观察者模式与Guava EventBus

观察者模式

结构图

代码实现

public abstract class Subject {

    private List<Observer> observerList = new ArrayList<Observer>();

    /**
     * 注册观察者
     * @param observer
     */
    public void register(Observer observer) {
        observerList.add(observer);
    }

    /**
     * 注销观察者
     *
     * @param observer
     */
    public void unregister(Observer observer) {
        observerList.remove(observer);
    }

    /**
     * 通知观察者更新
     */
    public void post() {
        for (Observer observer : observerList) {
            observer.update();
        }
    }

    /**
     * 获取被通知事件
     *
     * @return
     */
    public abstract Object getEvent();
}

public class ConcreteSubject1 extends Subject {

    /** 个性化的定制内容 */
    private String subjectState;

    public ConcreteSubject1(String subjectState) {
        this.subjectState = subjectState;
    }

    @Override
    public Object getEvent() {
        System.out.println("Custom ConcreteSubject1");
        return subjectState;
    }
}

public class ConcreteSubject2 extends Subject {

    private int subjectState;

    public ConcreteSubject2(int subjectState) {
        this.subjectState = subjectState;
    }

    @Override
    public Object getEvent() {
        System.out.println("Custom ConcreteSubject2");
        return subjectState;
    }
}

public abstract class Observer {

    /** 用于观察者获取被通知的事件 */
    protected Subject subject;

    /**
     * 用于给Subject通知时调用的更新方法
     */
    public abstract void update();
}

public class ConcreteObserver1 extends Observer {

    public ConcreteObserver1(Subject subject) {
        this.subject = subject;
    }

    @Override
    public void update() {
        System.out.println("Subject " + subject.getEvent() + " ConcreteObserver1");
    }
}

public class ConcreteObserver1 extends Observer {

    public ConcreteObserver1(Subject subject) {
        this.subject = subject;
    }

    @Override
    public void update() {
        System.out.println("Subject " + subject.getEvent() + " ConcreteObserver1");
    }
}

public class ConcreteObserver1 extends Observer {

    public ConcreteObserver1(Subject subject) {
        this.subject = subject;
    }

    @Override
    public void update() {
        System.out.println("Subject " + subject.getEvent() + " ConcreteObserver1");
    }
}

输出:

Custom ConcreteSubject1
Subject Sub1 ConcreteObserver1
Custom ConcreteSubject1
Subject Sub1 ConcreteObserver2

EventBus简单示例

EventBus是Guava提供的消息发布-订阅类库,它的工作机制类似于观察者模式,通过通知者去注册观察者,最后由通知者想观察者发布消息,示例代码

public class MsgCenter {

    /** EventBus的定位跟接近于消息中心,而他的post()方法跟接近于一个自定义的Subject */
    public static EventBus eventBus = new EventBus();
}

public class Observer1 {

    /**
     * 只有通过@Subscribe注解的方法才会被注册进EventBus
     * 而且方法有且只能有1个参数
     * @param msg
     */
    @Subscribe
    public void ob1Mthod1(String msg) {
        System.out.println(msg + " test1!");
    }

    @Subscribe
    public void ob1Method2(String msg) {
        System.out.println(msg + " test2!");
    }
}

public class Observer2 {
    @Subscribe
    public void ob2Method1(String msg) {
        System.out.println(msg + " test3!");
    }

    // 错误的基本型参数
    // @Subscribe
    // public void ob2Method2(int msg) {
    // System.out.println(msg + " test4!");
    // }
    /**
     * post() 不支持自动装箱功能,只能使用Integer,不能使用int,否则handlersByType的Class会是int而不是Intege
     * 而传入的int msg参数在post(int msg)的时候会被包装成Integer,导致无法匹配到
     */
    @Subscribe
    public void ob2Method2(Integer msg) {
        System.out.println(msg + " test4!");
    }
}

public class Test {

    public static void main(String[] args) throws InterruptedException {
        EventBus eventBus = new EventBus();
        Observer1 observer1 = new Observer1();
        Observer2 observer2 = new Observer2();

        eventBus.register(observer1);
        eventBus.register(observer2);

        // 只有注册的参数类型为String的方法会被调用
        eventBus.post("post string method");

        // 注销observer2
        eventBus.unregister(observer2);
        eventBus.post("post string method after unregister");
    }
}

输出

post string method test2!
post string method test1!
post string method test3!
post string method after unregister test2!
post string method after unregister test1!

