一、Condition接口

public interface Condition {
     /** 
      * 暂停此线程直至一下四种情况发生
      * 1.此Condition被signal()
      * 2.此Condition被signalAll()
      * 3.Thread.interrupt()
      * 4.伪wakeup
      * 以上情况.在能恢复方法执行时,当前线程必须要能获得锁
      */
    void await() throws InterruptedException;
    //跟上面类似,不过不响应中断
    void awaitUninterruptibly();
    //带超时时间的await()
    long awaitNanos(long nanosTimeout) throws InterruptedException;
    //带超时时间的await()
    boolean await(long time, TimeUnit unit) throws InterruptedException;
    //带deadline的await()
    boolean awaitUntil(Date deadline) throws InterruptedException;
    //唤醒某个等待在此condition的线程
    void signal();
    //唤醒所有等待在此condition的所有线程
    void signalAll();
}

二、ConditionObject解析

/**
 * ConditionObject的实现,其实其维护两个队列
 * 1)Condition队列,表示等待的队列,其waitStatus=Node.CONDITION,由firstWaiter和lastWaiter两个属性操控. 
 * 2)Sync队列,表示可以竞争锁的队列,这个跟AQS一致,waitStatus=0;
 * 3)await()把当前线程创建一个Node加入Condition队列,接着就一直循环查其在不在Sync队列
 *   如果当前节点在Sync队列里了,就可以竞争锁,恢复运行了.
 * 4)signal()方法就是把某个节点的nextWaiter设为null,再把其从Condition队列转到Sync队列
 */
public class ConditionObject implements Condition, java.io.Serializable {
	//condition队列的第一个节点
	private transient Node firstWaiter;
	//condition队列的最后一个节点
	private transient Node lastWaiter;
	public ConditionObject() { }
	/**
	 * 往condition队列中添加新元素
	 */
	private Node addConditionWaiter() {
		Node t = lastWaiter;
		// If lastWaiter is cancelled, clean out.
		if (t != null && t.waitStatus != Node.CONDITION) {
			unlinkCancelledWaiters();//遍历condition队列，移除其中非CONDITION状态的元素
			//lastWaiter非CONDITION状态，上一步遍历清除时lastWaiter发生了变化需要重新赋值
			t = lastWaiter;
		}
		//新建一个CONDITION状态的节点,并将其加在condition队列尾部
		Node node = new Node(Node.CONDITION);
		if (t == null)//lastWaiter为null
			firstWaiter = node;//firstWaiter指向新的node
		else
			t.nextWaiter = node;//lastWaiter不为null，node拼在lastWaiter后面
		lastWaiter = node;//lastWaiter指向新增加在末尾的元素
		return node;//返回新增元素
	}

	/**
	 * 暂停此线程,直至进入SyncQueue
	 * 线程唤醒时如果还是不在SyncQueue，会继续park
	 */
	public final void awaitUninterruptibly() {
		Node node = addConditionWaiter();//往condition队列中添加新元素
		int savedState = fullyRelease(node);//释放当前线程的锁
		boolean interrupted = false;
		while (!isOnSyncQueue(node)) {//node节点不在SyncQueue，就一直循环
			LockSupport.park(this);//阻塞node的线程
			//线程唤醒时，继续循环，如果还是不在SyncQueue，继续park休眠
			if (Thread.interrupted())
				interrupted = true;
		}
		//跳出来，说明线程进入SyncQueue了，acquireQueued作用：阻塞node
		if (acquireQueued(node, savedState) || interrupted)
			selfInterrupt();//中断线程
	}
	
	/**
	 * 释放信号
	 */
	public final void signal() {
		if (!isHeldExclusively())//不是独占模式(排它模式)抛出异常
			throw new IllegalMonitorStateException();
		//将等待队列的第一个节点出队列，并将其加入AQS的锁队列
		Node first = firstWaiter;
		if (first != null)
			doSignal(first);
	}
	
	/**
	 * 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)//第一个元素空了，说明condition队列空了
				lastWaiter = null;//最后一个元素引用置空
			//将first.nextWaiter置空，也就是从condition队列中出列
			first.nextWaiter = null;
		//如果转换队列不成功且condition队列不为null,继续循环
		} while (!transferForSignal(first) && (first = firstWaiter) != null);
	}

	/**
	 * 唤醒所有等待在此condition的所有线程
	 */
	public final void signalAll() {
		if (!isHeldExclusively())//非独占模式，抛出异常
			throw new IllegalMonitorStateException();
		Node first = firstWaiter;
		if (first != null)
			doSignalAll(first);
	}
	
