AQS-Condition详解

AtomicInteger解析：[url]http://donald-draper.iteye.com/blog/2359555[/url]
锁持有者管理器AbstractOwnableSynchronizer：[url]http://donald-draper.iteye.com/blog/2360109[/url]
AQS线程挂起辅助类：LockSupport：[url]http://donald-draper.iteye.com/blog/2360206[/url]
AQS详解-CLH队列，线程等待状态：[url]http://donald-draper.iteye.com/blog/2360256[/url]

/**
     * Condition implementation for a {@link
     * AbstractQueuedSynchronizer} serving as the basis of a {@link
     * Lock} implementation.
     * 作为AQS实现锁的一个基础实现Condition。
     * <p>Method documentation for this class describes mechanics,
     * not behavioral specifications from the point of view of Lock
     * and Condition users. Exported versions of this class will in
     * general need to be accompanied by documentation describing
     * condition semantics that rely on those of the associated
     * <tt>AbstractQueuedSynchronizer</tt>.
     *方法文档用于描述这个条件实现机制，不是锁和条件的使用者，可以使用的操作。
     此类的版本与AbstractQueuedSynchronizer相关联。
     * <p>This class is Serializable, but all fields are transient,
     * so deserialized conditions have no waiters.
     */
     //这个所有的all fields are transient，所以反序列化时，条件没有等待者。
    public class ConditionObject implements Condition, java.io.Serializable {
        private static final long serialVersionUID = 1173984872572414699L;
        /** First node of condition queue. */
	队列中第一个等待节点线程
        private transient Node firstWaiter;
        /** Last node of condition queue. */
	队列中最后一个等待条件的节点线程
        private transient Node lastWaiter;

        /**
         * Creates a new <tt>ConditionObject</tt> instance.
         */
	 //初始化实例
        public ConditionObject() { }

        // Internal methods

        /**
         * Adds a new waiter to wait queue.
         * @return its new wait node
         */
	//添加一个条件等待线程节点，到条件等待队列
        private Node addConditionWaiter() {
            Node t = lastWaiter;//取得队列的尾节点
            // If lastWaiter is cancelled, clean out.
            if (t != null && t.waitStatus != Node.CONDITION) {
	        //移除队列中非等待条件的线程节点
                unlinkCancelledWaiters();
                t = lastWaiter;
            }
	    //创建新节点条件线程等待节点
            Node node = new Node(Thread.currentThread(), Node.CONDITION);
	    //等队列为空，则新节点为头节点，否则加入到队尾
            if (t == null)
                firstWaiter = node;
            else
                t.nextWaiter = node;
            lastWaiter = node;
            return node;
        }
	/**
         * Unlinks cancelled waiter nodes from condition queue.
         * Called only while holding lock. This is called when
         * cancellation occurred during condition wait, and upon
         * insertion of a new waiter when lastWaiter is seen to have
         * been cancelled. This method is needed to avoid garbage
         * retention in the absence of signals. So even though it may
         * require a full traversal, it comes into play only when
         * timeouts or cancellations occur in the absence of
         * signals. It traverses all nodes rather than stopping at a
         * particular target to unlink all pointers to garbage nodes
         * without requiring many re-traversals during cancellation
         * storms.
         */
	//从条件队列，移除取消等待条件的节点线程。当线程只有锁时，会调用此方法。
        当在线程等待条件时，被取消，或者添加新节点时，发现尾节点已经取消等待，则
	调用此方法。在没有信号条件的时候，方法需要避免垃圾的产生。尽管可能需要遍历
	队列，当超时或取消条件等待时候，仍然会触发此方法。此方法会遍历会有节点，
	不会因为某个特殊事件的发生，不移除取消条件等待的垃圾节点。

        private void unlinkCancelledWaiters() {
	    //取得头结点
            Node t = firstWaiter;
            Node trail = null;
	    //遍历队列，移除非等待条件的节点
            while (t != null) {
                Node next = t.nextWaiter;
                if (t.waitStatus != Node.CONDITION) {
                    t.nextWaiter = null;
                    if (trail == null)
                        firstWaiter = next;//队列头
                    else
                        trail.nextWaiter = next; 
                    if (next == null)
                        lastWaiter = trail;//队列尾
                }
                else
                    trail = t;
                t = next;
            }
        }
 /**
         * 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
         */
        //唤醒等待队列中，第一个等待条件的线程节点。
        private void doSignal(Node first) {
            do {
                if ( (firstWaiter = first.nextWaiter) == null)
                    lastWaiter = null;
                first.nextWaiter = null;
		//
            } while (!transferForSignal(first) &&
                     (first = firstWaiter) != null);
        }

