ActiveMQ 的线程池

ActiveMQ 的线程池实质上也是 ThreadPoolExecutor，不过它的任务模型有自己的特点，我们先看一个例子：

public static void main(String[] args) throws InterruptedException {
    // TaskRunnerFactory 的作用是创建线程池
    TaskRunnerFactory factory = new TaskRunnerFactory();
    factory.init();
    // 创建 PooledTaskRunner
    TaskRunner taskRunner = factory.createTaskRunner(new Task() {
        // iterate 的返回值很重要，true表示继续，false表示停止
        public boolean iterate() {
            System.out.println("hello zhang");
            try {
                Thread.sleep(2000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            return true;
        }
    }, "zhang");

    // 调用线程池的 execute(runnable)
    taskRunner.wakeup();

    LockSupport.park();
}

Task 接口真正处理业务逻辑。factory.createTaskRunner 的作用只是创建一个命名的 PooledTaskRunner。

PooledTaskRunner 封装了线程池 executor 和任务 runnable，只有在调用 PooledTaskRunner.wakeup() 时，才会调用 executor.execute(runnable)，即真正执行任务。

以 Queue 类为例，它继承了 Task 接口，并且有自己的 taskRunner：

// org.apache.activemq.broker.region.Queue
public void initialize() throws Exception {
    // 省略其他代码
    // 创建queue的taskRunner
    this.taskRunner = taskFactory.createTaskRunner(this, "Queue:" + destination.getPhysicalName());
}

queue 的 iterate：

//org.apache.activemq.broker.region.Queue
@Override
public boolean iterate() {
    MDC.put("activemq.destination", getName());
    boolean pageInMoreMessages = false;
    synchronized (iteratingMutex) {

        // If optimize dispatch is on or this is a slave this method could be called recursively
        // we set this state value to short-circuit wakeup in those cases to avoid that as it
        // could lead to errors.
        iterationRunning = true;

        // do early to allow dispatch of these waiting messages
        synchronized (messagesWaitingForSpace) {
            Iterator<Runnable> it = messagesWaitingForSpace.values().iterator();
            while (it.hasNext()) {
                if (!memoryUsage.isFull()) {
                    Runnable op = it.next();
                    it.remove();
                    op.run();
                } else {
                    registerCallbackForNotFullNotification();
                    break;
                }
            }
        }

        if (firstConsumer) {
            firstConsumer = false;
            try {
                if (consumersBeforeDispatchStarts > 0) {
                    int timeout = 1000; // wait one second by default if
                                        // consumer count isn't reached
                    if (timeBeforeDispatchStarts > 0) {
                        timeout = timeBeforeDispatchStarts;
                    }
                    if (consumersBeforeStartsLatch.await(timeout, TimeUnit.MILLISECONDS)) {
                        LOG.debug("{} consumers subscribed. Starting dispatch.", consumers.size());
                    } else {
                        LOG.debug("{} ms elapsed and {} consumers subscribed. Starting dispatch.", timeout, consumers.size());
                    }
                }
                if (timeBeforeDispatchStarts > 0 && consumersBeforeDispatchStarts <= 0) {
                    iteratingMutex.wait(timeBeforeDispatchStarts);
                    LOG.debug("{} ms elapsed. Starting dispatch.", timeBeforeDispatchStarts);
                }
            } catch (Exception e) {
                LOG.error(e.toString());
            }
        }

        messagesLock.readLock().lock();
        try{
            pageInMoreMessages |= !messages.isEmpty();
        } finally {
            messagesLock.readLock().unlock();
        }

        pagedInPendingDispatchLock.readLock().lock();
        try {
            pageInMoreMessages |= !pagedInPendingDispatch.isEmpty();
        } finally {
            pagedInPendingDispatchLock.readLock().unlock();
        }

        // Perhaps we should page always into the pagedInPendingDispatch
        // list if
        // !messages.isEmpty(), and then if
        // !pagedInPendingDispatch.isEmpty()
        // then we do a dispatch.
        boolean hasBrowsers = browserDispatches.size() > 0;

        if (pageInMoreMessages || hasBrowsers || !redeliveredWaitingDispatch.isEmpty()) {
            try {
　　　　　　　　　 //分发消息
                pageInMessages(hasBrowsers);
            } catch (Throwable e) {
                LOG.error("Failed to page in more queue messages ", e);
            }
        }

        if (hasBrowsers) {
            ArrayList<MessageReference> alreadyDispatchedMessages = null;
            pagedInMessagesLock.readLock().lock();
            try{
                alreadyDispatchedMessages = new ArrayList<MessageReference>(pagedInMessages.values());
            }finally {
                pagedInMessagesLock.readLock().unlock();
            }

            Iterator<BrowserDispatch> browsers = browserDispatches.iterator();
            while (browsers.hasNext()) {
                BrowserDispatch browserDispatch = browsers.next();
                try {
                    MessageEvaluationContext msgContext = new NonCachedMessageEvaluationContext();
                    msgContext.setDestination(destination);

                    QueueBrowserSubscription browser = browserDispatch.getBrowser();

                    LOG.debug("dispatch to browser: {}, already dispatched/paged count: {}", browser, alreadyDispatchedMessages.size());
                    boolean added = false;
                    for (MessageReference node : alreadyDispatchedMessages) {
                        if (!((QueueMessageReference)node).isAcked() && !browser.isDuplicate(node.getMessageId()) && !browser.atMax()) {
                            msgContext.setMessageReference(node);
                            if (browser.matches(node, msgContext)) {
                                browser.add(node);
                                added = true;
                            }
                        }
                    }
                    // are we done browsing? no new messages paged
                    if (!added || browser.atMax()) {
                        browser.decrementQueueRef();
                        browserDispatches.remove(browserDispatch);
                    }
                } catch (Exception e) {
                    LOG.warn("exception on dispatch to browser: {}", browserDispatch.getBrowser(), e);
                }
            }
        }

        if (pendingWakeups.get() > 0) {
            pendingWakeups.decrementAndGet();
        }
        MDC.remove("activemq.destination");
        iterationRunning = false;

        return pendingWakeups.get() > 0;
    }
}

队列分发消息：

protected void pageInMessages(boolean force) throws Exception {
    doDispatch(doPageInForDispatch(force, true));
}