并发数据结构与线程（ArrayBlockingQueue）

今天在QQ群上抛出来一个问题，如下

我以Java自带的数据结构为例，用源码的形式说明，如何阻塞线程、通知线程的。

一、Lock & Condition
ArrayBlockingQueue以可重入锁和两个Condition对象来控制并发。

    /*
     * Concurrency control uses the classic two-condition algorithm
     * found in any textbook.
     */

    /** Main lock guarding all access */
    private final ReentrantLock lock;
    /** Condition for waiting takes */
    private final Condition notEmpty;
    /** Condition for waiting puts */
    private final Condition notFull;

构造函数中初始化了notEmpty和notFull.

    /**
     * Creates an <tt>ArrayBlockingQueue</tt> with the given (fixed)
     * capacity and the specified access policy.
     * @param capacity the capacity of this queue
     * @param fair if <tt>true</tt> then queue accesses for threads blocked
     *        on insertion or removal, are processed in FIFO order;
     *        if <tt>false</tt> the access order is unspecified.
     * @throws IllegalArgumentException if <tt>capacity</tt> is less than 1
     */
    public ArrayBlockingQueue(int capacity, boolean fair) {
        if (capacity <= 0)
            throw new IllegalArgumentException();
        this.items = new Object[capacity];
        lock = new ReentrantLock(fair);
        notEmpty = lock.newCondition();
        notFull =  lock.newCondition();
    }

二、线程阻塞
当ArrayBlockingQueue存储的元素是0个的时候，take()方法会阻塞.

    public Object take() throws InterruptedException {
        final ReentrantLock lock = this.lock;
        lock.lockInterruptibly();
        try {
            try {
                while (count == 0)
                    notEmpty.await();
            } catch (InterruptedException ie) {
                notEmpty.signal(); // propagate to non-interrupted thread
                throw ie;
            }
            Object x = extract();
            return x;
        } finally {
            lock.unlock();
        }
    }

这里take方法首先获得可重入锁lock,然后判断如果元素为空就执行notEmpty.await(); 这个时候线程挂起。

三、通知线程
比如使用put放入一个新元素，

    /**
     * Inserts the specified element at the tail of this queue, waiting
     * for space to become available if the queue is full.
     *
     * @throws InterruptedException {@inheritDoc}
     * @throws NullPointerException {@inheritDoc}
     */
    public void put(E e) throws InterruptedException {
        checkNotNull(e);
        final ReentrantLock lock = this.lock;
        lock.lockInterruptibly();
        try {
            while (count == items.length)
                notFull.await();
            enqueue(e);
        } finally {
            lock.unlock();
        }
    }

在enqueue方法中，

    /**
     * Inserts element at current put position, advances, and signals.
     * Call only when holding lock.
     */
    private void enqueue(E x) {
        // assert lock.getHoldCount() == 1;
        // assert items[putIndex] == null;
        final Object[] items = this.items;
        items[putIndex] = x;
        if (++putIndex == items.length)
            putIndex = 0;
        count++;
        notEmpty.signal();
    }

对刚才的notEmptyCondition进行通知。
四、ReentrantLock vs AbstractQueuedSynchronizer
ArrayBlockingQueue使用ReentrantLock来控制并发，同时也使用ArrayBlockingQueue的Condition对象来与线程交互。notEmpty和notFull都是由
ReentrantLock的成员变量sync生成的，

public Condition newCondition() {
        return sync.newCondition();
    }

sync可以认为是一个抽象类类型，Sync，它是在ReentrantLock内部定义的静态抽象类，抽象类实现了newCondition方法，

final ConditionObject newCondition() {
            return new ConditionObject();
        }

返回的类型是实现了Condition接口的ConditionObject类，这是在AbstractQueuedSynchronizer内部定义的类。在ArrayBlockingQueue中的notEmpty就是ConditionObject实例。