	/**
	 * 唤醒所有等待在此condition的所有线程
	 */
	private void doSignalAll(Node first) {
		lastWaiter = firstWaiter = null;
		do {//循环遍历condition队列，一个个塞到Sync队列中
			Node next = first.nextWaiter;
			first.nextWaiter = null;
			transferForSignal(first);
			first = next;
		} while (first != null);
	}

	/**
	 * 遍历condition队列，移除其中非CONDITION状态的元素
	 */
	private void unlinkCancelledWaiters() {
		Node t = firstWaiter;
		Node trail = null;
		while (t != null) {//遍历condition队列
			Node next = t.nextWaiter;
			if (t.waitStatus != Node.CONDITION) {
				t.nextWaiter = null;//这样t就从condition中移除了
				if (trail == null)
					firstWaiter = next;//指向下一个元素，再次遍历
				else//trail是上一个元素且为CONDITION状态
					trail.nextWaiter = next;//上一个元素与本元素的下一个元素相连，本元素被剔除
				if (next == null)
					lastWaiter = trail;//没有下一个元素了，lastWaiter指向trail
			} else {
				trail = t;
			}
			t = next;
		}
	}
	...
	/**
	 * 参考上面对awaitUninterruptibly(..)的分析
	 */
	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);
	}

	/**
	 * 参考上面对awaitUninterruptibly(..)的分析
	 */
	public final long awaitNanos(long nanosTimeout) throws InterruptedException {
		if (Thread.interrupted())
			throw new InterruptedException();
		// We don't check for nanosTimeout <= 0L here, to allow
		// awaitNanos(0) as a way to "yield the lock".
		final long deadline = System.nanoTime() + nanosTimeout;
		long initialNanos = nanosTimeout;
		Node node = addConditionWaiter();
		int savedState = fullyRelease(node);
		int interruptMode = 0;
		while (!isOnSyncQueue(node)) {
			if (nanosTimeout <= 0L) {
				transferAfterCancelledWait(node);
				break;
			}
			if (nanosTimeout > SPIN_FOR_TIMEOUT_THRESHOLD)
				LockSupport.parkNanos(this, nanosTimeout);
			if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
				break;
			nanosTimeout = deadline - System.nanoTime();
		}
		if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
			interruptMode = REINTERRUPT;
		if (node.nextWaiter != null)
			unlinkCancelledWaiters();
		if (interruptMode != 0)
			reportInterruptAfterWait(interruptMode);
		long remaining = deadline - System.nanoTime(); // avoid overflow
		return (remaining <= initialNanos) ? remaining : Long.MIN_VALUE;
	}

	/**
	 * 参考上面对awaitUninterruptibly(..)的分析
	 */
	public final boolean awaitUntil(Date deadline) throws InterruptedException {
		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;
	}

	/**
	 * 参考上面对awaitUninterruptibly(..)的分析
	 */
	public final boolean await(long time, TimeUnit unit)
			throws InterruptedException {
		long nanosTimeout = unit.toNanos(time);
		if (Thread.interrupted())
			throw new InterruptedException();
		// We don't check for nanosTimeout <= 0L here, to allow
		// await(0, unit) as a way to "yield the lock".
		final long deadline = System.nanoTime() + nanosTimeout;
		Node node = addConditionWaiter();
		int savedState = fullyRelease(node);
		boolean timedout = false;
		int interruptMode = 0;
		while (!isOnSyncQueue(node)) {
			if (nanosTimeout <= 0L) {
				timedout = transferAfterCancelledWait(node);
				break;
			}
			if (nanosTimeout > SPIN_FOR_TIMEOUT_THRESHOLD)
				LockSupport.parkNanos(this, nanosTimeout);
			if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
				break;
			nanosTimeout = deadline - System.nanoTime();
		}
		if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
			interruptMode = REINTERRUPT;
		if (node.nextWaiter != null)
			unlinkCancelledWaiters();
		if (interruptMode != 0)
			reportInterruptAfterWait(interruptMode);
		return !timedout;
	}
	...
	/**
	 * 获取condition队列中所有等待的线程并返回
	 */
	protected final Collection<Thread> getWaitingThreads() {
		if (!isHeldExclusively())
			throw new IllegalMonitorStateException();
		ArrayList<Thread> list = new ArrayList<>();
		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;
	}
}