SynchronousQueue是一种比较特殊的阻塞队列，不同于之前的阻塞队列，特点为：
1、 每次put必须有take的存在，也就是说生产者将一个元素put传递给一个消费者，消费者不存在就put不成功；
2、 内部没有容量来维持存放元素，所以size，迭代等一些方法没有意义；
3、 使用cas操作，没有使用锁；
4、 通过栈(非公平)，队列(公平)2中结构来支持公平\非公平策略；
newCachedThreadPool线程池使用了这种队列。
<span style="font-size:18px;">abstract static class Transferer {
/**
* put和take都调用这个函数
* e不为null，put操作，表示生产者将e转交给一个消费者
* e为null,take操作,表示消费者获取一个生产者转交的数据
* timed和nanos支持超时
*/
abstract Object transfer(Object e, boolean timed, long nanos);
}</span>这个是栈和队列的公共基类，所有的put，take操作最后都是调用这个transfer方法。
先来看下非公平的栈。
看下栈的SNode结构：
static final class SNode {
volatile SNode next; // next node in stack
volatile SNode match; // 本节点的匹配节点
volatile Thread waiter; // 等待线程，用于park，unpark
Object item; // put时data，take时null
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.
// 利用到到volatile语义的内存屏障
SNode(Object item) {
this.item = item;
}
boolean casNext(SNode cmp, SNode val) {
return cmp == next &&
UNSAFE.compareAndSwapObject(this, nextOffset, cmp, val);
}
/**
* 匹配当前和s节点
*/
boolean tryMatch(SNode s) {
if (match == null &&
UNSAFE.compareAndSwapObject(this, matchOffset, null, s)) { //match为null，则cas修改为s
Thread w = waiter; //如果节点有等待的线程，那就置null，unpark
if (w != null) { // waiters need at most one unpark
waiter = null;
LockSupport.unpark(w);
}
return true;
}
return match == s; //如果match已经存在或者cas失败，那就直接匹配match跟s
}
/**
* 匹配线程修改成自己
*/
void tryCancel() {
UNSAFE.compareAndSwapObject(this, matchOffset, null, this);
}
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);
}
}
}再看下栈的结构：
static final class TransferStack extends Transferer {
/* 节点状态 */
/** 未匹配的消费者，take的时候 */
static final int REQUEST = 0;
/** 未匹配的生产者，put的时候 */
static final int DATA = 1;
/** 有其他线程匹配 */
static final int FULFILLING = 2;
/** true:已经有节点匹配 */
static boolean isFulfilling(int m) { return (m & FULFILLING) != 0; }
/** The head (top) of the stack */
volatile SNode head;
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.
*/
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.
*/
Object transfer(Object e, boolean timed, long nanos) {
...
...
}
/**
* Spins/blocks until node s is matched by a fulfill operation.
*
* @param s the waiting node
* @param timed true if timed wait
* @param nanos timeout value
* @return matched node, or s if cancelled
*/
SNode awaitFulfill(SNode s, boolean timed, long nanos) {
...
...
}
/**
* 如果s节点是头结点、栈里面空的或者头结点有其他线程在匹配，那就自旋等等，说不定自旋一下，等下一次就有机会了
*/
boolean shouldSpin(SNode s) {
SNode h = head;
return (h == s || h == null || isFulfilling(h.mode));
}
/**
* Unlinks s from the stack.
*/
void clean(SNode s) {
...
...
}
// 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);
}
}
}transfer等几个相关方法，放上面太长了，单独提出来学习下：
Object transfer(Object e, boolean timed, long nanos) {
/*
* Basic algorithm is to loop trying one of three actions:
*
* 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.
*
* 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:
*
* 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.
