行为型设计模式：观察者模式以及guava中的使用

观察者模式是很常见的一种行为型设计模式。在Java原生态的实现方式中，观察者实现Observer接口，被观察者继承Observable。

下面编写一个使用Java api简单的实现。

观察者代码：

public class MyObserver implements Observer {

    public void update(Observable o, Object arg) {
        if (o instanceof MyObservable){
            System.out.println(arg);
        }

    }
}

被观察者：

public class MyObservable extends Observable {

    @Override
    public void notifyObservers(Object message){
        super.setChanged();
        super.notifyObservers(message);
    }

}

 

绑定主题类：

public class Subject {

    private Observable observable = new MyObservable();

    public void registerObserver(MyObserver observer) {
        observable.addObserver(observer);
    }

    public void removeObserver(MyObserver observer) {
        observable.deleteObserver(observer);
    }

    public void notifyObservers(String message) {
        observable.notifyObservers(message);
    }

}

测试代码

public static void main(String[] args) {
        Subject subject = new Subject();
        MyObserver observer = new MyObserver();
        subject.registerObserver(observer);
        subject.notifyObservers("hi, I am subject Observable");
}

java的实现方式，如果观察者使用异步来实现消息处理，会使业务代码和非业务代码耦合在一起。

guava封装了Java的观察者模式，并且方便的支持异步。talk is cheap，先看一下代码：

定义2个观察者：

public class AObserver {

    Logger logger = LoggerFactory.getLogger(getClass());

    @Subscribe
    public void handleMessage(String msg){
        logger.info("a obsesrver receive message:{}", msg);
    }
}

public class BObserver {

    Logger logger = LoggerFactory.getLogger(getClass());

    @Subscribe
    public void handleMessage(String msg){
        logger.info("b obsesrver receive message:{}", msg);
    }
}

 

EventBusUtil类

public class EventBusUtil {

    public static EventBus getEventBus(){
        return EventBusFactory.getAsyncInstance();
    }

    public static class EventBusFactory{
        private static EventBus asyncEventBus = new AsyncEventBus(LocalThreadPoolExecutor.getExecutor());
        private static EventBus syncEventBus = new AsyncEventBus(MoreExecutors.directExecutor());

        public static EventBus getAsyncInstance(){
            return asyncEventBus;
        }

        public static EventBus getyncInstance(){
            return syncEventBus;
        }

    }
}

注意：MoreExecutors.directExecutor()看起来是线程池，其实是单线程，看源码注解：

测试代码：

public class TestEventBus{

    public static void main(String[] args){
        EventBus eventBus = EventBusUtil.getEventBus();
        eventBus.register(new AObserver());
        eventBus.register(new BObserver());

        for (int j = 0; j < 2; j ++){
            eventBus.post("hi, observer" + j);
        }
    }
}

下面看一下guava中的实现：

1）EventBus中的注册，可以注册任意对象作为观察者

  /**
   * Registers all subscriber methods on {@code object} to receive events.
   *
   * @param object object whose subscriber methods should be registered.
   */
  public void register(Object object) {
    subscribers.register(object);
  }

 

所有的观察者类中，处理监听事件的方法加了注解@Subscribe，注册的时候，会查找类中加了这个注解的方法然后进行注册，见下面代码中的

findAllSubscribers方法

/** Registers all subscriber methods on the given listener object. */
  void register(Object listener) {
//查找所有包含@Subscribe注解的方法
    Multimap, Subscriber> listenerMethods = findAllSubscribers(listener);

    for (Entry, Collection> entry : listenerMethods.asMap().entrySet()) {
      Class eventType = entry.getKey();
      Collection eventMethodsInListener = entry.getValue();

      CopyOnWriteArraySet eventSubscribers = subscribers.get(eventType);

      if (eventSubscribers == null) {/还没有注册观察者
        CopyOnWriteArraySet newSet = new CopyOnWriteArraySet<>();
        eventSubscribers =
            MoreObjects.firstNonNull(subscribers.putIfAbsent(eventType, newSet), newSet);
      }

      eventSubscribers.addAll(eventMethodsInListener);
    }
  }

2）EventBus中的通知

/**
   * Posts an event to all registered subscribers. This method will return successfully after the
   * event has been posted to all subscribers, and regardless of any exceptions thrown by
   * subscribers.
   *
   * 
If no subscribers have been subscribed for {@code event}'s class, and {@code event} is not
   * already a {@link DeadEvent}, it will be wrapped in a DeadEvent and reposted.
   *
   * @param event event to post.
   */
  public void post(Object event) {
    Iterator eventSubscribers = subscribers.getSubscribers(event);
    if (eventSubscribers.hasNext()) {
      dispatcher.dispatch(event, eventSubscribers);
    } else if (!(event instanceof DeadEvent)) {
      // the event had no subscribers and was not itself a DeadEvent
      post(new DeadEvent(this, event));
    }
  }

从上面代码可以看出，通过dispatcher.dispatch方法进行通知，这个方法的代码看下面代码：

@Override
    void dispatch(Object event, Iterator subscribers) {
      checkNotNull(event);
      while (subscribers.hasNext()) {
        queue.add(new EventWithSubscriber(event, subscribers.next()));
      }

      EventWithSubscriber e;
      while ((e = queue.poll()) != null) {
        e.subscriber.dispatchEvent(e.event);
      }
    }

上面的代码能够看出，消息事件event和观察者subscriber封装成一个对象放入并发队列中，然后出队让观察者触发消息处理。

/** Dispatches {@code event} to this subscriber using the proper executor. */
  final void dispatchEvent(final Object event) {
    executor.execute(
        new Runnable() {
          @Override
          public void run() {
            try {
              invokeSubscriberMethod(event);
            } catch (InvocationTargetException e) {
              bus.handleSubscriberException(e.getCause(), context(event));
            }
          }
        });
  }

这儿的线程池正是我们在声明EventBus时传入的线程池变量。最后的事件触发使用了java的反射。

/**
   * Invokes the subscriber method. This method can be overridden to make the invocation
   * synchronized.
   */
  @VisibleForTesting
  void invokeSubscriberMethod(Object event) throws InvocationTargetException {
    try {
      method.invoke(target, checkNotNull(event));
    } catch (IllegalArgumentException e) {
      throw new Error("Method rejected target/argument: " + event, e);
    } catch (IllegalAccessException e) {
      throw new Error("Method became inaccessible: " + event, e);
    } catch (InvocationTargetException e) {
      if (e.getCause() instanceof Error) {
        throw (Error) e.getCause();
      }
      throw e;
    }
  }

代码分析就到这儿，guava详细代码请看这里：

https://github.com/google/guava

文中的示例代码请看这里

https://github.com/jinjunzhu/myguava.git

 

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