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!
endThread[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