*
* transfer主要处理3种情况:
* 1.栈为空或跟头结点模式相同，那就入栈，等待匹配
* 2.可以匹配，那就入栈修改头结点标记FULFILLING|mode，然后匹配出栈
* 3.发现其他线程在匹配，那就帮忙把匹配的节点出栈unlink
*/
SNode s = null; // constructed/reused as needed
int mode = (e == null) ? REQUEST : DATA; //根据e来决定mode，e入参取决于是put还是take操作
for (;;) {
SNode h = head;
if (h == null || h.mode == mode) { // empty or same-mode
if (timed && nanos <= 0) { // 超时了，
if (h != null && h.isCancelled())
casHead(h, h.next); // pop cancelled node
else
return null;
} else if (casHead(h, s = snode(s, e, h, mode))) {//没有超时或者不需要超时，那就新建node入栈
SNode m = awaitFulfill(s, timed, nanos); //入栈后就等待其他线程来匹配，匹配后返回匹配的节点
if (m == s) { // 如果返回的匹配节点就是自己，那说明节点被取消
clean(s); //清理，返回null
return null;
}
//上面s为head，这里head变化了，说明有其他线程入栈，然后匹配唤醒了s，则推进下
//考虑先transfer(e,true,10),然后在10时间内transfer(null,false,0)情况
if ((h = head) != null && h.next == s)
casHead(h, s.next); // help s's fulfiller
return (mode == REQUEST) ? m.item : s.item;
}
} else if (!isFulfilling(h.mode)) { // 在mode不同，头结点没有其他在匹配情况下
if (h.isCancelled()) // 头结点被取消，那就重新设置head
casHead(h, h.next); // pop and retry
else if (casHead(h, s=snode(s, e, h, FULFILLING|mode))) {//否则新节点入栈
for (;;) { // loop until matched or waiters disappear
SNode m = s.next; // m为s的匹配节点
if (m == null) { // m丢失了，可能被其他线程匹配了
casHead(s, null); // 出栈，重新走主流程
s = null; // use new node next time
break; // restart main loop
}
SNode mn = m.next;
if (m.tryMatch(s)) { //s和m匹配，方法修改m的match为s,并unpark m上等待线程
casHead(s, mn); // 弹出s和m节点
return (mode == REQUEST) ? m.item : s.item;
} else // 不匹配，说明有其他线程在匹配
s.casNext(m, mn); // help unlink
}
}
} else { // 到这里说明栈顶在匹配了，那就推进下匹配流程，类似上面那个else if流程，只是没返回
SNode m = h.next; // m is h's match
if (m == null) // waiter is gone
casHead(h, null); // pop fulfilling node
else {
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
}
}
}
}
/** 可用的处理器个数 */
static final int NCPUS = Runtime.getRuntime().availableProcessors();
/** 限时最大的空旋 */
static final int maxTimedSpins = (NCPUS < 2) ? 0 : 32;
/** 不限时最大空旋 */
static final int maxUntimedSpins = maxTimedSpins * 16;
/** 空旋的阈值 */
static final long spinForTimeoutThreshold = 1000L;
/**
* 自旋等待匹配或者cancel掉
*/
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.
* 阻塞park前先设置节点的waiter线程，这样匹配的时候可以唤醒该线程
*
* 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.
* 节点调用时，如果正好在栈顶，通过自旋运气好，说不定下一次就匹配了。多处理器时可能发生这种情况。
*
* 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.
*/
long lastTime = timed ? System.nanoTime() : 0;
Thread w = Thread.currentThread();
SNode h = head;
int spins = (shouldSpin(s) ?
(timed ? maxTimedSpins : maxUntimedSpins) : 0); //自旋次数
for (;;) { //自旋
if (w.isInterrupted())
s.tryCancel(); //线程被中断了，那就cancel,match设置成自己
SNode m = s.match;
if (m != null) // 匹配节点存在就返回
return m;
if (timed) {
long now = System.nanoTime();
nanos -= now - lastTime;
lastTime = now;
if (nanos <= 0) {
s.tryCancel(); //超时就cancel
continue;
}
}
if (spins > 0)
spins = shouldSpin(s) ? (spins-1) : 0; //空旋减1
else if (s.waiter == null)
s.waiter = w; // establish waiter so can park next iter 空旋结束还没有匹配，设置waiter，下一次会park
else if (!timed) //如果不限制超时，那就park
LockSupport.park(this);
else if (nanos > spinForTimeoutThreshold) //限制了超时，看看自旋的阈值，通俗讲就是看看，自旋划算还是park划算
LockSupport.parkNanos(this, nanos);
}
}
/**
* Returns true if node s is at head or there is an active
* fulfiller.
*/
boolean shouldSpin(SNode s) {
SNode h = head;
return (h == s || h == null || isFulfilling(h.mode));
}
/**
* 从栈中unlink节点s
*/
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.
*/
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())
casHead(p, p.next); //从头结点到past节点去掉连续的cancel节点
// Unsplice embedded nodes
while (p != null && p != past) { //上面是去掉连续的cancel节点，这里去掉不连续的cancel节点
SNode n = p.next;
if (n != null && n.isCancelled())
p.casNext(n, n.next);
else
p = n;
}
}处理还是挺复杂的，主要是不像之前那些用锁来控制。看的时候想下假如有多个线程，各自不同操作，会出现什么情况，然后跟代码走一遍。
公平的队列的结构不想看了，transfer的流程跟栈差不多，模式相同入队列，不同则从头结点匹配。参考里面那个哥们2种情况都有，真强悍。
这个阻塞队列看的太累，改天还要再看看。