}

//AQS
将一个条件队列节点，转移到同步等待队列

transferForSignal(Node node)

/**
     * 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).
     */
    //将一个条件队列节点，转移到同步等待队列
    final boolean transferForSignal(Node node) {
        /*
         * If cannot change waitStatus, the node has been cancelled.
         */

	 //将节点状态为等待条件，则将节点的状态，设置线程等待的初始状态0
        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);//添加到队列
        int ws = p.waitStatus;
        if (ws > 0 || !compareAndSetWaitStatus(p, ws, Node.SIGNAL))
	    //如果线程取消等待，或设置唤醒成功，则unpark线程
            LockSupport.unpark(node.thread);
        return true;
    }

    /**
     * CAS waitStatus field of a node.
     */
    //修改节点的等待状态
    private static final boolean compareAndSetWaitStatus(Node node,
                                                         int expect,
                                                         int update) {
        return unsafe.compareAndSwapInt(node, waitStatusOffset,
                                        expect, update);
    }


    /**
     * Inserts node into queue, initializing if necessary. See picture above.
     * @param node the node to insert
     * @return node's predecessor
     */
    //添加节点到等待队列，以CAS操作，将节点添加到等待队列中
    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;
                }
            }
        }
    }

小节：从上面可以看出，唤醒节点时，首先等待条件线程节点状态，
设置为线程等待的初始状态0，然后添加到等待队列，
如果线程取消等待，则移除节点线程，否则通知节点前驱，当前驱
节点线程释放锁时，unpark线程。

再回到ConditionObject

//ConditionObject       
        /**
         * Removes and transfers all nodes.
         * @param first (non-null) the first node on condition queue
         */
	 //唤醒队列中所有等待条件的节点
        private void doSignalAll(Node first) {
            lastWaiter = firstWaiter = null;
            do {
                Node next = first.nextWaiter;
                first.nextWaiter = null;
                transferForSignal(first);
                first = next;
            } while (first != null);
        }


        // public methods

        /**
         * 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}
         */
	 //如果线程持有独占锁，从头结点开始，唤醒第一个等待条件的线程节点
        public final void signal() {
            if (!isHeldExclusively())
                throw new IllegalMonitorStateException();
            Node first = firstWaiter;
            if (first != null)
                doSignal(first);
        }

        /**
         * Moves all threads from the wait queue for this condition to
         * the wait queue for the owning lock.
         *
         * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
         *         returns {@code false}
         */
	 //如果线程持有独占锁，从头结点开始，唤醒所有等待条件的线程节点
        public final void signalAll() {
            if (!isHeldExclusively())
                throw new IllegalMonitorStateException();
            Node first = firstWaiter;
            if (first != null)
                doSignalAll(first);
        }

        /**
         * Implements uninterruptible condition wait.
         * [list=1]不可中断条件等待
         * <li> Save lock state returned by {@link #getState}.
	 保存锁的当前state
         * <li> Invoke {@link #release} with
         *      saved state as argument, throwing
         *      IllegalMonitorStateException if it fails.
         * <li> Block until signalled.
	 调用此方法的线程，阻塞至被唤醒
         * <li> Reacquire by invoking specialized version of
         *      {@link #acquire} with saved state as argument.
         * [/list]
         */
        public final void awaitUninterruptibly() {
            Node node = addConditionWaiter();//添加条件等待节点
            int savedState = fullyRelease(node);
            boolean interrupted = false;
            while (!isOnSyncQueue(node)) {
                LockSupport.park(this);
                if (Thread.interrupted())
                    interrupted = true;
            }
            if (acquireQueued(node, savedState) || interrupted)
                selfInterrupt();
        }

