AbstractQueue简介： http://donald-draper.iteye.com/blog/2363608
ConcurrentLinkedQueue解析： http://donald-draper.iteye.com/blog/2363874
BlockingQueue接口的定义： http://donald-draper.iteye.com/blog/2363942
LinkedBlockingQueue解析： http://donald-draper.iteye.com/blog/2364007
ArrayBlockingQueue解析： http://donald-draper.iteye.com/blog/2364034
PriorityBlockingQueue解析： http://donald-draper.iteye.com/blog/2364100
SynchronousQueue是同步的队列，里面涉及到数据结构和一些算法的知识，今天我们虚心来看一下，能得到多少，是多少。欢迎网友，给出不同的建议，进行共同学习交流。
package java.util.concurrent;
import java.util.concurrent.locks.*;
import java.util.concurrent.atomic.*;
import java.util.*;
/**
* A {@linkplain BlockingQueue blocking queue} in which each insert
* operation must wait for a corresponding remove operation by another
* thread, and vice versa. A synchronous queue does not have any
* internal capacity, not even a capacity of one. You cannot
* <tt>peek</tt> at a synchronous queue because an element is only
* present when you try to remove it; you cannot insert an element
* (using any method) unless another thread is trying to remove it;
* you cannot iterate as there is nothing to iterate. The
* [i]head[/i] of the queue is the element that the first queued
* inserting thread is trying to add to the queue; if there is no such
* queued thread then no element is available for removal and
* <tt>poll()</tt> will return <tt>null</tt>. For purposes of other
* <tt>Collection</tt> methods (for example <tt>contains</tt>), a
* <tt>SynchronousQueue</tt> acts as an empty collection. This queue
* does not permit <tt>null</tt> elements.
*
SynchronousQueue阻塞队列，每次插入操作必须等待一个协同的移除线程，反之亦然。
SynchronousQueue同步队列没有容量，可以说，没有一个容量。由于队列中只有在消费线程，
尝试消费元素的时候，才会出现元素，所以不能进行peek操作；不能用任何方法，
生产元素，除非有消费者在尝试消费元素，同时由于队列中没有元素，所以不能迭代。
head是第一个生产线程尝试生产的元素；如果没有这样的生产线程，那么没有元素可利用，
remove和poll操作将会返回null。SynchronousQueue实际一个空集合类。同时同步队列不允许为null。
* <p>Synchronous queues are similar to rendezvous channels used in
* CSP and Ada. They are well suited for handoff designs, in which an
* object running in one thread must sync up with an object running
* in another thread in order to hand it some information, event, or
* task.
*
同步队列与CSP和Ada场景下的通道相似（具体CSP和Ada可以google，我查的意思
为CSP-Constraint Satisfaction Problem，只有这个意思看上去有点像，怎么感觉不对，
据说CSP在机器学习中很有用，Ada查的靠谱一点的意思为美国军方的程序设计语言，其他的
都不靠谱，看到这篇文章的网友，可以看一下，可以给我发私信或留言，探讨一下）。
同步队列适用于传输通道设计，一个线程同步或生产一个元素，消息，资源，同时
另一个线程消费这些资源或任务。
* <p> This class supports an optional fairness policy for ordering
* waiting producer and consumer threads. By default, this ordering
* is not guaranteed. However, a queue constructed with fairness set
* to <tt>true</tt> grants threads access in FIFO order.
*
同步队列支持生产者和消费者等待的公平性策略。默认情况下，不能保证生产消费的顺序。
如果一个同步队列构造为公平性，则可以线程以FIFO访问队列元素。
* <p>This class and its iterator implement all of the
* [i]optional[/i] methods of the {@link Collection} and {@link
* Iterator} interfaces.
*
实现了所有Collection和Iterator接口
* <p>This class is a member of the
* <a href="{@docRoot}/../technotes/guides/collections/index.html">
* Java Collections Framework</a>.
*
* @since 1.5
* @author Doug Lea and Bill Scherer and Michael Scott
* @param <E> the type of elements held in this collection
*/
public class SynchronousQueue<E> extends AbstractQueue<E>
implements BlockingQueue<E>, java.io.Serializable {
private static final long serialVersionUID = -3223113410248163686L;
/*
* This class implements extensions of the dual stack and dual
* queue algorithms described in "Nonblocking Concurrent Objects
* with Condition Synchronization", by W. N. Scherer III and
* M. L. Scott. 18th Annual Conf. on Distributed Computing,
* Oct. 2004 (see also
* http://www.cs.rochester.edu/u/scott/synchronization/pseudocode/duals.html).
