死磕Nacos系列：Nacos事件发布订阅模型-编程知识

前言

在Nacos源码中，你是否也经常看到NotifyCenter.publishEvent这样的代码块？

这个事件发布出去后，有哪些类接收到通知并进行了逻辑处理呢？

这里面的实现逻辑是什么呢？

如果你不太清楚，那我们一起来梳理下。Let’s go!

NotifyCenter

如果我们不靠其它资料，从代码层面去解读的话，还是得先从这个类入手，因为都是调用其静态方法完成通知的。

这个类的静态代码块里面有这样的逻辑：

static {// 从系统环境中取"nacos.core.notify.ring-buffer-size"，如果没有设置，则取默认值16384String ringBufferSizeProperty = "nacos.core.notify.ring-buffer-size";ringBufferSize = Integer.getInteger(ringBufferSizeProperty, 16384);// 从系统环境中取"nacos.core.notify.share-buffer-size"，如果没有设置，则取默认值1024String shareBufferSizeProperty = "nacos.core.notify.share-buffer-size";shareBufferSize = Integer.getInteger(shareBufferSizeProperty, 1024);// 通过SPI的方式加载EventPublisher的实现类final Collection<EventPublisher> publishers = NacosServiceLoader.load(EventPublisher.class);Iterator<EventPublisher> iterator = publishers.iterator();if (iterator.hasNext()) {clazz = iterator.next().getClass();} else {clazz = DefaultPublisher.class;}// 创建一个默认的EventPublisherFactory，用来产生EventPublisherDEFAULT_PUBLISHER_FACTORY = (cls, buffer) -> {try {EventPublisher publisher = clazz.newInstance();publisher.init(cls, buffer);return publisher;} catch (Throwable ex) {LOGGER.error("Service class newInstance has error : ", ex);throw new NacosRuntimeException(SERVER_ERROR, ex);}};try {// 创建一个默认共享的EventPublisherINSTANCE.sharePublisher = new DefaultSharePublisher();INSTANCE.sharePublisher.init(SlowEvent.class, shareBufferSize);} catch (Throwable ex) {LOGGER.error("Service class newInstance has error : ", ex);}// 添加一个ShutdownHookThreadUtils.addShutdownHook(NotifyCenter::shutdown);
}

从这里我们可以知道，有EventPublisherFactory和EventPublisher的概念，在EventPublisher中，还专门创建了一个DefaultSharePublisher，还知道了一个事件叫做SlowEvent。

那我们就看一下这几个模型：

模型

EventPublisherFactory

public interface EventPublisherFactory extends BiFunction<Class<? extends Event>, Integer, EventPublisher> {/*** Build an new {@link EventPublisher}.** @param eventType    eventType for {@link EventPublisher}* @param maxQueueSize max queue size for {@link EventPublisher}* @return new {@link EventPublisher}*/@OverrideEventPublisher apply(Class<? extends Event> eventType, Integer maxQueueSize);
}

可以看到，接口中就一个方法，就是通过事件类型和最大队列大小来得到一个EventPublisher。

其中有NamingEventPublisherFactory和TraceEventPublisherFactory两个实现类。

EventPublisher

public interface EventPublisher extends Closeable {// 初始化EventPublishervoid init(Class<? extends Event> type, int bufferSize);// 获取当前事件的数量long currentEventSize();// 添加一个订阅者void addSubscriber(Subscriber subscriber);// 删除一个订阅者void removeSubscriber(Subscriber subscriber);// 发布事件boolean publish(Event event);// 通知订阅者void notifySubscriber(Subscriber subscriber, Event event);}

其类图如下：

这里SharedEventPublisher看起来是做了更多的扩展，来看看呢：

public interface ShardedEventPublisher extends EventPublisher {// 给指定的一个事件添加订阅者void addSubscriber(Subscriber subscriber, Class<? extends Event> subscribeType);// 给指定的一个事件删除订阅者void removeSubscriber(Subscriber subscriber, Class<? extends Event> subscribeType);
}

这里新加入了个Subscriber，那我们来看看：

Subscriber

public abstract class Subscriber<T extends Event> {// 接收事件public abstract void onEvent(T event);// 获取当前订阅的事件类型public abstract Class<? extends Event> subscribeType();// 获取异步处理器public Executor executor() {return null;}// 忽略失效的事件public boolean ignoreExpireEvent() {return false;}// 获取事件是否匹配作用域public boolean scopeMatches(T event) {return true;}
}