下面来看fullyRelease(node);
//AQS

 /**
     * Invokes release with current state value; returns saved state.
     * Cancels node and throws exception on failure.
     * @param node the condition node for this wait
     * @return previous sync state
     */
     //释放锁的当前状态，返回保存值，如果是取消等待节点，则抛出异常
    final int fullyRelease(Node node) {
        boolean failed = true;
        try {
            int savedState = getState();//获取当前等待状态
            if (release(savedState)) {
                failed = false;
                return savedState;
            } else {
	        //释放失败，抛出异常
                throw new IllegalMonitorStateException();
            }
        } finally {
            if (failed)
	        //如果失释放败，设置节点状态为取消
                node.waitStatus = Node.CANCELLED;
        }
    }


 /**
     * Releases in exclusive mode.  Implemented by unblocking one or
     * more threads if {@link #tryRelease} returns true.
     * This method can be used to implement method {@link Lock#unlock}.
     * 释放独占模式锁，用unblock线程
     * @param arg the release argument.  This value is conveyed to
     *        {@link #tryRelease} but is otherwise uninterpreted and
     *        can represent anything you like.
     * @return the value returned from {@link #tryRelease}
     */
    public final boolean release(int arg) {
        if (tryRelease(arg)) {
	     //如果尝试释放锁成功
            Node h = head;
            if (h != null && h.waitStatus != 0)
	       //如果头结点不为null，且非初始等待状态0，则unpark头结点的后继
                unparkSuccessor(h);
            return true;
        }
        return false;
    }
//待子类扩展
 protected boolean tryRelease(int arg) {
        throw new UnsupportedOperationException();
    }

/**
     * Wakes up node's successor, if one exists.
     *唤醒接待的后继
     * @param node the node
     */
    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.
         */
	 //如果状态值为负，线程需要唤醒，尝试着清除状态值，
        int ws = node.waitStatus;
        if (ws < 0)
	    //设置线程状态为初始值
            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;
	    //从队尾遍历器前驱，找到最后一个状态值为负的节点
            for (Node t = tail; t != null && t != node; t = t.prev)
                if (t.waitStatus <= 0)
                    s = t;
        }
        if (s != null)
	    //unpark节点线程
            LockSupport.unpark(s.thread);
    }

回到 awaitUninterruptibly

public final void awaitUninterruptibly() {
            Node node = addConditionWaiter();//添加条件等待节点
            int savedState = fullyRelease(node);//释放节点持有的锁，唤醒队列第一个节点线程
            boolean interrupted = false;
	    //如果节点在同步等待队列上
            while (!isOnSyncQueue(node)) {
	        //park当前线程
                LockSupport.park(this);
                if (Thread.interrupted())
                    interrupted = true;
            }
            if (acquireQueued(node, savedState) || interrupted)
                selfInterrupt();
        }

看下面一句

isOnSyncQueue(node)

//AQS

/**
     * Returns true if a node, always one that was initially placed on
     * a condition queue, is now waiting to reacquire on sync queue.
     * @param node the node
     * @return true if is reacquiring
     */
     //如果一个节点，刚开始在条件队列，现在，再同步等待队列
    final boolean isOnSyncQueue(Node node) {
        if (node.waitStatus == Node.CONDITION || node.prev == null)
	   //条件等待节点线程，返回false
            return false;
        if (node.next != null) // If has successor, it must be on queue
            如果有前驱，代表在同步等待队列上
	    return true;
        /*
         * node.prev can be non-null, but not yet on queue because
         * the CAS to place it on queue can fail. So we have to
         * traverse from tail to make sure it actually made it.  It
         * will always be near the tail in calls to this method, and
         * unless the CAS failed (which is unlikely), it will be
         * there, so we hardly ever traverse much.
         */
	 节点的前驱可能为非null，但不在队列中，可能由于CAS操作失败。
	 所以我们不得不确定，，节点是否在队尾附近。
        return findNodeFromTail(node);
    }
     /**
     * Returns true if node is on sync queue by searching backwards from tail.
     * Called only when needed by isOnSyncQueue.
     * @return true if present
     */
     //从队尾遍历，查看节点是否在同步等待队列上
    private boolean findNodeFromTail(Node node) {
        Node t = tail;
        for (;;) {
            if (t == node)
                return true;
            if (t == null)
                return false;
            t = t.prev;
        }
    }

