nacos 源码阅读 发布订阅模式

 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

 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.
 *
 * 
Internally, use {@link ArrayBlockingQueue } as a message staging queue.
 *
 * @author liaochuntao
 * @author zongtanghu
 */
public class DefaultPublisher extends Thread implements EventPublisher {
    private volatile boolean initialized = false;
    private volatile boolean shutdown = false;
    private Classextends Event> eventType;
    protected final ConcurrentHashSet subscribers = new ConcurrentHashSet<>();
    private int queueMaxSize = -1;
    private BlockingQueue queue;
    protected volatile Long lastEventSequence = -1L; 
    private static final AtomicReferenceFieldUpdater UPDATER = AtomicReferenceFieldUpdater
            .newUpdater(DefaultPublisher.class, Long.class, "lastEventSequence");
    
    @Override
    public void init(Class 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。

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