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Condition在JUC框架下提供了传统Java监视器风格的wait、notify和notifyAll相似的功能。
Condition必须被绑定到一个独占锁上使用。ReentrantLock中获取Condition的方法为：
public Condition newCondition() { return sync.newCondition(); } final ConditionObject newCondition() { return new ConditionObject(); }
直接初始化并返回了一个AQS提供的ConditionObject对象。因此，Condition实际上是AQS框架的内容。ConditionObject通过维护两个成员变量：
/** First node of condition queue. */ private transient Node firstWaiter; /** Last node of condition queue. */ private transient Node lastWaiter;
来维护一个Condition等待队列，并通过signal操作将Condition队列中的线程移到Sync锁等待队列。
源代码：
public class ConditionObject implements Condition, java.io.Serializable { private static final long serialVersionUID = 1173984872572414699L; /** First node of condition queue. */ private transient Node firstWaiter; /** Last node of condition queue. */ private transient Node lastWaiter; /** * Creates a new {@code ConditionObject} instance. */ public ConditionObject() { } // Internal methods /** * Adds a new waiter to wait queue. * @return its new wait node */ private Node addConditionWaiter() { Node t = lastWaiter; // If lastWaiter is cancelled, clean out. if (t != null && t.waitStatus != Node.CONDITION) { unlinkCancelledWaiters(); t = lastWaiter; } Node node = new Node(Thread.currentThread(), Node.CONDITION); if (t == null) firstWaiter = node; else t.nextWaiter = node; lastWaiter = node; return node; } /** * 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); } /** * Removes and transfers all nodes. * @param first (non-null) the first node on condition queue */ private void doSignalAll(Node first) { lastWaiter = firstWaiter = null; do { Node next = first.nextWaiter; first.nextWaiter = null; transferForSignal(first); first = next; } while (first != null); } /** * 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. */ private void unlinkCancelledWaiters() { Node t = firstWaiter; Node trail = null; while (t != null) { Node next = t.nextWaiter; if (t.waitStatus != Node.CONDITION) { t.nextWaiter = null; if (trail == null) firstWaiter = next; else trail.nextWaiter = next; if (next == null) lastWaiter = trail; } else trail = t; t = next; } } // 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} */ 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} */ public final void signalAll() { if (!isHeldExclusively()) throw new IllegalMonitorStateException(); Node first = firstWaiter; if (first != null) doSignalAll(first); } /** * Implements uninterruptible condition wait. * <ol> * <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. * <li> Reacquire by invoking specialized version of * {@link #acquire} with saved state as argument. * </ol> */ public final void awaitUninterruptibly() { Node node = addConditionWaiter(); int savedState = fullyRelease(node); boolean interrupted = false; while (!isOnSyncQueue(node)) { LockSupport.park(this); if (Thread.interrupted()) interrupted = true; } if (acquireQueued(node, savedState) || interrupted) selfInterrupt(); } /* * 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. */ /** Mode meaning to reinterrupt on exit from wait */ private static final int REINTERRUPT = 1; /** Mode meaning to throw InterruptedException on exit from wait */ 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. */ private int checkInterruptWhileWaiting(Node node) { return Thread.interrupted() ? (transferAfterCancelledWait(node) ? THROW_IE : REINTERRUPT) : 0; } /** * Throws InterruptedException, reinterrupts current thread, or * does nothing, depending on mode. */ private void reportInterruptAfterWait(int interruptMode) throws InterruptedException { if (interruptMode == THROW_IE) throw new InterruptedException(); else if (interruptMode == REINTERRUPT) selfInterrupt(); } /** * 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); } /** * Implements timed 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, 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. * </ol> */ public final long awaitNanos(long nanosTimeout) throws InterruptedException { if (Thread.interrupted()) throw new InterruptedException(); Node node = addConditionWaiter(); int savedState = fullyRelease(node); final long deadline = System.nanoTime() + nanosTimeout; int interruptMode = 0; while (!isOnSyncQueue(node)) { if (nanosTimeout <= 0L) { transferAfterCancelledWait(node); break; } if (nanosTimeout >= spinForTimeoutThreshold) 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 deadline - System.nanoTime(); } /** * Implements absolute timed 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, 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. * </ol> */ 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; } /** * Implements timed 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, 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. * </ol> */ public final boolean await(long time, TimeUnit unit) throws InterruptedException { long nanosTimeout = unit.toNanos(time); if (Thread.interrupted()) throw new InterruptedException(); Node node = addConditionWaiter(); int savedState = fullyRelease(node); final long deadline = System.nanoTime() + nanosTimeout; 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; 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; } // support for instrumentation /** * Returns true if this condition was created by the given * synchronization object. * * @return {@code true} if owned */ final boolean isOwnedBy(AbstractQueuedSynchronizer sync) { return sync == AbstractQueuedSynchronizer.this; } /** * Queries whether any threads are waiting on this condition. * Implements {@link AbstractQueuedSynchronizer#hasWaiters(ConditionObject)}. * * @return {@code true} if there are any waiting threads * @throws IllegalMonitorStateException if {@link #isHeldExclusively} * returns {@code false} */ protected final boolean hasWaiters() { if (!isHeldExclusively()) throw new IllegalMonitorStateException(); for (Node w = firstWaiter; w != null; w = w.nextWaiter) { 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(ConditionObject)}. * * @return the estimated number of waiting threads * @throws IllegalMonitorStateException if {@link #isHeldExclusively} * returns {@code false} */ 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(ConditionObject)}. * * @return the collection of threads * @throws IllegalMonitorStateException if {@link #isHeldExclusively} * returns {@code false} */ 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; } }