回到 awaitUninterruptibly

public final void awaitUninterruptibly() {
            Node node = addConditionWaiter();//添加条件等待节点
            int savedState = fullyRelease(node);//释放节点持有的锁，唤醒队列第一个节点线程
            boolean interrupted = false;
	    //如果节点在同步等待队列上
            while (!isOnSyncQueue(node)) {
	        //park当前线程
                LockSupport.park(this);
                if (Thread.interrupted())
                    interrupted = true;
            }
            if (acquireQueued(node, savedState) || interrupted)
                selfInterrupt();
        }

//Thread

/**
     * Tests whether the current thread has been interrupted.  The
     * <i>interrupted status</i> of the thread is cleared by this method.  In
     * other words, if this method were to be called twice in succession, the
     * second call would return false (unless the current thread were
     * interrupted again, after the first call had cleared its interrupted
     * status and before the second call had examined it).
     *
     * <p>A thread interruption ignored because a thread was not alive
     * at the time of the interrupt will be reflected by this method
     * returning false.
     *
     判断当前线程是否已经被中断，这个方法可以清楚线程的中断状态。换种说法，
     当此方法成功调用两次时，第二次返回为false，
     * @return  <code>true</code> if the current thread has been interrupted;
     *          <code>false</code> otherwise.
     * @see #isInterrupted()
     * @revised 6.0
     */
    查看当前线程是否已经被可中断，如果当前线程处于非中断状态，可返回true，否
    返回else。
    public static boolean interrupted() {
        return currentThread().isInterrupted(true);
    }

回到 awaitUninterruptibly

public final void awaitUninterruptibly() {
            Node node = addConditionWaiter();//添加条件等待节点
            int savedState = fullyRelease(node);//释放节点持有的锁，唤醒队列第一个节点线程
            boolean interrupted = false;
	    //如果节点在同步等待队列上
            while (!isOnSyncQueue(node)) {
	        //park当前线程
                LockSupport.park(this);
                if (Thread.interrupted())
                    interrupted = true;
            }
            if (acquireQueued(node, savedState) || interrupted)
                selfInterrupt();
        }

再看这一段

if (acquireQueued(node, savedState) || interrupted)
                selfInterrupt();

//AQS

/*
     * Various flavors of acquire, varying in exclusive/shared and
     * control modes.  Each is mostly the same, but annoyingly
     * different.  Only a little bit of factoring is possible due to
     * interactions of exception mechanics (including ensuring that we
     * cancel if tryAcquire throws exception) and other control, at
     * least not without hurting performance too much.
     */
    不同的获取锁方式，对应着不同的控制模式，不如独占、共享。
    每一种大部分相同，但极少数不同。不同的，不如获取出现异常时的，取消
    方式。
    /**
     * Acquires in exclusive uninterruptible mode for thread already in
     * queue. Used by condition wait methods as well as acquire.
     *
     * @param node the node
     * @param arg the acquire argument
     * @return {@code true} if interrupted while waiting
     */
    final boolean acquireQueued(final Node node, int arg) {
        boolean failed = true;
        try {
            boolean interrupted = false;
            for (;;) {
                final Node p = node.predecessor();
		//如果节点的前继，是头节点则，尝试获取锁
                if (p == head && tryAcquire(arg)) {
                    setHead(node);
                    p.next = null; // help GC
                    failed = false;
                    return interrupted;
                }
		//前驱线程释放锁之后，是否应该唤醒后继节点，如果是，则
		//park当前线程
                if (shouldParkAfterFailedAcquire(p, node) &&
                    parkAndCheckInterrupt())
                    interrupted = true;
            }
        } finally {
            if (failed)
	        //如果失败，则取消线程
                cancelAcquire(node);
        }
    }

acquireQueued过程是这样的：
1. 如果当前节点是AQS队列的头结点（如果第一个节点是DUMP节点也就是傀儡节点，
那么第二个节点实际上就是头结点了），
就尝试在此获取锁tryAcquire(arg)。
如果成功就将头结点设置为当前节点（不管第一个结点是否是DUMP节点），返回中断位。否则进行2。
2. 检测当前节点是否应该park()，如果应该park()就挂起当前线程并且返回当前线程中断位。进行操作1。

    /**
     * Sets head of queue to be node, thus dequeuing. Called only by
     * acquire methods.  Also nulls out unused fields for sake of GC
     * and to suppress unnecessary signals and traversals.
     *设置头节点
     * @param node the node
     */
    private void setHead(Node node) {
        head = node;
        node.thread = null;
        node.prev = null;
    }

