Java并发包中的同步队列SynchronousQueue实现原理

转自:http://hugozhu.myalert.info/2013/03/05/java-SynchronousQueue-notes.html

介绍

Java 6的并发编程包中的SynchronousQueue是一个没有数据缓冲的BlockingQueue，生产者线程对其的插入操作put必须等待消费者的移除操作take，反过来也一样。

不像ArrayBlockingQueue或LinkedListBlockingQueue，SynchronousQueue内部并没有数据缓存空间，你不能调用peek()方法来看队列中是否有数据元素，因为数据元素只有当你试着取走的时候才可能存在，不取走而只想偷窥一下是不行的，当然遍历这个队列的操作也是不允许的。队列头元素是第一个排队要插入数据的线程，而不是要交换的数据。数据是在配对的生产者和消费者线程之间直接传递的，并不会将数据缓冲数据到队列中。可以这样来理解：生产者和消费者互相等待对方，握手，然后一起离开。

SynchronousQueue的一个使用场景是在线程池里。Executors.newCachedThreadPool()就使用了SynchronousQueue，这个线程池根据需要（新任务到来时）创建新的线程，如果有空闲线程则会重复使用，线程空闲了60秒后会被回收。

实现原理

同步队列的实现方法有许多：

阻塞算法实现

阻塞算法实现通常在内部采用一个锁来保证多个线程中的put()和take()方法是串行执行的。采用锁的开销是比较大的，还会存在一种情况是线程A持有线程B需要的锁，B必须一直等待A释放锁，即使A可能一段时间内因为B的优先级比较高而得不到时间片运行。所以在高性能的应用中我们常常希望规避锁的使用。

 
 
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();
  
   }
  
  }

信号量实现

经典同步队列实现采用了三个信号量，代码很简单，比较容易理解：

 
 
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();
  
   }
  
  }

在多核机器上，上面方法的同步代价仍然较高，操作系统调度器需要上千个时间片来阻塞或唤醒线程，而上面的实现即使在生产者put()时已经有一个消费者在等待的情况下，阻塞和唤醒的调用仍然需要。

Java 5实现

 
 
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的实现相对来说做了一些优化，只使用了一个锁，使用队列代替信号量也可以允许发布者直接发布数据，而不是要首先从阻塞在信号量处被唤醒。

Java6实现

Java 6的SynchronousQueue的实现采用了一种性能更好的无锁算法 – 扩展的“Dual stack and Dual queue”算法。性能比Java5的实现有较大提升。竞争机制支持公平和非公平两种：非公平竞争模式使用的数据结构是后进先出栈(Lifo Stack)；公平竞争模式则使用先进先出队列（Fifo Queue），性能上两者是相当的，一般情况下，Fifo通常可以支持更大的吞吐量，但Lifo可以更大程度的保持线程的本地化。

代码实现里的Dual Queue或Stack内部是用链表(LinkedList)来实现的，其节点状态为以下三种情况：

持有数据 - put()方法的元素
持有请求 - take()方法
空

这个算法的特点就是任何操作都可以根据节点的状态判断执行，而不需要用到锁。

其核心接口是Transfer，生产者的put或消费者的take都使用这个接口，根据第一个参数来区别是入列（栈）还是出列（栈）。

 
 
 /**
  
   * 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实现如下(摘自Java 6源代码)，入列和出列都基于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;
  
   }
  
   }
  
   }

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