同步队列实现拓展了双栈和双队列算法（条件同步的非阻塞并发对象），
在分布计算年刊中有具体描述，见下面连接
* The (Lifo) stack is used for non-fair mode, and the (Fifo)
* queue for fair mode. The performance of the two is generally
* similar. Fifo usually supports higher throughput under
* contention but Lifo maintains higher thread locality in common
* applications.
*
LIFO栈用于非公平模式，FIFO队列用于公平模式。两者的性能大体相同。
FIFO通常用于有高吞吐量存在竞争的场景，LIFO栈用于
Lifo maintains higher thread locality in common
applications.这句不翻译了，保持原味。
* A dual queue (and similarly stack) is one that at any given
* time either holds "data" -- items provided by put operations,
* or "requests" -- slots representing take operations, or is
* empty. A call to "fulfill" (i.e., a call requesting an item
* from a queue holding data or vice versa) dequeues a
* complementary node. The most interesting feature of these
* queues is that any operation can figure out which mode the
* queue is in, and act accordingly without needing locks.
*
双队列是一个在任何时候持有由put操作提供元素的data，slots表示的
take操作的请求，或为空队列，与栈相似。一个调用fulfill操作（请求队列中
的持有元素，即进行put操作），将会有一个不足元素出队列，反之亦然，
意思为一个take操作对一个put操作，一个put操作必须对应一个take操作。
这种队列最有趣的特点是，任何操作不根据锁，可以判断进队列的模式，
是非公平的LIFO栈stack还是公平的FIFO队列queue。
* Both the queue and stack extend abstract class Transferer
* defining the single method transfer that does a put or a
* take. These are unified into a single method because in dual
* data structures, the put and take operations are symmetrical,
* so nearly all code can be combined. The resulting transfer
* methods are on the long side, but are easier to follow than
* they would be if broken up into nearly-duplicated parts.
*
队列和栈继承了Transferer类，Transferer定义简单的方法（转换，转让）
做put或take操作。因为在双数据结构中，put和take操作是对称的，所以他们
统一定义在一个方法中，所以几乎所有的代码可以放在一起。
The resulting transfer
methods are on the long side, but are easier to follow than
they would be if broken up into nearly-duplicated parts.
这段不翻译保持原味。
* The queue and stack data structures share many conceptual
* similarities but very few concrete details. For simplicity,
* they are kept distinct so that they can later evolve
* separately.
*
队列和栈数据结构有许多概念上相同的属性，但也有一些具体的不同。
为了简单起见，他们保持着区别，确保later evolve separately。
* The algorithms here differ from the versions in the above paper
* in extending them for use in synchronous queues, as well as
* dealing with cancellation. The main differences include:
*
这个算法与上面论文中的算法有所不同，我们扩展为了论文中的算法用在同步
队列中，也用于处理cancellation。主要的不同包括：
* 1. The original algorithms used bit-marked pointers, but
* the ones here use mode bits in nodes, leading to a number
* of further adaptations.
* 2. SynchronousQueues must block threads waiting to become
* fulfilled.
* 3. Support for cancellation via timeout and interrupts,
* including cleaning out cancelled nodes/threads
* from lists to avoid garbage retention and memory depletion.
*
1.原始算法中用了bit标记指针，本同步队列实现算法中，在节点中使用bits模式，
将导致number进一步的调整。
2.同步队列必须阻塞线程等待变的可填充。
3.支持通过中断和超时取消等待策略，包括从等待队列中清除取消的节点或线程，
以避免产生垃圾，和内存泄漏。
* Blocking is mainly accomplished using LockSupport park/unpark,
* except that nodes that appear to be the next ones to become
* fulfilled first spin a bit (on multiprocessors only). On very
* busy synchronous queues, spinning can dramatically improve
* throughput. And on less busy ones, the amount of spinning is
* small enough not to be noticeable.
