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简单的观察者模式实现
import java.util.ArrayList; import java.util.Collections; import java.util.Iterator; import java.util.List;
/**
* 观察者模式中用到了回调:
* A. 观察者将自己注册到被观察者的监听者列表，且观察者类自身提供了一个回调函数
* B. 被观察者（Observable或Subject）维护观察者列表，并且可以注册和解注册观察者
* C. 一旦被观察者状态发生改变，它可以调用notifyObservers()，这个方法将遍历观察者列表并逐个调用
观察者提供的回调函数
* @author will
*
*/
public class SimpleObserverPattern {
public static void main(String[] args) {
SimpleObserverPattern sop = new SimpleObserverPattern();
List<IObserver> observers = new ArrayList<IObserver> ();
IObserver observerA = sop.new Observer("ObserverA");
IObserver observerB = sop.new Observer("ObserverB");
observers.add(observerA);
observers.add(observerB);
IObservable observable = sop.new Observable(observers);
observable.registerObserver(sop.new Observer("ObserverC"));
observable.changeState();
observable.close();
}
// 被观察者，有的地方叫Subject
interface IObservable {
void registerObserver(IObserver observer);
void unregisterObserver(IObserver observer);
void notifyObservers();
String getState();
void changeState();
void close();
}
class Observable implements IObservable {
private static final String NEW = "New";
private static final String CHANGED = "Changed";
private static final String CLOSED = "Closed";
private String state;
private List<IObserver> observers;
public Observable() {
this(null);
}
public Observable(List<IObserver> observers) {
if(observers == null) {
observers = new ArrayList<IObserver> ();
}
this.observers = Collections.synchronizedList(observers);
this.state = NEW;
}
@Override
public void registerObserver(IObserver observer) {
observers.add(observer);
}
@Override
public void unregisterObserver(IObserver observer) {
observers.remove(observer);
}
@Override
public void notifyObservers() {
Iterator<IObserver> iter = observers.iterator();
while(iter.hasNext()) {
iter.next().update(this);
}
}
@Override
public String getState() {
return state;
}
@Override
public void changeState() {
this.state = CHANGED;
notifyObservers();
}
@Override
public void close() {
this.state = CLOSED;
notifyObservers();
}
}
interface IObserver {
void update(IObservable observalbe);
}
class Observer implements IObserver {
private String name;
public Observer(String name) {
this.name = name;
}
@Override
public void update(IObservable observalbe) {
System.out.println(
String.format("%s receive observalbe's change, current observalbe's state is %s",
name, observalbe.getState()));
}
}
}
上面的实现直接将被观察者对象作为回调函数参数，这样做很不优雅，在简单的场景可能奏效。
但事实上更多情况下，一个被观察者有很多种事件或者状态，而每个观察者可能感兴趣的事件或状态都不相同，或者为了信息隐藏的目的，不想让每个观察者都能访问到Observable内部的所有状态。
这样我继续演化代码为下面这个版本，注意我这里没有很细致地考虑并发问题。
import java.util.Collections; import java.util.HashSet; import java.util.Hashtable; import java.util.Iterator; import java.util.Set;
public class MultiEventObserverPattern {
public static void main(String[] args) {
MultiEventObserverPattern meop = new MultiEventObserverPattern();
IObservable observable = meop.new Observable();
IObserver observerA = meop.new Observer("ObserverA");
IObserver observerB = meop.new Observer("ObserverB");
// 注册感兴趣的事件
observable.registerObserver(observable.getEventA(), observerA);
observable.registerObserver(observable.getEventB(), observerB);
// 改变被观察者状态
observable.changeStateA();
observable.changeStateB();
}
interface IEvent {
void eventChange();
String getState();
}
class EventA implements IEvent {
private static final String INITIALIZED = "Initialized";
