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Java並發包中的同步隊列SynchronousQueue實現原理
介紹
Java 6的並發編程包中的SynchronousQueue是一個沒有數據緩衝的BlockingQueue，生產者線程對其的插入操作put必須等待消費者的移除操作take，反過來也一樣。
不像ArrayBlockingQueue或LinkedListBlockingQueue，SynchronousQueue內部並沒有數據緩 存空間，你不能調用peek()方法來看隊列中是否有數據元素，因為數據元素隻有當你試著取走的時候才可能存在，不取走而隻想偷窺一下是不行的，當然遍曆 這個隊列的操作也是不允許的。隊列頭元素是第一個排隊要插入數據的線程，而不是要交換的數據。數據是在配對的生產者和消費者線程之間直接傳遞的，並不會將數據緩衝數據到隊列中。可以這樣來理解：生產者和消費者互相等待對方，握手，然後一起離開。
SynchronousQueue的一個使用場景是在線程池裏。Executors.newCachedThreadPool()就使用了 SynchronousQueue，這個線程池根據需要（新任務到來時）創建新的線程，如果有空閑線程則會重複使用，線程空閑了60秒後會被回收。
實現原理
阻塞隊列的實現方法有許多：
阻塞算法實現
阻塞算法實現通常在內部采用一個鎖來保證多個線程中的put()和take()方法是串行執行的。采用鎖的開銷是比較大的，還會存在一種情況是線程 A持有線程B需要的鎖，B必須一直等待A釋放鎖，即使A可能一段時間內因為B的優先級比較高而得不到時間片運行。所以在高性能的應用中我們常常希望規避鎖 的使用。
public class NativeSynchronousQueue<E> {
boolean putting = false;
E item = null;
public synchronized E take() throws InterruptedException {
while (item == null)
wait();
E e = item;
item = null;
notifyAll();
return e;
}
public synchronized void put(E e) throws InterruptedException {
if (e==null) return;
while (putting)
wait();
putting = true;
item = e;
notifyAll();
while (item!=null)
wait();
putting = false;
notifyAll();
}
}
信號量實現
經典同步隊列實現采用了三個信號量，代碼很簡單，比較容易理解：
public class SemaphoreSynchronousQueue<E> {
E item = null;
Semaphore sync = new Semaphore(0);
Semaphore send = new Semaphore(1);
Semaphore recv = new Semaphore(0);
public E take() throws InterruptedException {
recv.acquire();
E x = item;
sync.release();
send.release();
return x;
}
public void put (E x) throws InterruptedException{
send.acquire();
item = x;
recv.release();
sync.acquire();
}
}
在多核機器上，上麵方法的同步代價仍然較高，操作係統調度器需要上千個時間片來阻塞或喚醒線程，而上麵的實現即使在生產者put()時已經有一個消費者在等待的情況下，阻塞和喚醒的調用仍然需要。
Java 5實現
public class Java5SynchronousQueue<E> {
ReentrantLock qlock = new ReentrantLock();
Queue waitingProducers = new Queue();
Queue waitingConsumers = new Queue();
static class Node extends AbstractQueuedSynchronizer {
E item;
Node next;
Node(Object x) { item = x; }
void waitForTake() { /* (uses AQS) */ }
E waitForPut() { /* (uses AQS) */ }
}
public E take() {
Node node;
boolean mustWait;
qlock.lock();
node = waitingProducers.pop();
if(mustWait = (node == null))
node = waitingConsumers.push(null);
qlock.unlock();
if (mustWait)
return node.waitForPut();
else
return node.item;
}
public void put(E e) {
Node node;
boolean mustWait;
qlock.lock();
node = waitingConsumers.pop();
if (mustWait = (node == null))
node = waitingProducers.push(e);
qlock.unlock();
if (mustWait)
node.waitForTake();
else
node.item = e;
}
}
Java 5的實現相對來說做了一些優化，隻使用了一個鎖，使用隊列代替信號量也可以允許發布者直接發布數據，而不是要首先從阻塞在信號量處被喚醒。
Java6實現
Java 6的SynchronousQueue的實現采用了一種性能更好的無鎖算法 — 擴展的“Dual stack and Dual queue” 算法。性能比Java5的實現有較大提升。競爭機製支持公平和非公平兩種：非公平競爭模式使用的數據結構是後進先出棧(Lifo Stack)；公平競爭模式則使用先進先出隊列（Fifo Queue），性能上兩者是相當的，一般情況下，Fifo通常可以支持更大的吞吐量，但Lifo可以更大程度的保持線程的本地化。
代碼實現裏的Dual Queue或Stack內部是用鏈表(LinkedList)來實現的，其節點狀態為以下三種情況：
- 持有數據 – put()方法的元素
- 持有請求 – take()方法
- 空
這個算法的特點就是任何操作都可以根據節點的狀態判斷執行，而不需要用到鎖。
其核心接口是Transfer，生產者的put或消費者的take都使用這個接口，根據第一個參數來區別是入列（棧）還是出列（棧）。
/**
* Shared internal API for dual stacks and queues.