*
通过LockSupport的park/unpark方法，实现阻塞，除了在多处理器上，
下一个变得可填充的先自旋的节点或线程。在繁忙的同步队列中，自旋可以显著
提高吞吐量。在不繁忙时，自旋并不太多的消耗。
* Cleaning is done in different ways in queues vs stacks. For
* queues, we can almost always remove a node immediately in O(1)
* time (modulo retries for consistency checks) when it is
* cancelled. But if it may be pinned as the current tail, it must
* wait until some subsequent cancellation. For stacks, we need a
* potentially O(n) traversal to be sure that we can remove the
* node, but this can run concurrently with other threads
* accessing the stack.
*
在队列和栈中，清除操作有着不同的实现。在队列中，当一个节点或线程取消时，
我们大多数情况下，可以立即以常量1（一致性检查尝试次数的模）的时间移除一个节点或线程。
但是如果一直在队列的尾部，则必须等后来的线程节点取消。对于栈，
我们可能需要时间O(n)遍历已确定那个节点我们可以移除，但是这个可以与
其他线程并发访问栈。
* While garbage collection takes care of most node reclamation
* issues that otherwise complicate nonblocking algorithms, care
* is taken to "forget" references to data, other nodes, and
* threads that might be held on to long-term by blocked
* threads. In cases where setting to null would otherwise
* conflict with main algorithms, this is done by changing a
* node's link to now point to the node itself. This doesn't arise
* much for Stack nodes (because blocked threads do not hang on to
* old head pointers), but references in Queue nodes must be
* aggressively forgotten to avoid reachability of everything any
* node has ever referred to since arrival.
然而垃圾回收器必须关注其他复杂非阻塞算法的节点再生问题，数据，节点的引用
及线程也在会通过阻塞其他线程，以便长期持有锁。以防此类情况的发生，
引用将会为设置为null，以免与主要算法冲突，本算法姐姐方法是节点链接指向其自己。
这样不为引起大量的栈节点（因为阻塞线程，不能停留在head指针上），但是为了
避免其他的所有节点与以前引用的节点可达，队列节点的引用必须显示忘记索引。
*/
/**
* Shared internal API for dual stacks and queues.
双栈和队列共享内部API，队列和栈的父类
*/
abstract static class Transferer {
/**
* Performs a put or take.
*
执行一个put或take操作
* @param e if non-null, the item to be handed to a consumer;
* if null, requests that transfer return an item
* offered by producer.
如果元素为非空，则交给消费者处理，如果为null，请求生产者
生产一个元素，并返回元素
* @param timed if this operation should timeout 是否超时
* @param nanos the timeout, in nanoseconds 超时时间
* @return if non-null, the item provided or received; if null,
* the operation failed due to timeout or interrupt --
* the caller can distinguish which of these occurred
* by checking Thread.interrupted.
返回元素，如果非null，要不是队列中已经存在的，要不是生产者刚生产的。
如果为null，以为着由于超时，中断导致操作失败，调用可以通过检查线程中断位，
辨别放生了哪一种情况。
*/
abstract Object transfer(Object e, boolean timed, long nanos);
}
/** The number of CPUs, for spin control 获取运行时环境的处理个数*/
static final int NCPUS = Runtime.getRuntime().availableProcessors();
/**
* The number of times to spin before blocking in timed waits.
* The value is empirically derived -- it works well across a
* variety of processors and OSes. Empirically, the best value
* seems not to vary with number of CPUs (beyond 2) so is just
* a constant.
在超时等待阻塞前，自旋尝试的次数，这个值是一个，在不同处理器和系统性能上
良好工作的经验值。经验上来讲，最好的值，不要随着CPUS的个数/2的值变动，
所以它是一个常量，当处理器个数小于2，则为0，否则为32。
*/
static final int maxTimedSpins = (NCPUS < 2) ? 0 : 32;
/**
* The number of times to spin before blocking in untimed waits.
* This is greater than timed value because untimed waits spin
* faster since they don't need to check times on each spin.
在非超时等待阻塞之前，自旋的次数，最大非超时自旋时间大于最大自旋
时间，因为由于非超时自旋不需要在每次自旋时，不需要检查时间，所以，
非超时自旋非常快。
*/
static final int maxUntimedSpins = maxTimedSpins * 16;
/**
* The number of nanoseconds for which it is faster to spin
* rather than to use timed park. A rough estimate suffices.
