观察者模式(Observer Pattern)升级版

简介

前面的博客https://goo.gl/DFOz50中已经介绍过基本的观察者模式的代码实现，在实际应用中，我们往往希望代码更加简洁，同时也希望能与现有的框架，比如Spring，结合起来。下面介绍如何做抽象。

简单抽象

前面介绍过观察者模式有Subject，Observer，Client几种角色。在使用面向对象的时候Subject 和Observer都可以被抽象成接口，所以从原来的三种角色可以进化为6个角色。 (1) Subject : 被监控对象，抽象以后我们叫做BUS(总线，所有对被监控对象的操作都由它完成); (2) concreteSubject：具体的被监控对象 (3)Observer（Listener）：观察者 (4)concreteObserver：具体的观察者 (5)event : 监控对象和被监控对象交互时用到的消息

下面的类图用来解释新的观察者模式：

Note: （1） 图中引入了泛型T作为观察者，从而使得每一个Bus的具体实现对应一个Listener接口，这样一个Bus最终只对应到一类事件； （2）Listener和Bus之间交互使用一个单独的类Event实现； （3）ExecutorService的引入使得监听器的响应与主线程异步进行，提升效率。

如果对上面的图进一步抽象的话就编程了下面这样：

代码实现

Step 1

创建AbstractBus

public abstract class BaseBus<T> { private final List<T> listeners = new CopyOnWriteArrayList<T>(); private final ExecutorService executorService; protected BaseBus(ExecutorService executorService) { this.executorService = executorService; } public void iterate(final ListenerCallback<T> callback){ for(final T t:listeners){ this.executorService.submit(new Callable<Object>() { @Override public Object call() throws Exception { callback.doWithListener(t); return true; } }); } } public interface ListenerCallback<T> { Object doWithListener(T t); } public boolean addListener(T listener) { return this.listeners.add(listener); } public List<T> getListeners() { return listeners; } public void setListeners(List<T> listeners) { this.listeners.addAll(listeners); } }

Step 2

创建AbstractBus的具体实现：

public class ProfileChangeBus extends BaseBus<ProfileChangeListener> { public ProfileChangeBus(){ super(new ThreadPoolExecutor( 2, 10, 0L, TimeUnit.MILLISECONDS, new LinkedBlockingQueue<Runnable>(100), Executors.defaultThreadFactory(), new ThreadPoolExecutor.CallerRunsPolicy())); } public void fireCreate(final ProfileChangeEvent event) { event.setType(0); iterate(new ListenerCallback<ProfileChangeListener>() { @Override public Object doWithListener(ProfileChangeListener profileChangeListener) { profileChangeListener.onCreate(event); return true; } }); } public void fireUpdate(final ProfileChangeEvent event) { event.setType(0); this.iterate(new ListenerCallback<ProfileChangeListener>() { @Override public Object doWithListener(ProfileChangeListener profileChangeListener) { profileChangeListener.onUpdate(event); return true; } }); } public void fireDelete(final ProfileChangeEvent event) { event.setType(0); this.iterate(new ListenerCallback<ProfileChangeListener>() { @Override public Object doWithListener(ProfileChangeListener profileChangeListener) { profileChangeListener.onDelete(event); return true; } }); } }

Step 3

创建Listener

public interface ProfileChangeListener { void onCreate(ProfileChangeEvent profileChangeEvent); void onUpdate(ProfileChangeEvent profileChangeEvent); void onDelete(ProfileChangeEvent profileChangeEvent); }

Step 4

创建Listener的具体实现，这里有3个实现类： (1)

public class ProfileChangeListenerAdapter implements ProfileChangeListener { @Override public void onCreate(ProfileChangeEvent profileChangeEvent) { System.out.println("trigger create"); } @Override public void onUpdate(ProfileChangeEvent profileChangeEvent) { System.out.println("trigger update"); } @Override public void onDelete(ProfileChangeEvent profileChangeEvent) { System.out.println("trigger delete"); } }

(2)

public class ProfileChangeListenerAdapter4MQ extends ProfileChangeListenerAdapter { @Override public void onCreate(ProfileChangeEvent profileChangeEvent) { System.out.println("mq trigger create"); } @Override public void onUpdate(ProfileChangeEvent profileChangeEvent) { System.out.println("mq trigger update"); } @Override public void onDelete(ProfileChangeEvent profileChangeEvent) { System.out.println("mq trigger delete"); } }

(3)

public class ProfileChangeListener4DB implements ProfileChangeListener{ @Override public void onCreate(ProfileChangeEvent profileChangeEvent) { System.out.println("db trigger create"); } @Override public void onUpdate(ProfileChangeEvent profileChangeEvent) { System.out.println("db trigger create"); } @Override public void onDelete(ProfileChangeEvent profileChangeEvent) { System.out.println("db trigger create"); } }

Step 5

创建Event

public class ProfileChangeEvent { private int type; private String message; public ProfileChangeEvent(int type, String message){ this.type = type; this.message = message; } public int getType() { return type; } public void setType(int type) { this.type = type; } public String getMessage() { return message; } public void setMessage(String message) { this.message = message; } }

Step 6

将Listener注入到ConcreteBus

<bean id="profileChangeBus" class="designpattern.observer.ProfileChangeBus"> <property name="listeners"> <list> <bean class="designpattern.observer.ProfileChangeListener4DB" /> <bean class="designpattern.observer.ProfileChangeListenerAdapter4MQ"/> </list> </property> </bean>

Step 7

测试：

@RunWith(SpringJUnit4ClassRunner.class) @ContextConfiguration(locations = { "classpath:config/*.xml" }) public class ObserverPatternTest { @Autowired private ProfileChangeBus profileChangeBus; @Test public void test(){ profileChangeBus.fireCreate(new ProfileChangeEvent(1, "1")); } }