*/
static abstract class Transferer {
/**
* Performs a put or take.
*
* @param e if non-null, the item to be handed to a consumer;
* if null, requests that transfer return an item
* offered by producer.
* @param timed if this operation should timeout
* @param nanos the timeout, in nanoseconds
* @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.
*/
abstract Object transfer(Object e, boolean timed, long nanos);
}
TransferQueue實現如下(摘自Java 6源代碼)，入列和出列都基於Spin和CAS方法：
/**
* Puts or takes an item.
*/
Object transfer(Object e, boolean timed, long nanos) {
/* Basic algorithm is to loop trying to take either of
* two actions:
*
* 1. If queue apparently empty or holding same-mode nodes,
* try to add node to queue of waiters, wait to be
* fulfilled (or cancelled) and return matching item.
*
* 2. If queue apparently contains waiting items, and this
* call is of complementary mode, try to fulfill by CAS'ing
* item field of waiting node and dequeuing it, and then
* returning matching item.
*
* In each case, along the way, check for and try to help
* advance head and tail on behalf of other stalled/slow
* threads.
*
* The loop starts off with a null check guarding against
* seeing uninitialized head or tail values. This never
* happens in current SynchronousQueue, but could if
* callers held non-volatile/final ref to the
* transferer. The check is here anyway because it places
* null checks at top of loop, which is usually faster
* than having them implicitly interspersed.
*/
QNode s = null; // constructed/reused as needed
boolean isData = (e != null);
for (;;) {
QNode t = tail;
QNode h = head;
if (t == null || h == null) // saw uninitialized value
continue; // spin
if (h == t || t.isData == isData) { // empty or same-mode
QNode tn = t.next;
if (t != tail) // inconsistent read
continue;
if (tn != null) { // lagging tail
advanceTail(t, tn);
continue;
}
if (timed && nanos <= 0) // can't wait
return null;
if (s == null)
s = new QNode(e, isData);
if (!t.casNext(null, s)) // failed to link in
continue;
advanceTail(t, s); // swing tail and wait
Object x = awaitFulfill(s, e, timed, nanos);
if (x == s) { // wait was cancelled
clean(t, s);
return null;
}
if (!s.isOffList()) { // not already unlinked
advanceHead(t, s); // unlink if head
if (x != null) // and forget fields
s.item = s;
s.waiter = null;
}
return (x != null)? x : e;
} else { // complementary-mode
QNode m = h.next; // node to fulfill
if (t != tail || m == null || h != head)
continue; // inconsistent read
Object x = m.item;
if (isData == (x != null) || // m already fulfilled
x == m || // m cancelled
!m.casItem(x, e)) { // lost CAS
advanceHead(h, m); // dequeue and retry
continue;
}
advanceHead(h, m); // successfully fulfilled
LockSupport.unpark(m.waiter);
return (x != null)? x : e;
}
}
}
文章轉自 並發編程網-ifeve.com
最後更新：2017-05-22 20:04:52