 /**
     * Checks and updates status for a node that failed to acquire.
     * Returns true if thread should block. This is the main signal
     * control in all acquire loops.  Requires that pred == node.prev
     *如果一个节点获取锁失败，则检查和更新节点状态
     如果节点应该block，返回true
     * @param pred node's predecessor holding status
     * @param node the node
     * @return {@code true} if thread should block
     */
    private static boolean shouldParkAfterFailedAcquire(Node pred, Node node) {
        int ws = pred.waitStatus;
        if (ws == Node.SIGNAL)
            /*
             * This node has already set status asking a release
             * to signal it, so it can safely park.
	     先驱节点释放锁时，需要唤醒后继节点
             */
            return true;
        if (ws > 0) {
            /*
             * Predecessor was cancelled. Skip over predecessors and
             * indicate retry.
             */
            do {
                node.prev = pred = pred.prev;
		如果先驱节点，等待被取消，向前遍历，找到第一个等待条件的节点
            } 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.
             */
	    设置先驱节点状态为SIGNAL，当先驱释放锁时，需要唤醒后继节点线程
            compareAndSetWaitStatus(pred, ws, Node.SIGNAL);
        }
        return false;
    }


    /**
     * Convenience method to park and then check if interrupted
     * park当前线程
     * @return {@code true} if interrupted
     */
    private final boolean parkAndCheckInterrupt() {
        LockSupport.park(this);
        return Thread.interrupted();
    }

  // Utilities for various versions of acquire

    /**
     * Cancels an ongoing attempt to acquire.
     *取消一个尝试获取锁的线程
     * @param node the node
     */
    private void cancelAcquire(Node node) {
        // Ignore if node doesn't exist
        if (node == null)
            return;
        //清空节点线程
        node.thread = null;

        // Skip cancelled predecessors
        Node pred = node.prev;
	//找到第一个，等待条件的节点
        while (pred.waitStatus > 0)
            node.prev = pred = pred.prev;

        // predNext is the apparent node to unsplice. CASes below will
        // fail if not, in which case, we lost race vs another cancel
        // or signal, so no further action is necessary.
        Node predNext = pred.next;

        // Can use unconditional write instead of CAS here.
        // After this atomic step, other Nodes can skip past us.
        // Before, we are free of interference from other threads.
	//设置节点状态，为取消状态
        node.waitStatus = Node.CANCELLED;

        // If we are the tail, remove ourselves.
        if (node == tail && compareAndSetTail(node, pred)) {
            compareAndSetNext(pred, predNext, null);
        } else {
            // If successor needs signal, try to set pred's next-link
            // so it will get one. Otherwise wake it up to propagate.
            int ws;
            if (pred != head &&
                ((ws = pred.waitStatus) == Node.SIGNAL ||
                 (ws <= 0 && compareAndSetWaitStatus(pred, ws, Node.SIGNAL))) &&
                pred.thread != null) {
                Node next = node.next;
                if (next != null && next.waitStatus <= 0)
                    compareAndSetNext(pred, predNext, next);
            } else {
                unparkSuccessor(node);
            }
            //节点指向自己，等待gc回收
            node.next = node; // help GC
        }
    }

再来看

 selfInterrupt();

  /**
     * Convenience method to interrupt current thread.
     */
    private static void selfInterrupt() {
        Thread.currentThread().interrupt();
    }

在回到这个方法

public final void awaitUninterruptibly() {
            Node node = addConditionWaiter();//添加条件等待节点
            int savedState = fullyRelease(node);//释放节点持有的锁，唤醒队列第一个节点线程
            boolean interrupted = false;
	    //如果节点在同步等待队列上
            while (!isOnSyncQueue(node)) {
	        //park当前线程
                LockSupport.park(this);
                if (Thread.interrupted())
                    interrupted = true;
            }
            if (acquireQueued(node, savedState) || interrupted)
                selfInterrupt();
        }

小节：非中断模式等待，首先添加新的等待条件线程节点，到等待条件线程队列；
释放节点状态，释放节点锁状态过程，待子类扩展，在过程中，同时唤醒等待队列
头结点；再判断节点是在同步等待队列上，如果不在，则park当前线程；
如果线程已经中断，则取消线程中断状态，即线程非中断等待条件。


