摘要：今天在群上拋出來(lái)一個(gè)問(wèn)題，如下我以自帶的數(shù)據(jù)結(jié)構(gòu)為例，用源碼的形式說(shuō)明，如何阻塞線程通知線程的。一以可重入鎖和兩個(gè)對(duì)象來(lái)控制并發(fā)。四使用來(lái)控制并發(fā)，同時(shí)也使用的對(duì)象來(lái)與線程交互。

今天在QQ群上拋出來(lái)一個(gè)問(wèn)題，如下

我以Java自帶的數(shù)據(jù)結(jié)構(gòu)為例，用源碼的形式說(shuō)明，如何阻塞線程、通知線程的。

一、Lock & Condition
ArrayBlockingQueue以可重入鎖和兩個(gè)Condition對(duì)象來(lái)控制并發(fā)。

    /*
     * 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;

構(gòu)造函數(shù)中初始化了notEmpty和notFull.

    /**
     * Creates an ArrayBlockingQueue with the given (fixed)
     * capacity and the specified access policy.
     * @param capacity the capacity of this queue
     * @param fair if true then queue accesses for threads blocked
     *        on insertion or removal, are processed in FIFO order;
     *        if false the access order is unspecified.
     * @throws IllegalArgumentException if capacity 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、線程阻塞
當(dāng)ArrayBlockingQueue存儲(chǔ)的元素是0個(gè)的時(shí)候，take()方法會(huì)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();
        }
    }

這里take方法首先獲得可重入鎖lock,然后判斷如果元素為空就執(zhí)行notEmpty.await(); 這個(gè)時(shí)候線程掛起。

三、通知線程
比如使用put放入一個(gè)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();
    }

對(duì)剛才的notEmptyCondition進(jìn)行通知。
四、ReentrantLock vs AbstractQueuedSynchronizer
ArrayBlockingQueue使用ReentrantLock來(lái)控制并發(fā)，同時(shí)也使用ArrayBlockingQueue的Condition對(duì)象來(lái)與線程交互。notEmpty和notFull都是由
ReentrantLock的成員變量sync生成的，

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

sync可以認(rèn)為是一個(gè)抽象類(lèi)類(lèi)型，Sync，它是在ReentrantLock內(nèi)部定義的靜態(tài)抽象類(lèi)，抽象類(lèi)實(shí)現(xiàn)了newCondition方法，

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

返回的類(lèi)型是實(shí)現(xiàn)了Condition接口的ConditionObject類(lèi)，這是在AbstractQueuedSynchronizer內(nèi)部定義的類(lèi)。在ArrayBlockingQueue中的notEmpty就是ConditionObject實(shí)例。

阻塞：
當(dāng)ArrayBlockingQueue為空時(shí)，notEmpty.await()將自己掛起，如ConditionObject的await方法，

        /**
         * Implements interruptible condition wait.
         * 

         * 
 If current thread is interrupted, throw InterruptedException.
         * 
 Save lock state returned by {@link #getState}.
         * 
 Invoke {@link #release} with saved state as argument,
         *      throwing IllegalMonitorStateException if it fails.
         * 
 Block until signalled or interrupted.
         * 
 Reacquire by invoking specialized version of
         *      {@link #acquire} with saved state as argument.
         * 
 If interrupted while blocked in step 4, throw InterruptedException.
         * 
         */
        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是將當(dāng)前線程作為一個(gè)node加入到ConditionObject的隊(duì)列中，隊(duì)列是用鏈表實(shí)現(xiàn)的。
如果是初次加入隊(duì)列的情況，node.waitStatus == Node.CONDITION成立，方法isOnSyncQueue返回false，那么就將線程park。

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

至此線程被掛起，LockSupport.park(this);這里this是指ConditionObject,是notEmpty.
通知：
當(dāng)新的元素put進(jìn)入ArrayBlockingQueue后，notEmpty.signal()通知在這上面等待的線程，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一開(kāi)始接收到的參數(shù)就是firstWaiter這個(gè)參數(shù)，在內(nèi)部實(shí)現(xiàn)中用了do..while的形式，首先將first的的nextWaiter找出來(lái)保存到firstWaiter此時(shí)（first和firstWaiter不是一回事），在while的比較條件中可調(diào)用了transferForSignal方法，
整個(gè)while比較條件可以看著短路邏輯，如果transferForSignal結(jié)果為true,后面的first = firstWaiter就不執(zhí)行了，整個(gè)while循環(huán)就結(jié)束了。
參照注釋?zhuān)?/p>

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;
    }

首先確保想要被signal的等待node還是處于Node.CONDITION狀態(tài)，然后調(diào)整狀態(tài)為Node.SIGNAL，這兩個(gè)都是采用CAS方法，最后調(diào)用的是

LockSupport.unpark(node.thread);

五、LockSupport
至此，我們已經(jīng)知道了線程的掛起和通知都是使用LockSupport來(lái)完成的，并發(fā)數(shù)據(jù)結(jié)構(gòu)與線程直接的交互最終也是需要LockSupport。那么關(guān)于LockSupport，我們又可以了解多少呢？

Ref:
Java中的ReentrantLock和synchronized兩種鎖定機(jī)制的對(duì)比
Java的LockSupport.park()實(shí)現(xiàn)分析