Nacos server事件分發EventDispatcher代碼小結

EventDispatcher類，作爲事件分發器。

AbstractEventListener靜態內部抽象類，繼承該類後實例化的子類，將在該類中的構造方法被註冊到EventDispatcher中，這一點以前我沒有想到，挺好的寫法，設計模式是監聽器模式，通過事件調用具體事件的監聽類。對於Spring監聽器的實現也是類似，只不過spring監聽器將事件作爲參數與監聽器綁定，通過監聽器輪詢反射獲取參數類型進行比對，而nacos事件分發監聽是直接通過監聽器與事件類進行成員變量的綁定，註冊監聽事件的過程有點繞，過程總結如下：

實例化子類監聽器-》註冊子類監聽器，註冊過程同時要調用子類監聽器的interest方法，返回事件類型，將事件類型與監聽器進行多對一的註冊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類的成員變量：
        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;
            }
        }
    }

使用過程通過調用：

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);
            }
        }
    }

完整事件分配的代碼：

 
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>();
        }

        @Override
        public boolean equals(Object obj) {
            if (null == obj || obj.getClass() != getClass()) {
                return false;
            }
            if (this == obj) {
                return true;
            }
            return eventType == ((Entry)obj).eventType;
        }

        @Override
        public 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);
        }

        /**
         * 感興趣的事件列表
         *
         * @return event list
         */
        abstract public List<Class<? extends Event>> interest();

        /**
         * 處理事件
         *
         * @param event event
         */
        abstract public void onEvent(Event event);
    }

}