阻塞：
当ArrayBlockingQueue为空时，notEmpty.await()将自己挂起，如ConditionObject的await方法，

        /**
         * Implements interruptible condition wait.
         * <ol>
         * <li> If current thread is interrupted, throw InterruptedException.
         * <li> Save lock state returned by {@link #getState}.
         * <li> Invoke {@link #release} with saved state as argument,
         *      throwing IllegalMonitorStateException if it fails.
         * <li> Block until signalled or interrupted.
         * <li> Reacquire by invoking specialized version of
         *      {@link #acquire} with saved state as argument.
         * <li> If interrupted while blocked in step 4, throw InterruptedException.
         * </ol>
         */
        public final void await() throws InterruptedException {
            if (Thread.interrupted())
                throw new InterruptedException();
            Node node = addConditionWaiter();
            int savedState = fullyRelease(node);
            int interruptMode = 0;
            while (!isOnSyncQueue(node)) {
                LockSupport.park(this);
                if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
                    break;
            }
            if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
                interruptMode = REINTERRUPT;
            if (node.nextWaiter != null) // clean up if cancelled
                unlinkCancelledWaiters();
            if (interruptMode != 0)
                reportInterruptAfterWait(interruptMode);
        }

addConditionWaiter是将当前线程作为一个node加入到ConditionObject的队列中，队列是用链表实现的。
如果是初次加入队列的情况，node.waitStatus == Node.CONDITION成立，方法isOnSyncQueue返回false，那么就将线程park。

while (!isOnSyncQueue(node)) {
                LockSupport.park(this);
                ....
}

至此线程被挂起，LockSupport.park(this);这里this是指ConditionObject,是notEmpty.
通知：
当新的元素put进入ArrayBlockingQueue后，notEmpty.signal()通知在这上面等待的线程，如ConditionObject的signal方法，

/**
         * Moves the longest-waiting thread, if one exists, from the
         * wait queue for this condition to the wait queue for the
         * owning lock.
         *
         * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
         *         returns {@code false}
         */
        public final void signal() {
            if (!isHeldExclusively())
                throw new IllegalMonitorStateException();
            Node first = firstWaiter;
            if (first != null)
                doSignal(first);
        }

doSignal方法，

        /**
         * Removes and transfers nodes until hit non-cancelled one or
         * null. Split out from signal in part to encourage compilers
         * to inline the case of no waiters.
         * @param first (non-null) the first node on condition queue
         */
        private void doSignal(Node first) {
            do {
                if ( (firstWaiter = first.nextWaiter) == null)
                    lastWaiter = null;
                first.nextWaiter = null;
            } while (!transferForSignal(first) &&
                     (first = firstWaiter) != null);
        }

doSignal一开始接收到的参数就是firstWaiter这个参数，在内部实现中用了do..while的形式，首先将first的的nextWaiter找出来保存到firstWaiter此时（first和firstWaiter不是一回事），在while的比较条件中可调用了transferForSignal方法，
整个while比较条件可以看着短路逻辑，如果transferForSignal结果为true,后面的first = firstWaiter就不执行了，整个while循环就结束了。
参照注释，看

transferForSignal方法，

    /**
     * Transfers a node from a condition queue onto sync queue.
     * Returns true if successful.
     * @param node the node
     * @return true if successfully transferred (else the node was
     * cancelled before signal)
     */
    final boolean transferForSignal(Node node) {
        /*
         * If cannot change waitStatus, the node has been cancelled.
         */
        if (!compareAndSetWaitStatus(node, Node.CONDITION, 0))
            return false;

        /*
         * Splice onto queue and try to set waitStatus of predecessor to
         * indicate that thread is (probably) waiting. If cancelled or
         * attempt to set waitStatus fails, wake up to resync (in which
         * case the waitStatus can be transiently and harmlessly wrong).
         */
        Node p = enq(node);
        int ws = p.waitStatus;
        if (ws > 0 || !compareAndSetWaitStatus(p, ws, Node.SIGNAL))
            LockSupport.unpark(node.thread);
        return true;
    }

首先确保想要被signal的等待node还是处于Node.CONDITION状态，然后调整状态为Node.SIGNAL，这两个都是采用CAS方法，最后调用的是

LockSupport.unpark(node.thread);

五、LockSupport
至此，我们已经知道了线程的挂起和通知都是使用LockSupport来完成的，并发数据结构与线程直接的交互最终也是需要LockSupport。那么关于LockSupport，我们又可以了解多少呢？

Ref:
Java中的ReentrantLock和synchronized两种锁定机制的对比
Java的LockSupport.park()实现分析