下面我们就来分析下Condition的工作流程。
一、await 在条件变量上等待
分别是Condition队列的头结点和尾节点。Condition在调用await方法之前，必须先获取锁，注意，这个锁必须是一个独占锁。我们先来看一下await中用到的几个方法：
addConditionWaiter：
private Node addConditionWaiter() { Node t = lastWaiter; // If lastWaiter is cancelled, clean out. if (t != null && t.waitStatus != Node.CONDITION) { unlinkCancelledWaiters(); t = lastWaiter; } Node node = new Node(Thread.currentThread(), Node.CONDITION); if (t == null) firstWaiter = node; else t.nextWaiter = node; lastWaiter = node; return node; }
顾名思义，此方法在Condition队列中添加一个等待线程。首先，方法先检查一下队列尾节点是否还在等待Condition（如果被signal或者中断，waitStatus会被修改为0或者CANCELLED）。如果尾节点被取消或者中断，调用unlinkCancelledWaiters方法删除Condition队列中被cancel的节点。然后将当前线程封装在一个Node中，添加到Condition队列的尾部。这里由于我们在操纵Condition队列的时候已经获取了一个独占锁，因此不会发生竞争。
值得注意的是，Condition队列与Sync队列（锁等待队列）有几点不同：①Condition队列是一个单向链表，而Sync队列是一个双向链表；②Sync队列在初始化的时候，会在队列头部添加一个空的dummy节点，它不持有任何线程，而Condition队列初始化时，头结点就开始持有等待线程了。
我们有必要在这里提一下Node对象中的nextWaiter成员、SHARED成员和EXCLUSIVE成员：
/** Marker to indicate a node is waiting in shared mode */ static final Node SHARED = new Node(); /** Marker to indicate a node is waiting in exclusive mode */ static final Node EXCLUSIVE = null; Node nextWaiter;
nextWaiter在共享模式下，被设置为SHARED，SHARED为一个final的空节点，用来表示当前模式是共享模式；默认情况下nextWaiter是null，EXCLUSIVE成员是一个final的null，因此默认模式是独占模式。在Condition队列中nextWaiter被用来指向队列里的下一个等待线程。在一个线程从Condition队列中被移除之后，nextWaiter被设置为空（EXCLUSIVE）。这再次表明：Condition必须被绑定在一个独占锁上使用。
我们来看一下unlinkCancelledWaiters方法：
private void unlinkCancelledWaiters() { Node t = firstWaiter; Node trail = null; while (t != null) { Node next = t.nextWaiter; if (t.waitStatus != Node.CONDITION) { t.nextWaiter = null; if (trail == null) firstWaiter = next; else trail.nextWaiter = next; if (next == null) lastWaiter = trail; } else trail = t; t = next; } }
unlinkCancelledWaiters方法很简单，从头到尾遍历Condition队列，移除被cancel或被中断的节点。由于这里我们在操纵Condition队列的时候已经获取了所绑定的独占锁，因此不用担心竞争的发生。
我们再来看一下fullyRelease方法，这个方法用来释放锁：
final int fullyRelease(Node node) { boolean failed = true; try { int savedState = getState(); if (release(savedState)) { failed = false; return savedState; } else { throw new IllegalMonitorStateException(); } } finally { if (failed) node.waitStatus = Node.CANCELLED; } }
方法首先获取了state的值，这个值表示可锁被“重入”深度，并调用release释放全部的重入获取，如果成功，返货这个深度，如果失败，要将当前线程的waitStatus设置为CANCELLED。
我们再来看一下isOnSyncQueue方法，这个方法返节点是否在Sync队列中等待锁：
final boolean isOnSyncQueue(Node node) { if (node.waitStatus == Node.CONDITION || node.prev == null) return false; if (node.next != null) // If has successor, it must be on queue 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. */ return findNodeFromTail(node); }
node从Condition队列移除的第一步，就是设置waitStatus为其他值，因此是否等于Node.CONDITON可以作为判断标志，如果等于，说明还在Condition队列中，即不再Sync队列里。在node被放入Sync队列时，第一步就是设置node的prev为当前获取到的尾节点，所以如果发现node的prev为null的话，可以确定node尚未被加入Sync队列。
相似的，node被放入Sync队列的最后一步是设置node的next，如果发现node的next不为null，说明已经完成了放入Sync队列的过程，因此可以返回true。
当我们执行完两个if而仍未返回时，node的prev一定不为null，next一定为null，这个时候可以认为node正处于放入Sync队列的执行CAS操作执行过程中。而这个CAS操作有可能失败，因此我们再给node一次机会，调用findNodeFromTail来检测：
