node.EXCLUSIVE), arg)