回到ConditionObject的其他方法
//ConditionObject

  /*
         * For interruptible waits, we need to track whether to throw
         * InterruptedException, if interrupted while blocked on
         * condition, versus reinterrupt current thread, if
         * interrupted while blocked waiting to re-acquire.
         */
	 当线程是可中断等待时，我们需要捕捉是否抛出中断异常，
	 当阻塞在条件上时，则中断

        /** Mode meaning to reinterrupt on exit from wait */
	//在此模式上表示，当退出等待条件时，需要二次中断，即消除中断状态
        private static final int REINTERRUPT =  1;
        /** Mode meaning to throw InterruptedException on exit from wait */
	//此模式用于描述，当退出等待条件时，需要抛出异常
        private static final int THROW_IE    = -1;

        /**
         * Checks for interrupt, returning THROW_IE if interrupted
         * before signalled, REINTERRUPT if after signalled, or
         * 0 if not interrupted.
         */
	 //检查节点线程，是否需要消除中断，抛出异常，还是不需要中断
        private int checkInterruptWhileWaiting(Node node) {
            return Thread.interrupted() ?
                (transferAfterCancelledWait(node) ? THROW_IE : REINTERRUPT) :
                0;
        }

//AQS

/**
     * Transfers node, if necessary, to sync queue after a cancelled
     * wait. Returns true if thread was cancelled before being
     * signalled.
     * @param current the waiting thread
     * @param node its node
     * @return true if cancelled before the node was signalled
     */
    在线程被唤醒之前，取消等待则返回true。如果需要的话，
    当线程等待被取消时，将线程节点放到，同步等待队列中
    final boolean transferAfterCancelledWait(Node node) {
        if (compareAndSetWaitStatus(node, Node.CONDITION, 0)) {
	    //如果设置节点为初始化状态成功，则添加到同步等待队列
            enq(node);
            return true;
        }
        /*
         * If we lost out to a signal(), then we can't proceed
         * until it finishes its enq().  Cancelling during an
         * incomplete transfer is both rare and transient, so just
         * spin.
         */
        while (!isOnSyncQueue(node))
            Thread.yield();
        return false;
    }

//Thread
该方法与sleep()类似，只是不能由用户指定暂停多长时间，
并且yield（）方法只能让同优先级的线程有执行的机会。

/**
     * A hint to the scheduler that the current thread is willing to yield
     * its current use of a processor. The scheduler is free to ignore this
     * hint.
     *
     * <p> Yield is a heuristic attempt to improve relative progression
     * between threads that would otherwise over-utilise a CPU. Its use
     * should be combined with detailed profiling and benchmarking to
     * ensure that it actually has the desired effect.
     *
     * <p> It is rarely appropriate to use this method. It may be useful
     * for debugging or testing purposes, where it may help to reproduce
     * bugs due to race conditions. It may also be useful when designing
     * concurrency control constructs such as the ones in the
     * {@link java.util.concurrent.locks} package.
     */
    public static native void yield();

//ConditionObject

        /**
         * Implements interruptible condition wait.
         * [list=1]
         * <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.
         * [/list]
         */
	 //可中断的条件等待
        public final void await() throws InterruptedException {
            if (Thread.interrupted())
	    //如果线程中断，则抛出异常
                throw new InterruptedException();
	   //添加新的线程节点到等待条件队列
            Node node = addConditionWaiter();
	    //释放节点锁状态，unpark等待队列头节点
            int savedState = fullyRelease(node);
            int interruptMode = 0;
            while (!isOnSyncQueue(node)) {
	        //如果节点不在同步等待队列，则park
                LockSupport.park(this);
                if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
                    break;
            }
	    //判断线程等待条件后，是中断，还是抛出异常
            if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
                interruptMode = REINTERRUPT;
	    //移除取消等待条件的节点
            if (node.nextWaiter != null) // clean up if cancelled
                unlinkCancelledWaiters();
            if (interruptMode != 0)
                reportInterruptAfterWait(interruptMode);
        }
         /**
         * Throws InterruptedException, reinterrupts current thread, or
         * does nothing, depending on mode.
         */
	//根据线程当前中断模式，确定中断，还是抛出异常
        private void reportInterruptAfterWait(int interruptMode)
            throws InterruptedException {
            if (interruptMode == THROW_IE)
	       //抛出异常
                throw new InterruptedException();
            else if (interruptMode == REINTERRUPT)
	        //中断
                selfInterrupt();
        }
        /**
         * Implements timed condition wait.
         * [list=1]
         * <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, interrupted, or timed out.
         * <li> Reacquire by invoking specialized version of
         *      {@link #acquire} with saved state as argument.
         * <li> If interrupted while blocked in step 4, throw InterruptedException.
         * [/list]
         */
	 超时等待条件，如果当前线程处于中断，则抛出中断异常，
	 线程阻塞到，知道唤醒，或时间超时，或被中断。
        public final long awaitNanos(long nanosTimeout)
                throws InterruptedException {
            if (Thread.interrupted())
                throw new InterruptedException();
            Node node = addConditionWaiter();
            int savedState = fullyRelease(node);
            long lastTime = System.nanoTime();
            int interruptMode = 0;
            while (!isOnSyncQueue(node)) {
                if (nanosTimeout <= 0L) {
                    transferAfterCancelledWait(node);
                    break;
                }
                LockSupport.parkNanos(this, nanosTimeout);
                if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
                    break;