快速自旋的时间，而不是park的时间，一个粗略的估计值。
*/
static final long spinForTimeoutThreshold = 1000L;
}
下面来看dual队列和栈的实现
先看栈：
/** Dual stack */
static final class TransferStack extends Transferer {
/*
* This extends Scherer-Scott dual stack algorithm, differing,
* among other ways, by using "covering" nodes rather than
* bit-marked pointers: Fulfilling operations push on marker
* nodes (with FULFILLING bit set in mode) to reserve a spot
* to match a waiting node.
本stack实现的是算法是拓展了Scherer-Scott双栈的算法，所不同的时，用
covering节点，而不是bit-marked指针：在bit集填充模式下，填充操作将会为
匹配一个等待节点保留资源，生产一个标记节点。
*/
/* Modes for SNodes, ORed together in node fields */
/** Node represents an unfulfilled consumer REQUEST节点表示一个未填充的消费者*/
static final int REQUEST = 0;
/** Node represents an unfulfilled producer DATA节点表示一个未填充的生产者*/
static final int DATA = 1;
/** Node is fulfilling another unfulfilled DATA or REQUEST
FULFILLING节点表示生产者正在给等待资源的消费者补给资源，或生产者在等待消费者消费资源/
static final int FULFILLING = 2;
/** Return true if m has fulfilling bit set
如果m是一个填充为单元，则返回true*/
static boolean isFulfilling(int m) { return (m & FULFILLING) != 0; }
/** Node class for TransferStacks. 栈节点 */
static final class SNode {
volatile SNode next; // next node in stack 节点的后继
volatile SNode match; // the node matched to this 匹配节点
volatile Thread waiter; // to control park/unpark 等待者线程
Object item; // data; or null for REQUESTs 数据，消费者消费的资源
int mode;//节点模式
// Note: item and mode fields don't need to be volatile
// since they are always written before, and read after,
// other volatile/atomic operations.
//元素item和mode需要要可见，由于他们总是在其他可见/原子操作写之前，读之后
SNode(Object item) {
this.item = item;
}
//设置节点后继
boolean casNext(SNode cmp, SNode val) {
return cmp == next &&
UNSAFE.compareAndSwapObject(this, nextOffset, cmp, val);
}
/**
* Tries to match node s to this node, if so, waking up thread.
* Fulfillers call tryMatch to identify their waiters.
* Waiters block until they have been matched.
*
尝试匹配目标节点与本节点，如果匹配，可以唤醒线程。补给者调用tryMatch方法
确定它们的等待线程。等待线程阻塞到它们自己被匹配。如果匹配返回true。
* @param s the node to match
* @return true if successfully matched to s
*/
boolean tryMatch(SNode s) {
if (match == null &&
UNSAFE.compareAndSwapObject(this, matchOffset, null, s)) {
Thread w = waiter;
//如果等待者不为null，则unpark等待线程
if (w != null) { // waiters need at most one unpark
waiter = null;
LockSupport.unpark(w);
}
return true;
}
return match == s;
}
/**
* Tries to cancel a wait by matching node to itself.节点尝试取消等待
*/
void tryCancel() {
UNSAFE.compareAndSwapObject(this, matchOffset, null, this);
}
//match指向自己，则取消等待
boolean isCancelled() {
return match == this;
}
// Unsafe mechanics
private static final sun.misc.Unsafe UNSAFE;
private static final long matchOffset;
private static final long nextOffset;
static {
try {
UNSAFE = sun.misc.Unsafe.getUnsafe();
Class k = SNode.class;
matchOffset = UNSAFE.objectFieldOffset
(k.getDeclaredField("match"));
nextOffset = UNSAFE.objectFieldOffset
(k.getDeclaredField("next"));
} catch (Exception e) {
throw new Error(e);
}
}
}
/** The head (top) of the stack 栈头节点*/
volatile SNode head;
//CAS操作nh为当前head，并比较head旧值是否为h
boolean casHead(SNode h, SNode nh) {
return h == head &&
UNSAFE.compareAndSwapObject(this, headOffset, h, nh);
}
/**
* Creates or resets fields of a node. Called only from transfer
* where the node to push on stack is lazily created and
* reused when possible to help reduce intervals between reads
* and CASes of head and to avoid surges of garbage when CASes
* to push nodes fail due to contention.
