SynchronousQueue 的联想

SynchronousQueue介绍

SynchronousQueue是一种阻塞队列，该队列没有任务的容量。内部实现采用了一种性能更好的无锁算法。

代码实现里的Dual Queue，其中每一个put对应一个take方法。

简单测试代码

public class SynchronousQueueExample {
	public static void main(String args[]) {

		final SynchronousQueue queue = new SynchronousQueue();
		new Thread(new QueueProducer(queue)).start();
		new Thread(new QueueConsumer(queue)).start();
	}
}

public class QueueProducer implements Runnable {

	private SynchronousQueue queue;

	public QueueProducer(SynchronousQueue queue) {
		this.queue = queue;
	}

	@Override
	public void run() {

		String event = "FIRST_EVENT";
		String another_event = "SECOND_EVENT";
		try {
			queue.put(event);
			System.out.printf("[%s] producer event : %s %n", Thread
					.currentThread().getName(), event);

			queue.put(another_event);
			System.out.printf("[%s] producer event : %s %n", Thread
					.currentThread().getName(), another_event);

		} catch (InterruptedException e) {
			e.printStackTrace();
		}
	}
}

public class QueueConsumer implements Runnable {
	private SynchronousQueue queue;

	public QueueConsumer(SynchronousQueue queue) {
		this.queue = queue;
	}

	@Override
	public void run() {
		try {
			String event = (String) queue.take();
			// thread will block here
			System.out.printf("[%s] consumed event : %s %n", Thread
					.currentThread().getName(), event);
		} catch (InterruptedException e) {
			e.printStackTrace();
		}

	}
}

--------------------------
[Thread-0] producer event : FIRST_EVENT
[Thread-1] consumed event : FIRST_EVENT
--------------------------

生产者每生产一个，如果没有消费者消费那就发生阻塞上面例子中。结果只打印了FIRST_EVENT ，因为SECOND_EVENT没有调用 queue.take()方法 ，所以没有打印。

绑定 put和take方法

  /**
         * Puts or takes an item.
         */
        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.
             */

            SNode s = null; // constructed/reused as needed
            int mode = (e == null)? REQUEST : DATA;

            for (;;) {
                SNode h = head;
                if (h == null || h.mode == mode) {  // empty or same-mode
                    if (timed && nanos <= 0) {      // can't wait
                        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))) {
                        SNode m = awaitFulfill(s, timed, nanos);
                        if (m == s) {               // wait was cancelled
                            clean(s);
                            return null;
                        }
                        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)) { // try to fulfill
                    if (h.isCancelled())            // already cancelled
                        casHead(h, h.next);         // pop and retry
                    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
                            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;
                            if (m.tryMatch(s)) {
                                casHead(s, mn);     // pop both s and m
                                return (mode == REQUEST)? m.item : s.item;
                            } else                  // lost match
                                s.casNext(m, mn);   // help unlink
                        }
                    }
                } else {                            // help a fulfiller
                    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
                    }
                }
            }
        }

说到SynchronousQueue不由的想到LinkedBlockingQueue，ArrayBlockingQueue，PriorityBlockingQueue

• ArrayBlockingQueue
• LinkedBlockingQueue
• PriorityBlockingQueue
• SynchronousQueue

根据不同的需要BlockingQueue有4种具体实现：

• (1)ArrayBlockingQueue：规定大小的BlockingQueue，其构造函数必须带一个int参数来指明其大小。其所含的对象是以FIFO（先入先出）顺序排序的。
• (2)LinkedBlockingQueue：大小不定的BlockingQueue，若其构造函数带一个规定大小的参数，生成的BlockingQueue有大小限制， 若不带大小参数，所生成的BlockingQueue的大小由Integer.MAX_VALUE来决定。其所含的对象是以FIFO（先入先出）顺序排序的。LinkedBlockingQueue和ArrayBlockingQueue比较起来，它们背后所用的数据结构不一样， 导致LinkedBlockingQueue的数据吞吐量要大于ArrayBlockingQueue，但在线程数量很大时其性能的可预见性低于ArrayBlockingQueue。
• (3)PriorityBlockingQueue：类似于LinkedBlockingQueue，但其所含对象的排序不是FIFO，而是依据对象的自然排序顺序或者是构造函数所带的Comparator决定的顺序。
• (4)SynchronousQueue：特殊的BlockingQueue，对其的操作必须是放和取交替完成的。

