事件驱动模型的简单Java实现

事件驱动模型的原理不再赘述，Swing是不错的实现。别人也有不错的博文来说明原理。

本文的目的是提供一种简单的，可供参考的简短代码，用来帮助理解该模型。

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Event

事件通用接口：

package org.joshua.event.events;

public interface Event {
}

Click事件:

package org.joshua.event.events;

public class ClickEvent implements Event {

}

Double click事件：

package org.joshua.event.events;

public class DblClickEvent implements Event {

}

Listener

事件监听器通用接口：

package org.joshua.event.listener;

import org.joshua.event.events.Event;

public interface EventListener<T extends Event> {

    public void handleEvent(T event);

}

Click事件监听器：

package org.joshua.event.listener;

import org.joshua.event.events.ClickEvent;


public interface ClickEventHandler extends EventListener<ClickEvent> {

}

Double Click事件监听器：

package org.joshua.event.listener;

import org.joshua.event.events.DblClickEvent;

public interface DblClickEventHandler extends EventListener<DblClickEvent> {

}

Event Source

事件源通用接口：

package org.joshua.event.source;

import org.joshua.event.events.Event;
import org.joshua.event.listener.EventListener;

public interface EventSource {

    void addEventListener(EventListener<? extends Event> listener);

    void removeEventListener(EventListener<? extends Event> listener);

    void notifyListeners(Event event);

}

模拟的按钮控件：

package org.joshua.event.source;

import java.util.LinkedList;
import java.util.List;

import org.joshua.event.events.Event;
import org.joshua.event.listener.EventListener;

public class Button implements EventSource {

    protected List<EventListener<? extends Event>> listeners = new LinkedList<EventListener<? extends Event>>();

    @Override
    public void addEventListener(EventListener<? extends Event> listener) {
        listeners.add(listener);
    }

    @Override
    public void removeEventListener(EventListener<? extends Event> listener) {
        listeners.remove(listener);
    }

    @Override
    public void notifyListeners(Event event) {
        for (EventListener listener : listeners) {
            try {
                listener.handleEvent(event);
            } catch (ClassCastException e) {
            }
        }
    }

}

Client

package org.joshua.event;

import org.joshua.event.events.ClickEvent;
import org.joshua.event.events.DblClickEvent;
import org.joshua.event.events.Event;
import org.joshua.event.listener.ClickEventHandler;
import org.joshua.event.listener.DblClickEventHandler;
import org.joshua.event.source.Button;
import org.junit.Before;
import org.junit.Test;

public class Client {

    private Event currentEvent;

    private Button button;


    @Before
    public void initComponent() {

        button = new Button();

        button.addEventListener(new ClickEventHandler() {
            @Override
            public void handleEvent(ClickEvent event) {
                System.out.println("Button was clicked!");
            }
        });

        button.addEventListener(new DblClickEventHandler() {
            @Override
            public void handleEvent(DblClickEvent event) {
                System.out.println("Button was double clicked!");
            }
        });

    }

    @Test
    public void testCommonEvents() {
        currentEvent = new ClickEvent();
        button.notifyListeners(currentEvent);

        currentEvent = new DblClickEvent();
        button.notifyListeners(currentEvent);
    }

}

 Button类中的notifyListener方法实现起来虽方便，利用了一把异常机制，但着实不推荐大家在项目中这样做。且不说性能问题，Listener的handleEvent方法里所有抛出的ClassCastException都需要重新包装。当然，我们可以使用annotation、限定类名等方式相对优雅地解决event和对应listener的mapping问题。