创建或重新设置节点的fields。在节点入栈懒创建，在当可能需要保证减少intervals（间隔）
读和head的CAS操或避免由于竞争CAS操作节点入栈引起的垃圾时，此方法会被transfer调用
*/
static SNode snode(SNode s, Object e, SNode next, int mode) {
if (s == null) s = new SNode(e);
s.mode = mode;
s.next = next;
return s;
}
/**
* Puts or takes an item.
put或take一个元素
*/
Object transfer(Object e, boolean timed, long nanos) {
/*
* Basic algorithm is to loop trying one of three actions:
*
算法的基本步骤是，循环尝试一下3步
* 1. If apparently empty or already containing nodes of same
* mode, try to push node on stack and wait for a match,
* returning it, or null if cancelled.
*
1.如果队列为空或已经包含相同模式的节点，则尝试节点入栈，等待匹配，
返回，如果取消返回null。
* 2. If apparently containing node of complementary mode,
* try to push a fulfilling node on to stack, match
* with corresponding waiting node, pop both from
* stack, and return matched item. The matching or
* unlinking might not actually be necessary because of
* other threads performing action 3:
*
2.如果包含一个互补模式的节点（take(REQUEST)->put(DATA)；put(DATA)->take(REQUEST)），
则尝试一个FULFILLING节点入栈，同时匹配等待的协同节点，两个节点同时出栈，返回匹配的元素。
由于其他线程执行步骤3，实际匹配和解除链接指针动作不会发生。
* 3. If top of stack already holds another fulfilling node,
* help it out by doing its match and/or pop
* operations, and then continue. The code for helping
* is essentially the same as for fulfilling, except
* that it doesn't return the item.
3.如果栈顶存在另外一个FULFILLING的节点，则匹配节点，并出栈。这段的代码
与fulfilling相同，除非没有元素返回
*/
SNode s = null; // constructed/reused as needed
//根据元素判断节点模式，元素不为null，则为DATA，否则为REQUEST
int mode = (e == null) ? REQUEST : DATA;
for (;;) {
SNode h = head;
if (h == null || h.mode == mode) { // empty or same-mode
//如果是空队列，或栈头节点的模式与要放入的节点模式相同
if (timed && nanos <= 0) { // can't wait
//如果超时，则取消等待，出栈，设置栈头为其后继
if (h != null && h.isCancelled())
casHead(h, h.next); // pop cancelled node
else
//否则返回null
return null;
} else if (casHead(h, s = snode(s, e, h, mode))) {
//如果非超时，则将创建的新节点入栈成功，即放在栈头，自旋等待匹配节点（timed决定超时，不超时）
SNode m = awaitFulfill(s, timed, nanos);
if (m == s) { // wait was cancelled
//如果返回的是自己，节点取消等待，从栈中移除，并遍历栈移除取消等待的节点
clean(s);
return null;
}
if ((h = head) != null && h.next == s)
//s节点匹配成功，则设置栈头为s的后继
casHead(h, s.next); // help s's fulfiller
//匹配成功，REQUEST模式返回，匹配到的节点元素（DATA），DATA模式返回当前节点元素
return (mode == REQUEST) ? m.item : s.item;
}
} else if (!isFulfilling(h.mode)) { // try to fulfill
//如果栈头节点模式不为Fulfilling，判断是否取消等待，是则出栈
if (h.isCancelled()) // already cancelled
casHead(h, h.next); // pop and retry
//非取消等待，则是节点入栈
else if (casHead(h, s=snode(s, e, h, FULFILLING|mode))) {
for (;;) { // loop until matched or waiters disappear
SNode m = s.next; // m is s's match
//后继节点为null，则出栈
if (m == null) { // all waiters are gone
casHead(s, null); // pop fulfill node
s = null; // use new node next time
break; // restart main loop
}
SNode mn = m.next;
//尝试匹配是s节点
if (m.tryMatch(s)) {
//匹配成功两个节点则出栈，
casHead(s, mn); // pop both s and m
return (mode == REQUEST) ? m.item : s.item;
} else // lost match
//否则，跳过s的后继节点
s.casNext(m, mn); // help unlink
}
}
} else { // help a fulfiller
//如果栈头节点模式为Fulfilling,找出栈头的匹配节点
SNode m = h.next; // m is h's match
if (m == null) // waiter is gone
//如果无后继等待节点，则栈头出栈
casHead(h, null); // pop fulfilling node
else {
//尝试匹配，如果匹配成功，栈头和匹配节点出栈，否则跳过后继节点
SNode mn = m.next;
if (m.tryMatch(h)) // help match
casHead(h, mn); // pop both h and m
else // lost match
h.casNext(m, mn); // help unlink
}
}
}
}
/**
* Spins/blocks until node s is matched by a fulfill operation.