ThreadPoolExecutor

 /**
     * Creates a new <tt>ThreadPoolExecutor</tt> with the given initial
     * parameters and default thread factory.
     *
     * @param corePoolSize the number of threads to keep in the
     * pool, even if they are idle.
     * @param maximumPoolSize the maximum number of threads to allow in the
     * pool.
     * @param keepAliveTime when the number of threads is greater than
     * the core, this is the maximum time that excess idle threads
     * will wait for new tasks before terminating.
     * @param unit the time unit for the keepAliveTime
     * argument.
     * @param workQueue the queue to use for holding tasks before they
     * are executed. This queue will hold only the <tt>Runnable</tt>
     * tasks submitted by the <tt>execute</tt> method.
     * @param handler the handler to use when execution is blocked
     * because the thread bounds and queue capacities are reached.
     * @throws IllegalArgumentException if corePoolSize or
     * keepAliveTime less than zero, or if maximumPoolSize less than or
     * equal to zero, or if corePoolSize greater than maximumPoolSize.
     * @throws NullPointerException if <tt>workQueue</tt>
     * or <tt>handler</tt> are null.
     */
    public ThreadPoolExecutor(int corePoolSize,
                              int maximumPoolSize,
                              long keepAliveTime,
                              TimeUnit unit,
                              BlockingQueue<Runnable> workQueue,
                              RejectedExecutionHandler handler) {
        this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
             Executors.defaultThreadFactory(), handler);
    }

上面的每一个参数很详细的介绍了ThreadPoolExecutor的用法，保持线程的数量，最大化线程的数量，调度时间的间隔，用到的线程队列等。

主要的execute方法。

  /**
     * Executes the given task sometime in the future.  The task
     * may execute in a new thread or in an existing pooled thread.
     *
     * If the task cannot be submitted for execution, either because this
     * executor has been shutdown or because its capacity has been reached,
     * the task is handled by the current <tt>RejectedExecutionHandler</tt>.
     *
     * @param command the task to execute
     * @throws RejectedExecutionException at discretion of
     * <tt>RejectedExecutionHandler</tt>, if task cannot be accepted
     * for execution
     * @throws NullPointerException if command is null
     */
    public void execute(Runnable command) {
        if (command == null)
            throw new NullPointerException();
        if (poolSize >= corePoolSize || !addIfUnderCorePoolSize(command)) {
            if (runState == RUNNING && workQueue.offer(command)) {
                if (runState != RUNNING || poolSize == 0)
                    ensureQueuedTaskHandled(command);
            }
            else if (!addIfUnderMaximumPoolSize(command))
                reject(command); // is shutdown or saturated
        }
    }

在线程池中每一个任务被包装成Runnable 类型，传入到execute方法中 ， 该方法中会判断是否超过最大线程，是否有空余线程，当调用停止或者达到最大容量会调用RejectedExecutionHandler。

    /**
     * Rechecks state after queuing a task. Called from execute when
     * pool state has been observed to change after queuing a task. If
     * the task was queued concurrently with a call to shutdownNow,
     * and is still present in the queue, this task must be removed
     * and rejected to preserve shutdownNow guarantees.  Otherwise,
     * this method ensures (unless addThread fails) that there is at
     * least one live thread to handle this task
     * @param command the task
     */
    private void ensureQueuedTaskHandled(Runnable command) {
        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        boolean reject = false;
        Thread t = null;
        try {
            int state = runState;
            if (state != RUNNING && workQueue.remove(command))
                reject = true;
            else if (state < STOP &&
                     poolSize < Math.max(corePoolSize, 1) &&
                     !workQueue.isEmpty())
                t = addThread(null);
        } finally {
            mainLock.unlock();
        }
        if (reject)
            reject(command);
        else if (t != null)
            t.start();
    }

    /**
     * Invokes the rejected execution handler for the given command.
     */
    void reject(Runnable command) {
        handler.rejectedExecution(command, this);
    }

网上的一个测试

public class Test {
    static ExecutorService e = Executors.newFixedThreadPool(2);
    static int N = 1000000;

    public static void main(String[] args) throws Exception {
        for (int i = 0; i < 10; i++) {
            int length = (i == 0) ? 1 : i * 5;
            System.out.print(length + "\t");
            System.out.print(doTest(new LinkedBlockingQueue<Integer>(length), N) + "\t");
            System.out.print(doTest(new ArrayBlockingQueue<Integer>(length), N) + "\t");
            System.out.print(doTest(new SynchronousQueue<Integer>(), N));
            System.out.println();
        }

        e.shutdown();
    }

    private static long doTest(final BlockingQueue<Integer> q, final int n) throws Exception {
        long t = System.nanoTime();

        e.submit(new Runnable() {
            public void run() {
                for (int i = 0; i < n; i++)
                    try { q.put(i); } catch (InterruptedException ex) {}
            }
        });    

        Long r = e.submit(new Callable<Long>() {
            public Long call() {
                long sum = 0;
                for (int i = 0; i < n; i++)
                    try { sum += q.take(); } catch (InterruptedException ex) {}
                return sum;
            }
        }).get();
        t = System.nanoTime() - t;

        return (long)(1000000000.0 * N / t); // Throughput, items/sec
    }
}

参考

具体使用那个一个消息队列要看使用场景，多个生产者一个消费者，多个生产者多个消费者以及并发量的大小。