EventDispatcher类，作为事件分发器。

AbstractEventListener静态内部抽象类，继承该类后实例化的子类，将在该类中的构造方法被注册到EventDispatcher中，这一点以前我没有想到，挺好的写法，设计模式是监听器模式，通过事件调用具体事件的监听类。对于Spring监听器的实现也是类似，只不过spring监听器将事件作为参数与监听器绑定，通过监听器轮询反射获取参数类型进行比对，而nacos事件分发监听是直接通过监听器与事件类进行成员变量的绑定，注册监听事件的过程有点绕，过程总结如下：

实例化子类监听器-》注册子类监听器，注册过程同时要调用子类监听器的interest方法，返回事件类型，将事件类型与监听器进行多对一的注册Entry入口中

 /*** add event listener*/static public void addEventListener(AbstractEventListener listener) {for (Class<? extends Event> type : listener.interest()) {getEntry(type).listeners.addIfAbsent(listener);}}

Entry类的成员变量：
final Class<? extends Event> eventType;
final CopyOnWriteArrayList<AbstractEventListener> listeners;

static Entry getEntry(Class<? extends Event> eventType) {for (; ; ) {for (Entry entry : LISTENER_HUB) {if (entry.eventType == eventType) {return entry;}}Entry tmp = new Entry(eventType);/***  false means already exists*/if (LISTENER_HUB.addIfAbsent(tmp)) {return tmp;}}}

使用过程通过调用：

EventDispatcher.fireEvent(new ConfigDataChangeEvent(false, dataId, group, tenant, time.getTime()));

/*** fire event, notify listeners.*/static public void fireEvent(Event event) {if (null == event) {throw new IllegalArgumentException();}for (AbstractEventListener listener : getEntry(event.getClass()).listeners) {try {listener.onEvent(event);} catch (Exception e) {log.error(e.toString(), e);}}}

完整事件分配的代码：

 
public class EventDispatcher {/*** add event listener*/static public void addEventListener(AbstractEventListener listener) {for (Class<? extends Event> type : listener.interest()) {getEntry(type).listeners.addIfAbsent(listener);}}/*** fire event, notify listeners.*/static public void fireEvent(Event event) {if (null == event) {throw new IllegalArgumentException();}for (AbstractEventListener listener : getEntry(event.getClass()).listeners) {try {listener.onEvent(event);} catch (Exception e) {log.error(e.toString(), e);}}}/*** For only test purpose*/static public void clear() {LISTENER_HUB.clear();}/*** get event listener for eventType. Add Entry if not exist.*/static Entry getEntry(Class<? extends Event> eventType) {for (; ; ) {for (Entry entry : LISTENER_HUB) {if (entry.eventType == eventType) {return entry;}}Entry tmp = new Entry(eventType);/***  false means already exists*/if (LISTENER_HUB.addIfAbsent(tmp)) {return tmp;}}}static private class Entry {final Class<? extends Event> eventType;final CopyOnWriteArrayList<AbstractEventListener> listeners;Entry(Class<? extends Event> type) {eventType = type;listeners = new CopyOnWriteArrayList<AbstractEventListener>();}@Overridepublic boolean equals(Object obj) {if (null == obj || obj.getClass() != getClass()) {return false;}if (this == obj) {return true;}return eventType == ((Entry)obj).eventType;}@Overridepublic int hashCode() {return super.hashCode();}}static private final Logger log = LoggerFactory.getLogger(EventDispatcher.class);static final CopyOnWriteArrayList<Entry> LISTENER_HUB = new CopyOnWriteArrayList<Entry>();public interface Event {}static public abstract class AbstractEventListener {public AbstractEventListener() {/*** automatic register*/EventDispatcher.addEventListener(this);}/*** 感兴趣的事件列表** @return event list*/abstract public List<Class<? extends Event>> interest();/*** 处理事件** @param event event*/abstract public void onEvent(Event event);}}