private boolean findNodeFromTail(Node node) { Node t = tail; for (;;) { if (t == node) return true; if (t == null) return false; t = t.prev; } }
findNodeFromTail方法从尾部遍历Sync队列，如果检查node是否在队列中，如果还不在，此时node也许在CAS自旋中，在不久的将来可能会进到Sync队列里。但我们已经等不了了，直接放回false。
我们再来看一下checkInterruptWhileWaiting方法：
/** Mode meaning to reinterrupt on exit from wait */ private static final int REINTERRUPT = 1; /** Mode meaning to throw InterruptedException on exit from wait */ 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. */ private int checkInterruptWhileWaiting(Node node) { return Thread.interrupted() ? (transferAfterCancelledWait(node) ? THROW_IE : REINTERRUPT) : 0; }
此方法在线程从park中醒来后调用，它的返回值有三种：0代表在park过程中没有发生中断；THORW_IE（1）代表发生了中断，且在后续我们需要抛出中断异常；REINTERRUPT表示发生了中断，但在后续我们不抛出中断异常，而是“补上”这次中断。当没有发生中断时，我们返回0即可，当中断发生时，返回THROW_IE or REINTERRUPT由transferAfterCancelledWait方法判断：
final boolean transferAfterCancelledWait(Node node) { if (compareAndSetWaitStatus(node, Node.CONDITION, 0)) { 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; }
transferAfterCancelledWait方法并不在ConditionObject中定义，而是由AQS提供。这个方法根据是否中断发生时，是否有signal操作来“掺和”来返回结果。方法调用CAS操作将node的waitStatus从CONDITION设置为0，如果成功，说明当中断发生时，说明没有signal发生（signal的第一步是将node的waitStatus设置为0），在调用enq将线程放入Sync队列后直接返回true，表示中断先于signal发生，即中断在await等待过程中发生，根据await的语义，在遇到中断时需要抛出中断异常，返回true告诉上层方法返回THROW_IT，后续会根据这个返回值做抛出中断异常的处理。
如果CAS操作失败，是否说明中断后于signal发生呢？只能说这时候我们不能确定中断和signal到底谁先发生，只是在我们做CAS操作的时候，他们俩已经都发生了（中断->interrupted检测->signal->CAS，或者signal->中断->interrupted检测->CAS都有可能），这时候我们无法判断到底顺序是怎样，这里的处理是不管怎样都返回false告诉上层方法返回REINTERRUPT，当做是signal先发生（线程被signal唤醒）来处理，后续根据这个返回值做“补上”中断的处理。在返回false之前，我们要先做一下等待，直到当前线程被成功放入Sync锁等待队列。
因此，我们可以这样总结：transferAfterCancelledWait的返回值表示了线程是否因为中断从park中唤醒。
至此，我们终于可以正式来看await方法了：
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); }
await方法是及时响应中断的。它首先检查了一下中断标志。然后调用addConditionWaiter将当前线程放入Condition队列的尾，并顺手清理了一下队列里的无用节点。紧接着调用fullyRelease方法释放当前线程持有的锁。然后是一个while循环，这个循环会循环检测线程的状态，直到线程被signal或者中断唤醒且被放入Sync锁等待队列。如果中断发生的话，还需要调用checkInterruptWhileWaiting方法，根据中断发生的时机确定后去处理这次中断的方式，如果发生中断，退出while循环。
退出while循环后，我们调用acquireQueued方法来获取锁，注意，acquireQueued方法的返回值表示在等待获取锁的过程中是否发生中断，如果发生中断 且 原来没有需要做抛出处理的中断发生时，我们将后续处理方式设置为REINTERRUPT（如果原来在await状态有中断发生，即interrruptMode==THROW_IE，依然保持THROW_IE）。
如果是应为中断从park中唤醒（interruptMode==THROT_IE），当前线程仍在Condition队列中，但waitStatus已经变成0了，这里在调用unlinkCancelledWaiters做一次清理。
最后，根据interruptMode的值，调用reportInterruptAfterWait做出相应处理：
private void reportInterruptAfterWait(int interruptMode) throws InterruptedException { if (interruptMode == THROW_IE) throw new InterruptedException(); else if (interruptMode == REINTERRUPT) selfInterrupt(); }
如果interruptMod==0，donothing，如果是THROW_IE，说明在await状态下发生中断，抛出中断异常，如果是REINTERRUPT，说明是signal“掺和”了中断，我们无法分辨具体的先后顺序，于是统一按照先signal再中断来处理，即成功获取锁之后要调用selfInterrupt“补上”这次中断。
二、awaitNanos 限时的在条件变量上等待
public final long awaitNanos(long nanosTimeout) throws InterruptedException { if (Thread.interrupted()) throw new InterruptedException(); Node node = addConditionWaiter(); int savedState = fullyRelease(node); final long deadline = System.nanoTime() + nanosTimeout; int interruptMode = 0; while (!isOnSyncQueue(node)) { if (nanosTimeout <= 0L) { transferAfterCancelledWait(node); break; } if (nanosTimeout >= spinForTimeoutThreshold) 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 deadline - System.nanoTime(); }