自旋或阻塞，直到节点被一个fulfill操作匹配
*
* @param s the waiting node 等待被匹配的节点
* @param timed true if timed wait 是否超时等待
* @param nanos timeout value 时间值
* @return matched node, or s if cancelled 如果匹配返回节点，否则取消等待
*/
SNode awaitFulfill(SNode s, boolean timed, long nanos) {
/*
* When a node/thread is about to block, it sets its waiter
* field and then rechecks state at least one more time
* before actually parking, thus covering race vs
* fulfiller noticing that waiter is non-null so should be
* woken.
*
当一个节点线程将要阻塞时，在实际park之前，设置等待线程的field，重新至少检查
自身状态一次，这样可以避免在fulfiller注意到有等待线程非null，可以操作时，掩盖了竞争。
* When invoked by nodes that appear at the point of call
* to be at the head of the stack, calls to park are
* preceded by spins to avoid blocking when producers and
* consumers are arriving very close in time. This can
* happen enough to bother only on multiprocessors.
*
当awaitFulfill被栈头节点调用时，通过自旋park一段时间，以免在刚要阻塞的时刻，
有生产者或消费者到达。这在多处理机上将会发生。
* The order of checks for returning out of main loop
* reflects fact that interrupts have precedence over
* normal returns, which have precedence over
* timeouts. (So, on timeout, one last check for match is
* done before giving up.) Except that calls from untimed
* SynchronousQueue.{poll/offer} don't check interrupts
* and don't wait at all, so are trapped in transfer
* method rather than calling awaitFulfill.
主循环检查返回的顺序将会反应，在正常返回时，中断是否处理，还是超时处理。
（在放弃匹配之前，及最后一次检查，正好超时），除非调用SynchronousQueue的
非超时poll/offer操作，不会检查中断，不等待，那么将调用transfer方法中的其他部分逻辑，
而不是调用awaitFulfill。
*/
long lastTime = timed ? System.nanoTime() : 0;
Thread w = Thread.currentThread();
SNode h = head;
//获取自旋的次数
int spins = (shouldSpin(s) ?
(timed ? maxTimedSpins : maxUntimedSpins) : 0);
for (;;) {
if (w.isInterrupted())
//如果线程被中断，则取消等待
s.tryCancel();
SNode m = s.match;
if (m != null)
//如果节点的匹配节点不为null，则返回匹配节点
return m;
if (timed) {
long now = System.nanoTime();
nanos -= now - lastTime;
lastTime = now;
if (nanos <= 0) {
//如果超时，则取消等待
s.tryCancel();
continue;
}
}
if (spins > 0)
//如果自旋次数大于零，且可以自旋，则自旋次数减1
spins = shouldSpin(s) ? (spins-1) : 0;
else if (s.waiter == null)
//如果节点S的等待线程为空，则设置当前节点为S节点的等待线程，以便可以park后继节点。
s.waiter = w; // establish waiter so can park next iter
else if (!timed)
//非超时等在者，park当前线程
LockSupport.park(this);
else if (nanos > spinForTimeoutThreshold)
//如果超时时间大于，最大自旋阈值，则超时park当前线程
LockSupport.parkNanos(this, nanos);
}
}
/**
* Returns true if node s is at head or there is an active
* fulfiller.
如果节点在栈头或栈头为FULFILLING的节点，则返回true
*/
boolean shouldSpin(SNode s) {
SNode h = head;
return (h == s || h == null || isFulfilling(h.mode));
}
/**
* Unlinks s from the stack.
*/
void clean(SNode s) {
s.item = null; // forget item
s.waiter = null; // forget thread
/*
* At worst we may need to traverse entire stack to unlink
* s. If there are multiple concurrent calls to clean, we
* might not see s if another thread has already removed
* it. But we can stop when we see any node known to
* follow s. We use s.next unless it too is cancelled, in
* which case we try the node one past. We don't check any
* further because we don't want to doubly traverse just to
* find sentinel.