实际上EventBus要表达的意图很简单,就是将post(Object arg)这里的arg当做参数传入到已注册的方法(被@Subscribe)的方法里,并调用该方法,所以当post(String)的时候,调用的参数类型为String的注册方法,当post(int)的时候,调用则是参数类型为Integer的注册方法

EventBus的实现方式

eventbus的实现方式实际上类似于上例写的简单的观察者模式,不同点在于它实现了泛化的注册方法以及泛化的方法调用,另外还考虑到了多线程的问题,对多线程使用时做了一些优化

register(Object listener)

register()方法注册一个任意类型的实例并将其使用了@Subscribe的方法注册到一个Map中,这个Map使用方法的参数类型Class为Key,值为一个Set,这个Set包含了所有参数类型为Key的EventHandler,Eventhandler是EventBus定义的一个数据结构,由listener(方法拥有者instance,例如上例中的observer1)和这个listener的@Subscribe方法构成

]这个Map的结构示意图如下

其实现代码如下

  /**
   * Registers all handler methods on {@code object} to receive events.
   * Handler methods are selected and classified using this EventBus's
   * {@link HandlerFindingStrategy}; the default strategy is the
   * {@link AnnotatedHandlerFinder}.
   *
   * @param object  object whose handler methods should be registered.
   */
  public void register(Object object) {
    handlersByType.putAll(finder.findAllHandlers(object));
  }

  /**
   * {@inheritDoc}
   *
   * This implementation finds all methods marked with a {@link Subscribe} annotation.
   */
  @Override
  public Multimap<Class<?>, EventHandler> findAllHandlers(Object listener) {
    Multimap<Class<?>, EventHandler> methodsInListener = HashMultimap.create();
    Class<?> clazz = listener.getClass();
    Set<? extends Class<?>> supers = TypeToken.of(clazz).getTypes().rawTypes();

    for (Method method : clazz.getMethods()) {
      /*
       * Iterate over each distinct method of {@code clazz}, checking if it is annotated with
       * @Subscribe by any of the superclasses or superinterfaces that declare it.
       */
      for (Class<?> c : supers) {
        try {
          Method m = c.getMethod(method.getName(), method.getParameterTypes());
          if (m.isAnnotationPresent(Subscribe.class)) {
            Class<?>[] parameterTypes = method.getParameterTypes();
            if (parameterTypes.length != 1) {
              throw new IllegalArgumentException("Method " + method
                  + " has @Subscribe annotation, but requires " + parameterTypes.length
                  + " arguments.  Event handler methods must require a single argument.");
            }
            Class<?> eventType = parameterTypes[0];
            EventHandler handler = makeHandler(listener, method);

            methodsInListener.put(eventType, handler);
            break;
          }
        } catch (NoSuchMethodException ignored) {
          // Move on.
        }
      }
    }
    return methodsInListener;
  }

post(Object event)

post(Object event)方法用于向已注册的方法传递一个参数,并以此调用参数类型为event.class的所有方法,调用的时候会使用一个ThreadLocal的Queue来进行任务分发,这样的结果就是在多线程情况下,线程间共享注册方法的Map(上面提到那个),当时在发送消息时线程会保有自己独立的一个Post任务的Queue,保证了线程执行post()方法时候的独立性而不会相互影响,下面是多线程执行post()时的示意图

真正在执行post(Object event)的时候,会将msg与所有event.class对应的set里的所有method组合成一个EventBus.EventWithHandler对象并加入到ThreadLocal的Queue中,最后再将Queue出队依次执行这些方法,最后清空ThreadLocal的Queue,EventBus的实现代码如下