awaitNanos方法与await方法大致相同，区别在于每次park是定时的，当被唤醒时，比较一下剩余等待时间Timeout与spinForTimeoutThreshold阈值的大小，如果小于，将不再park，spinForTimeoutThreshold阈值的作用在笔者的另一篇博文Semaphore源码学习笔记中已经分析过，作用是提高短时长的等待的相应效率。
注意：当已经到达等待的deadline时，调用transferAfterCancelledWait方法，注意，此时可能发生中断（上次调用checkInterruptWhileWaiting之后被中断），再次的，我们无法判断这次中断与到时这两个的先后顺序，我们在这里的处理方式是直接忽略这次中断，统一认为是先到时后中断（体现在没有记录transferAfterCancelledWait方法的返回值），但在transferAfterCancelledWait方法中的处理是考虑了被中断的情况的，只不过这个中断标志位没有检测，留给后续来处理了。这个中断标志将会在调用acquireQueued方法并成功获取锁之后被检测并返回，最终影响interruptMode的值，并在reportInterruptAfterWait方法中被处理。可见，这次中断最终没有被遗漏，只是我们先处理的signal，回过头来再去处理它。
最后方法的返回值是拍唤醒后的剩余等待时间，这个时间可能小于0。
await(long time, TimeUnit unit)方法与awaitNanos方法十分类似，不再赘述。
三、awaitUtil 指定结束时刻的在条件变量上等待
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; }
awaitUtil方法在原理上与awaitNanos方法是也十分相似，只不过park操作调用的是LockSupportparkUtil方法，且没有spinForTimeoutThreshold阈值的应用。在返回值上也有些许差别：返回值timedout记录了transferAfterCancelledWait方法的返回值——线程是否因为中断从park中唤醒，如果是的话，表示还没有到等待的deadline。
四、signal 唤醒Condition队列的头节点持有的线程
public final void signal() { if (!isHeldExclusively()) throw new IllegalMonitorStateException(); Node first = firstWaiter; if (first != null) doSignal(first); }
调用signal之前也需要获取锁，因此signal方法首先检测了一下当前线程是否获取了独占锁。然后调用doSignal唤醒队列中第一个等待线程。注意，这里的“唤醒”意思是将线程从Condition队列移到Sync队列，表示已经完成Condition的等待，具有了去竞争锁的资格。至此，我们可以发现，由于await会直接把线程放入Condition等待队列的尾部，因此Condition是公平的，即按照入列的顺序来signal。
private void doSignal(Node first) { do { if ( (firstWaiter = first.nextWaiter) == null) lastWaiter = null; first.nextWaiter = null; } while (!transferForSignal(first) && (first = firstWaiter) != null); }
doSignal方法先将first节点从队列中摘下，然后调用transferForSignal去改变first节点的waitStatus（所谓唤醒线程），这个方法有可能失败，因为等待线程可能已经到时或者被中断，因此while循环这个操作直到成功唤醒或队列为空。我们来看下transferForSignal方法：
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; }
这个方法并不在ConditionObject中定义，而是由AQS提供。方法首先调用CAS操作修改node的waitStatus，如果失败，表示线程已经放弃等待（到时或被中断），直接返回false。如果成功，调用enq方法将它放入Sync锁等待队列，返回值p是node在Sync队列中的前驱节点。紧接着检测一下前驱p的waitStatus，如果发现不为SIGNAL，需要将node持有的线程（注意不是当前线程）unpark，这里必须搞清楚，node线程是在哪里park的，显然，他还在await方法的那个while循环里。unpark之后，node线程将会从while循环中退出，然后去调用acquireQueued方法，这个方法是一个自旋，弄得线程会在自旋过程中清除已经为CANCELLED状态的前驱，然后注册前驱节点的waitStatus为SIGNAL。
至此，signal方法已经完成了所有该做的，“唤醒”的线程已经成功加入Sync队列并已经参与锁的竞争了，返回true。
五、signalAll 唤醒Condition队列的所有等待线程
public final void signalAll() { if (!isHeldExclusively()) throw new IllegalMonitorStateException(); Node first = firstWaiter; if (first != null) doSignalAll(first); }
signalAll方法同样先检测是否持有独占锁，然后对奥用doSignalAll方法：
private void doSignalAll(Node first) { lastWaiter = firstWaiter = null; do { Node next = first.nextWaiter; first.nextWaiter = null; transferForSignal(first); first = next; } while (first != null); }
doSignalAll方法循环调用transferForSignal方法“唤醒”队列的头结点，直到队列为空。
总结：ConditionObject由AQS提供，它实现了类似wiat、notify和notifyAll类似的功能。Condition必须与一个独占锁绑定使用，在await或signal之前必须现持有独占锁。Condition队列是一个单向链表，他是公平的，按照先进先出的顺序从队列中被“唤醒”，所谓唤醒指的是完成Condition对象上的等待，被移到Sync锁等待队列中，有参与竞争锁的资格（Sync队列有公平&非公平两种模式，注意区别）。