锁持有者管理器AbstractOwnableSynchronizer： http://donald-draper.iteye.com/blog/2360109
AQS线程挂起辅助类：LockSupport： http://donald-draper.iteye.com/blog/2360206
AQS详解-CLH队列，线程等待状态： http://donald-draper.iteye.com/blog/2360256
/**
* 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。
* 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
* AbstractQueuedSynchronizer.
*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。
*
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 ConditionObject 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
* 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
* interrupted status 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).
*
* 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 true if the current thread has been interrupted;
* false 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.
*
* 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.
*
*
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]
* 总结：
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。