观察者模式又叫发布订阅模式，通常用在事件监听和数据变更通知场景中。当一个对象状态发生改变时，所有依赖于它的对象都会得到变更通知，时一种对象间一对多关系。

案例：当天气发生变化时，电视台播放天气预告，手机推送天气消息。

不使用第三方类库实现

public interface Subject<M> {

    /**
     * 添加监听者
     */
    void register(Observer<M> observer);

    /**
     * 移除监听者
     */
    void unregister(Observer<M> observer);

    void notifyObservers();

    /**
     * 事件通知
     */
    void notifyObservers(M msg);

}

// 通用 Subject 封装
public class Observable<M> implements Subject<M> {

    /**
     * executor != null 时，异步通知
     */
    private final Executor executor;
    private final List<Observer<M>> observers;

    public Observable() {
        this.executor = null;
        this.observers = new CopyOnWriteArrayList<>();
    }

    public Observable(final Executor executor) {
        this.executor = executor;
        this.observers = new CopyOnWriteArrayList<>();
    }

    @Override
    public void register(final Observer<M> observer) {
        observers.add(observer);
    }

    @Override
    public void unregister(final Observer<M> observer) {
        observers.remove(observer);
    }

    @Override
    public void notifyObservers() {
        this.notifyObservers(null);
    }

    @Override
    public void notifyObservers(M msg) {
        if (executor == null) {
            observers.forEach(item -> item.update(msg));
            return;
        }
        observers.forEach(item -> executor.execute(() -> item.update(msg)));
    }
}

// 具体 Subject 实现
public class Weather extends Observable<WeatherType> {

    private static final AtomicInteger curr = new AtomicInteger(0);

    public void weatherChange() {
        final WeatherType[] weatherTypes = WeatherType.values();
        WeatherType currentWeather = weatherTypes[curr.getAndIncrement() % weatherTypes.length];
        System.out.println("天气发生变化：" + currentWeather);
        notifyObservers(currentWeather);
    }
}

// 观察者
public interface Observer<M> {

    void update(M msg);

}

public class TvWeatherObserver implements Observer<WeatherType> {

    @Override
    public void update(final WeatherType msg) {
        System.out.println("电视台天气预告，当前天气：" + msg.getDescription());
    }
}

public class PhoneWeatherObserver implements Observer<WeatherType> {

    @Override
    public void update(final WeatherType msg) {
        System.out.println("手机推送，当前天气：" + msg.getDescription());
    }
}

测试类

public class WeatherApp {

    public static void main(String[] args) {
        Weather weather = new Weather();
        weather.register(new TvWeatherObserver());
        weather.register(new PhoneWeatherObserver());
        weather.weatherChange();
        weather.weatherChange();
        weather.weatherChange();
        weather.weatherChange();
    }
}

使用guava实现

public class GuavaWeather {

    private final EventBus bus = new EventBus("weather");
    private static final AtomicInteger curr = new AtomicInteger(0);

    public void register(final Object observer) {
        bus.register(observer);
    }

    public void unregister(final Object observer) {
        bus.unregister(observer);
    }

    public void weatherChange() {
        final WeatherType[] weatherTypes = WeatherType.values();
        WeatherType currentWeather = weatherTypes[curr.getAndIncrement() % weatherTypes.length];
        System.out.println("天气发生变化：" + currentWeather);
        bus.post(currentWeather);
    }
}

public class GuavaTvWeatherObserver {

    @Subscribe
    public void update(final WeatherType msg) {
        System.out.println("电视台天气预告，当前天气：" + msg.getDescription());
    }
}

public class GuavaPhoneWeatherObserver {

    @Subscribe
    public void update(WeatherType msg) {
        System.out.println("手机推送，当前天气：" + msg.getDescription());
    }
}

测试类

public class GuavaWeatherApp {
    public static void main(String[] args) {
        final GuavaWeather weather = new GuavaWeather();
        weather.register(new GuavaTvWeatherObserver());
        weather.register(new GuavaPhoneWeatherObserver());
        weather.weatherChange();
        weather.weatherChange();
        weather.weatherChange();
        weather.weatherChange();
        weather.weatherChange();
    }
}

EventBus源码可以查看https://github.com/google/guava/blob/master/guava/src/com/google/common/eventbus/EventBus.java (opens new window)

另外guava还提供了异步支持AsyncEventBus (opens new window)