有个特殊的SmartSubscriber，它是Subscriber的子类，是一个抽象类：

public abstract class SmartSubscriber extends Subscriber {// 返回订阅的事件列表public abstract List<Class<? extends Event>> subscribeTypes();@Overridepublic final Class<? extends Event> subscribeType() {return null;}// 默认不忽略过期的事件@Overridepublic final boolean ignoreExpireEvent() {return false;}
}

Event

public abstract class Event implements Serializable {private static final long serialVersionUID = -3731383194964997493L;private static final AtomicLong SEQUENCE = new AtomicLong(0);private final long sequence = SEQUENCE.getAndIncrement();// 获取事件序号public long sequence() {return sequence;}// 获取事件作用域public String scope() {return null;}// 判断是否是一个插件的事件public boolean isPluginEvent() {return false;}
}

在SlowEvent中，仅仅是将sequence统一默认为0。

public abstract class SlowEvent extends Event {@Overridepublic long sequence() {return 0;}
}

综上，我们通过接口关系就可以大概得出一个结论：

EventPublisher可通过EventPublisherFactory创建得来，EventPublisher可以给Event添加Subscriber，并发布事件让对应的Subscriber执行。这不妥妥的观察者模式嘛！

我们来画张图：

了解了NotifyCenter大概的模型之后，我们来看看内部的实现。

实现方法

registerToPublisher

给时间注册一个EventPublisher。

public static EventPublisher registerToPublisher(final Class<? extends Event> eventType,final EventPublisherFactory factory, final int queueMaxSize) {if (ClassUtils.isAssignableFrom(SlowEvent.class, eventType)) {return INSTANCE.sharePublisher;}final String topic = ClassUtils.getCanonicalName(eventType);synchronized (NotifyCenter.class) {// MapUtils.computeIfAbsent is a unsafe method.MapUtil.computeIfAbsent(INSTANCE.publisherMap, topic, factory, eventType, queueMaxSize);}return INSTANCE.publisherMap.get(topic);
}

诶，如果事件是SlowEvent，就会统一返回一个sharePublisher，这个就是静态代码块中初始化的DefaultSharePublisher。

好，（敲黑板），所有SlowEvent的事件共用一个DefaultSharePublisher。

接着给NotifyCenter上锁，如果publisherMap中没有当前事件的EventPublisher，那就用EventPublisherFactory创建一个EventPublisher，并存入publisherMap中，再返回。

registerSubscriber

给事件注册一个订阅者。

public static void registerSubscriber(final Subscriber consumer, final EventPublisherFactory factory) {// 如果订阅者是一个SmartSubscriber，可以订阅多个事件if (consumer instanceof SmartSubscriber) {for (Class<? extends Event> subscribeType : ((SmartSubscriber) consumer).subscribeTypes()) {// 如果事件是SlowEvent，则添加到sharePublisher中if (ClassUtils.isAssignableFrom(SlowEvent.class, subscribeType)) {INSTANCE.sharePublisher.addSubscriber(consumer, subscribeType);} else {// 其他事件则添加到对应的EventPublisher中addSubscriber(consumer, subscribeType, factory);}}return;}// 其他类型的Subscriberfinal Class<? extends Event> subscribeType = consumer.subscribeType();if (ClassUtils.isAssignableFrom(SlowEvent.class, subscribeType)) {// 如果事件是SlowEvent，则添加到sharePublisher中INSTANCE.sharePublisher.addSubscriber(consumer, subscribeType);return;}// 其他事件则添加到对应的EventPublisher中addSubscriber(consumer, subscribeType, factory);
}

publishEvent

发送事件。

private static boolean publishEvent(final Class<? extends Event> eventType, final Event event) {\// 如果事件是SlowEvent，则用sharePublisher发送if (ClassUtils.isAssignableFrom(SlowEvent.class, eventType)) {return INSTANCE.sharePublisher.publish(event);}final String topic = ClassUtils.getCanonicalName(eventType);// 根据事件找到对应的EventPublisher进行发送                                                  EventPublisher publisher = INSTANCE.publisherMap.get(topic);if (publisher != null) {return publisher.publish(event);}if (event.isPluginEvent()) {return true;}LOGGER.warn("There are no [{}] publishers for this event, please register", topic);return false;
}