最糟糕的情况是我们需要遍历整个栈，unlink节点s。如果有多个线程同时访问
clean方法，由于其他线程可能移除s节点，我们也许看不到s节点。但是我们可以停止
操作，当发现一个节点的后继为s。我们可以用s节点的后继，除非s节点取消，否则，
我们可越过s节点。我们不会进一步地检查，因为我们不想仅仅为了发现s节点，遍历两次。
*/
SNode past = s.next;
if (past != null && past.isCancelled())
past = past.next;
// Absorb cancelled nodes at head
SNode p;
while ((p = head) != null && p != past && p.isCancelled())
//设置栈头节点的后继为第一个非取消等待的节点
casHead(p, p.next);
// Unsplice embedded nodes，遍历栈，移除取消等待的节点
while (p != null && p != past) {
SNode n = p.next;
if (n != null && n.isCancelled())
p.casNext(n, n.next);
else
p = n;
}
}
// Unsafe mechanics
private static final sun.misc.Unsafe UNSAFE;
private static final long headOffset;
static {
try {
UNSAFE = sun.misc.Unsafe.getUnsafe();
Class k = TransferStack.class;
headOffset = UNSAFE.objectFieldOffset
(k.getDeclaredField("head"));
} catch (Exception e) {
throw new Error(e);
}
}
}
}
自此同步队列的TransferStack已经看完，由于同步队列的内容量较大，我们这一篇先分析到这，下一篇再看TransferQueue和其他部分，先小节一下吧：
SynchronousQueue阻塞队列，每次插入操作必须等待一个协同的移除线程，反之亦然。SynchronousQueue同步队列没有容量，可以说，没有一个容量。由于队列中只有在消费线程，尝试消费元素的时候，才会出现元素，所以不能进行peek操作；不能用任何方法，生产元素，除非有消费者在尝试消费元素，同时由于队列中没有元素，所以不能迭代。head是第一个生产线程尝试生产的元素；如果没有这样的生产线程，那么没有元素可利用，remove和poll操作将会返回null。SynchronousQueue实际一个空集合类。同时同步队列不允许为null。同步队列支持生产者和消费者等待的公平性策略。默认情况下，不能保证生产消费的顺序。如果一个同步队列构造为公平性，则可以线程以FIFO访问队列元素。当时非公平策略用的是TransferStack，公平策略用的是TransferQueue；TransferStack和TransferQueue是存放等待操作线程的描述，从TransferStack中Snode节点可以看出：节点关联一个等待线程waiter，后继next，匹配节点match，节点元素item和模式mode；模式由三种，REQUEST节点表示消费者等待消费资源，DATA表示生产者等待生产资源。FULFILLING节点表示生产者正在给等待资源的消费者补给资源，或生产者在等待消费者消费资源。当有线程take/put操作时，查看栈头，如果是空队列，或栈头节点的模式与要放入的节点模式相同；如果是超时等待，判断时间是否小于0，小于0则取消节点等待；如果非超时，则将创建的新节点入栈成功，即放在栈头，自旋等待匹配节点（timed决定超时，不超时）；如果匹配返回的是自己，节点取消等待，从栈中移除，并遍历栈移除取消等待的节点；匹配成功，两个节点同时出栈，REQUEST模式返回，匹配到的节点元素（DATA），DATA模式返回返回当前节点元素）。如果与栈头节点的模式不同且不为FULFILLING，匹配节点，成功者，两个节点同时出栈，REQUEST模式返回，匹配到的节点元素（DATA），DATA（put）模式返回返回当前节点元素。如果栈头为FULFILLING，找出栈头的匹配节点，栈头与匹配到的节点同时出栈。从分析非公平模式下的TransferStack，可以看出一个REQUEST操作必须同时伴随着一个DATA操作，一个DATA操作必须同时伴随着一个REQUEST操作，这也是同步队列的命名中含Synchronous原因。这也应了这句话
SynchronousQueue像一个管道，一个操作必须等待另一个操作的发生。
SynchronousQueue解析下-TransferQueue： http://donald-draper.iteye.com/blog/2364842