    /**
     * Posts an event to all registered handlers.  This method will return
     * successfully after the event has been posted to all handlers, and
     * regardless of any exceptions thrown by handlers.
     *
     * <p>If no handlers have been subscribed for {@code event}'s class, and
     * {@code event} is not already a {@link com.google.common.eventbus.DeadEvent}, it will be wrapped in a
     * DeadEvent and reposted.
     *
     * @param event  event to post.
     */
    @SuppressWarnings("deprecation") // only deprecated for external subclasses
    public void post(Object event) {
        Set<Class<?>> dispatchTypes = flattenHierarchy(event.getClass());

        boolean dispatched = false;
        // 将event和所有event.class对应的方法组合成EventWithHandler并入队
        for (Class<?> eventType : dispatchTypes) {
            Set<EventHandler> wrappers = getHandlersForEventType(eventType);

            if (wrappers != null && !wrappers.isEmpty()) {
                dispatched = true;
                for (EventHandler wrapper : wrappers) {
                    enqueueEvent(event, wrapper);
                }
            }
        }

        if (!dispatched && !(event instanceof DeadEvent)) {
            post(new DeadEvent(this, event));
        }

        dispatchQueuedEvents();
    }

    /**
     * Queue the {@code event} for dispatch during
     * {@link #dispatchQueuedEvents()}. Events are queued in-order of occurrence
     * so they can be dispatched in the same order.
     */
    void enqueueEvent(Object event, EventHandler handler) {
        eventsToDispatch.get().offer(new EventWithHandler(event, handler));
    }

    /**
     * Drain the queue of events to be dispatched. As the queue is being drained,
     * new events may be posted to the end of the queue.
     *
     * @deprecated This method should not be overridden outside of the eventbus package. It is
     *     scheduled for removal in Guava 14.0.
     */
    @Deprecated
    protected void dispatchQueuedEvents() {
        // don't dispatch if we're already dispatching, that would allow reentrancy
        // and out-of-order events. Instead, leave the events to be dispatched
        // after the in-progress dispatch is complete.
        if (isDispatching.get()) {
            return;
        }

        isDispatching.set(true);
        try {
            while (true) {
                EventWithHandler eventWithHandler = eventsToDispatch.get().poll();
                if (eventWithHandler == null) {
                    break;
                }

                dispatch(eventWithHandler.event, eventWithHandler.handler);
            }
        } finally {
            isDispatching.set(false);
        }
    }

斜体加粗部分即为关键部分

EventBus多线程使用示例

public class Test2 {

    public static void main(String[] args) throws InterruptedException {

        Thread t1 = new Thread() {

            @Override
            public void run() {
                System.out.println(Thread.currentThread());
                System.out.println("start");
                Observer1 observer1 = new Observer1();
                MsgCenter.eventBus.register(observer1);
                MsgCenter.eventBus.post("post string");
                System.out.println("end");
                System.out.println();
            }
        };

        Thread t2 = new Thread() {

            @Override
            public void run() {
                System.out.println(Thread.currentThread());
                System.out.println("start");
                Observer2 observer2 = new Observer2();
                MsgCenter.eventBus.register(observer2);
                MsgCenter.eventBus.post("post string2");
                System.out.println("end");
                System.out.println();
            }
        };

        ExecutorService exec = Executors.newFixedThreadPool(2);

        exec.execute(t1);

        // 为何忽略多线程是run()方法的线程安全问题,让两个任务分开执行
        TimeUnit.MILLISECONDS.sleep(500);

        exec.execute(t2);

        exec.shutdown();
    }
}

输出

Thread[pool-1-thread-1,5,main]
start
post string test1!
post string test2!
end

Thread[pool-1-thread-2,5,main]
start
post string2 test1!
post string2 test2!
post string2 test3!
end

这里为了让执行结果更清晰,并没有让两个线程并发执行,但可以清楚看到他们是共享同一个注册方法的Map的,而由于post()分发消息时间的Queue是ThreadLocal的,这个Queue由每个线程独有

对于这里使用ThreadLocal的Queue的个人理解就是假如不使用ThreadLocal,共享同一个队列,就有可能由Thread1入队的EventWithHandler会由Thread2来执行,而由Thread2入队的EventWithHandler又有可能会由Thread1来执行,而造成执行秩序混乱.

而使用ThreadLocal的Queue,则这些EventWithHandler示例是由哪个线程入队就由哪个线程执行,同时不需要去考虑共享队列的入队和出队时的线程安全问题,也可以提升效率