nacos 源码阅读发布订阅模式

女爷i 2024-04-02 03:49 62阅读 0赞

nacos中很多地方都用到了发布订阅模式，这点我们可以从**Subscriber**继承树上来看到

![d634dd3ea1c84efb8439c47f50ef1bbf.png][]

这里发布订阅模式的使用也很有借鉴意义，我们首先看下核心代码

package com.alibaba.nacos.common.notify;
    
    import com.alibaba.nacos.common.notify.listener.Subscriber;
    import com.alibaba.nacos.common.utils.CollectionUtils;
    import com.alibaba.nacos.common.utils.ConcurrentHashSet;
    import com.alibaba.nacos.common.utils.ThreadUtils;
    import org.slf4j.Logger;
    import org.slf4j.LoggerFactory;
    
    import java.util.concurrent.ArrayBlockingQueue;
    import java.util.concurrent.BlockingQueue;
    import java.util.concurrent.Executor;
    import java.util.concurrent.atomic.AtomicReferenceFieldUpdater;
    
    import static com.alibaba.nacos.common.notify.NotifyCenter.ringBufferSize;
    
    /**
     * The default event publisher implementation.
     *
     * <p>Internally, use {@link ArrayBlockingQueue <Event/>} as a message staging queue.
     *
     * @author <a href="mailto:liaochuntao@live.com">liaochuntao</a>
     * @author zongtanghu
     */
    public class DefaultPublisher extends Thread implements EventPublisher {
        private volatile boolean initialized = false;
        private volatile boolean shutdown = false;
        private Class<? extends Event> eventType;
        protected final ConcurrentHashSet<Subscriber> subscribers = new ConcurrentHashSet<>();
        private int queueMaxSize = -1;
        private BlockingQueue<Event> queue;
        protected volatile Long lastEventSequence = -1L; 
        private static final AtomicReferenceFieldUpdater<DefaultPublisher, Long> UPDATER = AtomicReferenceFieldUpdater
                .newUpdater(DefaultPublisher.class, Long.class, "lastEventSequence");
        
        @Override
        public void init(Class<? extends Event> type, int bufferSize) {
            setDaemon(true);
            setName("nacos.publisher-" + type.getName());
            this.eventType = type;
            this.queueMaxSize = bufferSize;
            this.queue = new ArrayBlockingQueue<>(bufferSize);
            start();
        }
        
        
        @Override
        public synchronized void start() {
            if (!initialized) {
                // start just called once
                super.start();
                if (queueMaxSize == -1) {
                    queueMaxSize = ringBufferSize;
                }
                initialized = true;
            }
        }
        
        
        @Override
        public void run() {
            openEventHandler();
        }
        
        void openEventHandler() {
            try {
                
                // This variable is defined to resolve the problem which message overstock in the queue.
                int waitTimes = 60;
                // To ensure that messages are not lost, enable EventHandler when
                // waiting for the first Subscriber to register
                for (; ; ) {
                    if (shutdown || hasSubscriber() || waitTimes <= 0) {
                        break;
                    }
                    ThreadUtils.sleep(1000L);
                    waitTimes--;
                }
                
                for (; ; ) {
                    if (shutdown) {
                        break;
                    }
                    final Event event = queue.take();
                    receiveEvent(event);
                    UPDATER.compareAndSet(this, lastEventSequence, Math.max(lastEventSequence, event.sequence()));
                }
            } catch (Throwable ex) {
                LOGGER.error("Event listener exception : ", ex);
            }
        }
        
        
        /**
         * Receive and notifySubscriber to process the event.
         *
         * @param event {@link Event}.
         */
        void receiveEvent(Event event) {
            final long currentEventSequence = event.sequence();
            
            if (!hasSubscriber()) {
                LOGGER.warn("[NotifyCenter] the {} is lost, because there is no subscriber.", event);
                return;
            }
            
            // Notification single event listener
            for (Subscriber subscriber : subscribers) {
                if (!subscriber.scopeMatches(event)) {
                    continue;
                }
                
                // Whether to ignore expiration events
                if (subscriber.ignoreExpireEvent() && lastEventSequence > currentEventSequence) {
                    LOGGER.debug("[NotifyCenter] the {} is unacceptable to this subscriber, because had expire",
                            event.getClass());
                    continue;
                }
                
                // Because unifying smartSubscriber and subscriber, so here need to think of compatibility.
                // Remove original judge part of codes.
                notifySubscriber(subscriber, event);
            }
        }
        
        @Override
        public void notifySubscriber(final Subscriber subscriber, final Event event) {
            
            LOGGER.debug("[NotifyCenter] the {} will received by {}", event, subscriber);
            
            final Runnable job = () -> subscriber.onEvent(event);
            final Executor executor = subscriber.executor();
            
            if (executor != null) {
                executor.execute(job);
            } else {
                try {
                    job.run();
                } catch (Throwable e) {
                    LOGGER.error("Event callback exception: ", e);
                }
            }
        }
    }

这里省略了一些常规代码和注释。

我们可以看到，**DefaultPublisher**这继承了**Thread**类，并覆写了**start()**和**run()**方法。在**run()**方法的具体内容中，通过**for(;;)\{...\}**来持续遍历**事件队列queue**。

对于线程的生命管理，有通过一个**shutdown**的**线程阀门**，当其为true时即线程结束。

这里比较优秀的一点是在**存储Event**时使用了**BlockingQueue**，可以在线程的执行过程中**避免无意义的notify()。**

同时我们可以看到，在**notifySubscriber()**方法中，判断了消息订阅者是否使用了线程池。这里的实现方式对**发布者**处理效率至关重要。

欢迎小伙伴们在评论区分享自己的见解。

[d634dd3ea1c84efb8439c47f50ef1bbf.png]: https://image.dandelioncloud.cn/pgy_files/images/2024/04/02/942f6df68d214509b8c68896fba99aff.png