多线程事件处理机制

思路是用队列暂存事件，然后若干个事件分发器将事件分发给指定数量的事件处理线程处理。

package com.joshua.test.event;

import java.util.concurrent.BlockingQueue;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.LinkedBlockingQueue;

import com.joshua.test.event.event.Event;
import com.joshua.test.event.event.EventType;
import com.joshua.test.event.handler.CreateEventHandler;

public class EventManager {

    private static final int EVENT_QUEUE_LENGTH = 1000;
    private static final int DISPATCHER_NUM = 2;
    private static final int EVENT_HANDLER_NUM = 10;

    public BlockingQueue<Event> eventQueue = new LinkedBlockingQueue<Event>(EVENT_QUEUE_LENGTH);
    private ExecutorService eventHandlerPool;

    protected EventDispatcher createDispatcher() {
        EventDispatcher dispatcher = new EventDispatcher(this.eventQueue, this.eventHandlerPool);
        dispatcher.register(EventType.CREATE, CreateEventHandler.class);
        return dispatcher;
    }

    public void init() {
        eventHandlerPool = Executors.newFixedThreadPool(EVENT_HANDLER_NUM);
    }

    public void start() {
        for (int i = 0; i < DISPATCHER_NUM; i++) {
            createDispatcher().start();
        }
    }

    public void notify(Event event) {
        try {
            eventQueue.put(event);
        } catch (InterruptedException e) {
        }
    }

}

package com.joshua.test.event;

import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.BlockingQueue;
import java.util.concurrent.ExecutorService;

import com.joshua.test.event.event.Event;
import com.joshua.test.event.event.EventType;
import com.joshua.test.event.handler.EventHandler;

@SuppressWarnings("rawtypes")
public class EventDispatcher {

    private final BlockingQueue<Event> eventQueue;
    private final ExecutorService eventHandlerPool;
    protected final Map<EventType, Class<? extends EventHandler>> eventDispatchers = new HashMap<EventType, Class<? extends EventHandler>>();
    private Thread eventHandlingThread;

    private volatile boolean stopped = false;

    public EventDispatcher(BlockingQueue<Event> eventQueue, ExecutorService eventHandlerPool) {
        this.eventQueue = eventQueue;
        this.eventHandlerPool = eventHandlerPool;
        System.out.println("Event dispatcher starting...");
    }

    Runnable createThread() {
        return new Runnable() {
            @Override
            public void run() {
                while (!stopped && !Thread.currentThread().isInterrupted()) {
                    Event event;
                    try {
                        event = eventQueue.take();
                    } catch (InterruptedException ie) {
                        if (!stopped) {
                            System.out.println("Dispatcher thread interrupted");
                            ie.printStackTrace();
                        }
                        return;
                    }
                    if (event != null) {
                        dispatch(event);
                    }
                }
            }
        };
    }

    @SuppressWarnings("unchecked")
    protected void dispatch(Event event) {
        EventType type = event.getType();
        try {
            Class<? extends EventHandler> handlerClazz = eventDispatchers
                    .get(type);
            if (handlerClazz != null) {
                EventHandler handler = handlerClazz.newInstance();
                handler.setEvent(event);
                eventHandlerPool.submit(handler);
            } else {
                throw new Exception("No handler for registered for " + type);
            }
        } catch (Throwable t) {
            System.err.println("Error in dispatcher thread");
            t.printStackTrace();
            System.exit(-1);
        }
    }

    public void register(EventType eventType,
            Class<? extends EventHandler> handler) {
        Class<? extends EventHandler> registeredHandler = eventDispatchers
                .get(eventType);
        System.out.println("Registering " + eventType + " for "
                + handler);
        if (registeredHandler == null) {
            eventDispatchers.put(eventType, handler);
        }
    }

    public void start() {
        eventHandlingThread = new Thread(createThread());
        eventHandlingThread.setName("AsyncDispatcher event handler");
        eventHandlingThread.start();
        System.out.println("Event dispatcher started!");
    }

    public void stop() {
        stopped = true;
        if (eventHandlingThread != null) {
            eventHandlingThread.interrupt();
            try {
                eventHandlingThread.join();
            } catch (InterruptedException ie) {
                System.out.println("Interrupted Exception while stopping");
                ie.printStackTrace();
            }
        }
    }

}