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