                long now = System.nanoTime();
                nanosTimeout -= now - lastTime;
                lastTime = now;
            }
            if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
                interruptMode = REINTERRUPT;
            if (node.nextWaiter != null)
                unlinkCancelledWaiters();
            if (interruptMode != 0)
                reportInterruptAfterWait(interruptMode);
            return nanosTimeout - (System.nanoTime() - lastTime);
        }

        /**
         * Implements absolute timed condition wait.
         * [list=1]
         * <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, interrupted, or timed out.
         * <li> Reacquire by invoking specialized version of
         *      {@link #acquire} with saved state as argument.
         * <li> If interrupted while blocked in step 4, throw InterruptedException.
         * <li> If timed out while blocked in step 4, return false, else true.
         * [/list]
         */
	 超时等待条件，如果当前线程处于中断，则抛出中断异常，
	 线程阻塞到，知道唤醒，或到指定时间，或被中断。
        public final boolean awaitUntil(Date deadline)
                throws InterruptedException {
            if (deadline == null)
                throw new NullPointerException();
            long abstime = deadline.getTime();
            if (Thread.interrupted())
                throw new InterruptedException();
            Node node = addConditionWaiter();
            int savedState = fullyRelease(node);
            boolean timedout = false;
            int interruptMode = 0;
            while (!isOnSyncQueue(node)) {
                if (System.currentTimeMillis() > abstime) {
                    timedout = transferAfterCancelledWait(node);
                    break;
                }
                LockSupport.parkUntil(this, abstime);
                if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
                    break;
            }
            if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
                interruptMode = REINTERRUPT;
            if (node.nextWaiter != null)
                unlinkCancelledWaiters();
            if (interruptMode != 0)
                reportInterruptAfterWait(interruptMode);
            return !timedout;
        }

        /**
         * Implements timed condition wait.
         * [list=1]
         * <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, interrupted, or timed out.
         * <li> Reacquire by invoking specialized version of
         *      {@link #acquire} with saved state as argument.
         * <li> If interrupted while blocked in step 4, throw InterruptedException.
         * <li> If timed out while blocked in step 4, return false, else true.
         * [/list]
         */
	 超时等待条件，如果当前线程处于中断，则抛出中断异常，
	 线程阻塞到，知道唤醒，或到时间超时，或被中断。
        public final boolean await(long time, TimeUnit unit)
                throws InterruptedException {
            if (unit == null)
                throw new NullPointerException();
            long nanosTimeout = unit.toNanos(time);
            if (Thread.interrupted())
                throw new InterruptedException();
            Node node = addConditionWaiter();
            int savedState = fullyRelease(node);
            long lastTime = System.nanoTime();
            boolean timedout = false;
            int interruptMode = 0;
            while (!isOnSyncQueue(node)) {
                if (nanosTimeout <= 0L) {
                    timedout = transferAfterCancelledWait(node);
                    break;
                }
                if (nanosTimeout >= spinForTimeoutThreshold)
                    LockSupport.parkNanos(this, nanosTimeout);
                if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
                    break;
                long now = System.nanoTime();
                nanosTimeout -= now - lastTime;
                lastTime = now;
            }
            if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
                interruptMode = REINTERRUPT;
            if (node.nextWaiter != null)
                unlinkCancelledWaiters();
            if (interruptMode != 0)
                reportInterruptAfterWait(interruptMode);
            return !timedout;
        }

