并发编程（4）——AbstractQueuedSynchronizer

AQS

内部类Node

等待队列是CLH有锁队列的变体。

waitStatus的几种状态：

  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;

以下面的测试程序为例，简单介绍一下同步队列的变化：

	@Test
    public void test() {
        CountDownLatch countDownLatch = new CountDownLatch(1);
        ReentrantLock lock = new ReentrantLock();
        try {
            for (int i = 0; i < 5; i++) {

                new Thread(new Runnable() {
                    @Override
                    public void run() {
                        lock.lock();
                    }
                }, "线程 " + i
                ).start();
            }

            countDownLatch.await();
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
//            lock.unlock();
        }

我们发现，ReentrantLock的lock方法如下：

		final void lock() {
            if (compareAndSetState(0, 1))
                setExclusiveOwnerThread(Thread.currentThread());
            else
                acquire(1);
        }

由于是独占的获取，因此只有一个线程会通过CAS成功获取state，因此其它四个线程都会进入acquire(1)方法。acquire(int arg)是AQS的模板方法，方法内容如下：

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

以非公平锁为例，tryAcquire实际调用nonfairTryAcquire.该方法可以看出，首先还是通过CAS来获取state，如果是owner是之前的那个线程的话，允许重入，acquire加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;
        }

继续回到刚才的acquire方法，会发现tryAcquire方法返回false，调用addWaiter方法：

	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;
    }

假设最开始是线程0获取了state，后面来的依次是线程1、线程2、线程3、线程4.

线程1进入addWaiter方法，tail为空，进入enq方法，这里会初始化AQS中的head和tail，例子里的话head是一个new Node对象，tail的Node对象是new Node(“线程1”, mode)对象。

	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;
                }
            }
        }
    }

继续，执行完addWaiter方法之后会进入acquireQueued方法：

	final boolean acquireQueued(final Node node, int arg) {
        boolean failed = true;
        try {
            boolean interrupted = false;
            for (;;) {
            	// 找到node的前辈节点
                final Node p = node.predecessor();
                // 如果线程0不释放，则该不会进入
                // 如果线程0释放state，并且p是head，也就是同步队列中的第一个任务，这个时候获取state成功，将node设置为AQS的head，返回false，结束acquire方法。
                if (p == head && tryAcquire(arg)) {
                    setHead(node);
                    p.next = null; // help GC
                    failed = false;
                    return interrupted;
                }
                // 第一个判断，判断node的前辈节点是否为-1或者大于0，否则设置状态为-1，再下一次循环时，返回true进入第二个判断
                // 第二个判断，将node对应的线程park，即设置为wait状态
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    interrupted = true;
            }
        } finally {
            if (failed)
                cancelAcquire(node);
        }
    }

其余的线程2/3/4依次在同步队列上，类似于：

+-----------+ +-----------+ + -----------+ + -----------+ +-----------+

| head | | 线程1 | | 线程2 | | 线程3 | | 线程4|

+-----------+ +-----------+ + -----------+ + -----------+ +-----------+

以下面测试程序为例，再看unlock方法(顺便提一下，idea调试多线程需要将断点处的all改为thread， 程序中的countdownlatch是为了不让test线程结束，导致无法调试)调试时看到一个线程进入release方法，其余四个线程处于wait状态，说明程序正确了。

	@Test
    public void test() {
        CountDownLatch countDownLatch = new CountDownLatch(1);
        ReentrantLock lock = new ReentrantLock();
        try {
            for (int i = 0; i < 5; i++) {
                new Thread(new Runnable() {
                    @Override
                    public void run() {
                        lock.lock();
                        try {
                            Thread.sleep(1000);
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                        if ( lock.isHeldByCurrentThread()) {

                            lock.unlock();
                        }
                    }
                }, "线程 " + i
                ).start();
            }

            countDownLatch.await();
        } catch (InterruptedException e) {
            e.printStackTrace();
        } finally {
        }
    }

讲完了lock()方法，再看unlock()方法，调用release(int arg)方法

	public final boolean release(int arg) {
        if (tryRelease(arg)) {
            Node h = head;
            if (h != null && h.waitStatus != 0)
                unparkSuccessor(h);
            return true;
        }
        return false;
    }

tryRelease(arg)不再赘述，不过是释放获得的许可，将state设置为0（一般情况下，有些是重入，需要多调用几次unlock才行),置空独占线程。

进入if内部，调用unparkSuccessor方法

	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);
    }

正常情况下，唤醒同步队列中的第一个任务线程

acquireShared

上面讲的是独占获取，接下来看一下共享获取

这里以ReentrantReadWriteLock为例

简单介绍一下内部类,包含一个同步器Sync，以及公平及非公平类FairSync与NonfairSync，ReadLock和WriteLock

因为读锁非独占，因此lock方法对应的是sync.tryAcquireShared(1),写锁则相反。

其他

AQS使用了模板方法设计模式。