        //  support for instrumentation

        /**
         * Returns true if this condition was created by the given
         * synchronization object.
         *
         * @return {@code true} if owned
         */
	 //判断条件是否为AQS创建
        final boolean isOwnedBy(AbstractQueuedSynchronizer sync) {
            return sync == AbstractQueuedSynchronizer.this;
        }

        /**
         * Queries whether any threads are waiting on this condition.
         * Implements {@link AbstractQueuedSynchronizer#hasWaiters}.
         *
         * @return {@code true} if there are any waiting threads
         * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
         *         returns {@code false}
         */
	 //判断时候还有线程在等待这个条件
        protected final boolean hasWaiters() {
            if (!isHeldExclusively())
	         //如果当前线程非独占锁持有者，抛出异常
                throw new IllegalMonitorStateException();
            for (Node w = firstWaiter; w != null; w = w.nextWaiter) {
	       //如果等待条件队列中，有等在次条件的节点线程，则返回true
                if (w.waitStatus == Node.CONDITION)
                    return true;
            }
            return false;
        }

        /**
         * Returns an estimate of the number of threads waiting on
         * this condition.
         * Implements {@link AbstractQueuedSynchronizer#getWaitQueueLength}.
         *
         * @return the estimated number of waiting threads
         * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
         *         returns {@code false}
         */
	 //返回等待此条件的节点线程数
        protected final int getWaitQueueLength() {
            if (!isHeldExclusively())
                throw new IllegalMonitorStateException();
            int n = 0;
            for (Node w = firstWaiter; w != null; w = w.nextWaiter) {
                if (w.waitStatus == Node.CONDITION)
                    ++n;
            }
            return n;
        }

        /**
         * Returns a collection containing those threads that may be
         * waiting on this Condition.
         * Implements {@link AbstractQueuedSynchronizer#getWaitingThreads}.
         *
         * @return the collection of threads
         * @throws IllegalMonitorStateException if {@link #isHeldExclusively}
         *         returns {@code false}
         */
	 //获取等待此条件的线程
        protected final Collection<Thread> getWaitingThreads() {
            if (!isHeldExclusively())
                throw new IllegalMonitorStateException();
            ArrayList<Thread> list = new ArrayList<Thread>();
            for (Node w = firstWaiter; w != null; w = w.nextWaiter) {
                if (w.waitStatus == Node.CONDITION) {
                    Thread t = w.thread;
                    if (t != null)
                        list.add(t);
                }
            }
            return list;
        }
    }

总结：
[color=green]ConditionObject是AQS的一个内部类，其实现是基于AQS；ConditionObject中的条件等待队列中的节点与同步队列中的节点基本相同，最大的不同时，同步等待队列
中的节点有先驱pre和后继next，而条件等待队列中的节点只有后继nextWaiter。
条件等待有两种方法，一种为非中断等待awaitUninterruptibly，在线程等待条件时不可中断线程；另外一种，可中断等待await，等待后，确定中断当前线程，还是抛出异常。
非中断模式等待，首先添加新的等待条件线程节点，到等待条件线程队列；
释放节点状态，释放节点锁状态过程，待子类扩展，在过程中，同时唤醒等待队列
头结点；再判断节点是在同步等待队列上，如果不在，则park当前线程；
如果线程已经中断，则取消线程中断状态，即线程非中断等待条件。
唤醒有两种，一种是唤醒等待条件队列中的头结点，另一种，唤醒条件等待队列中，
所有等待此条件的节点线程。唤醒节点时，首先设置线程等待的节点状态的初始状态0，
然后添加到同步等待队列，如果线程取消等待，则移除线程，否则通知节点前驱，当前驱
节点线程释放锁时，unpark线程。
在唤醒等待条件的线程时，为什么要将，节点移动同步等待线程节点能，这是因为条件时锁的一部分，先创建锁，再创建条件；当线程被唤醒时，只能说明，线程条件发生，能处于park状态，要获取锁，才能unpark线程。[/color]