private static final String PENDING = "Pending";
private String state;
public EventA() {
this.state = INITIALIZED;
}
@Override
public void eventChange() {
System.out.println("EventA change");
this.state = PENDING;
}
@Override
public String toString() {
return "EventA";
}
@Override
public String getState() {
return state;
}
}
class EventB implements IEvent {
private static final String NEW = "New";
private static final String IDLE = "Idle";
private String state;
public EventB() {
this.state = NEW;
}
@Override
public void eventChange() {
System.out.println("EventB change");
this.state = IDLE;
}
@Override
public String toString() {
return "EventB";
}
@Override
public String getState() {
return state;
}
}
// 被观察者(Observable)，有的地方叫Subject
interface IObservable {
void registerObserver(IEvent event, IObserver observer);
void unregisterObserver(IEvent event, IObserver observer);
// 通知观察者某个事件发生了
void notifyObservers(IEvent event);
void changeStateA();
void changeStateB();
IEvent getEventA();
IEvent getEventB();
}
class Observable implements IObservable {
private IEvent eventA;
private IEvent eventB;
private Hashtable<IEvent, Set<IObserver>> eventObserverMapping;
public Observable() {
this(null);
}
// 这里如果evenObserverMapping传入的某些Set<IObserver>是未被同步修饰的，那么也没办法
public Observable(Hashtable<IEvent, Set<IObserver>> eventObserverMapping) {
if(eventObserverMapping == null) {
eventObserverMapping = new Hashtable<IEvent, Set<IObserver>> ();
}
this.eventObserverMapping = new Hashtable<IEvent, Set<IObserver>> ();
this.eventA = new EventA();
this.eventB = new EventB();
}
@Override
public void registerObserver(IEvent event, IObserver observer) {
Set<IObserver> observers = eventObserverMapping.get(event);
if(observers == null) {
observers = Collections.synchronizedSet(new HashSet<IObserver> ());
observers.add(observer);
eventObserverMapping.put(event, observers);
}
else {
observers.add(observer);
}
}
@Override
public void unregisterObserver(IEvent event, IObserver observer) {
Set<IObserver> observers = eventObserverMapping.get(event);
if(observers != null) {
observers.remove(observer);
}
}
@Override
public void notifyObservers(IEvent event) {
Set<IObserver> observers = eventObserverMapping.get(event);
if(observers != null && observers.size() > 0) {
Iterator<IObserver> iter = observers.iterator();
while(iter.hasNext()) {
iter.next().update(event);
}
}
}
@Override
public void changeStateA() {
// 改变状态A会触发事件A
eventA.eventChange();
notifyObservers(eventA);
}
@Override
public void changeStateB() {
// 改变状态B会触发事件B
eventB.eventChange();
notifyObservers(eventB);
}
@Override
public IEvent getEventA() {
return eventA;
}
@Override
public IEvent getEventB() {
return eventB;
}
}
interface IObserver {
void update(IEvent event);
}
class Observer implements IObserver {
private String name;
public Observer(String name) {
this.name = name;
}
@Override
public void update(IEvent event) {
System.out.println(
String.format("%s receive %s's change, current observalbe's state is %s",
name, event, event.getState()));
}
}
}
似乎看起来挺完美了，但还是不够完美。因为事件被硬编码为被观察者类的属性。这样事件类型在编译时期就被定死了，如果要增加新的事件类型就不得不修改IObservable接口和Observable类，这大大削减了灵活性。
相当于被观察者耦合于这些具体的事件，那么我们如何来打破这个限制呢？
答案是引入一个新的组件，让那个组件来管理事件、观察者、被观察者之间的关系，事件发生时也由那个组件来调用观察者的回调函数。这也是一种解耦吧，有点类似Spring的IOC容器。
至于具体实现，我觉得Guava EventBus做得已经蛮好了，可以参考我前面提到的链接。
PS：本帖不是为Guava EventBus做广告，只是自己的思路一步步推进，逐渐地就和Guava EventBus的设计思路吻合了。
下面继续看看JDK标准类实现观察者模式的例子，然后分析下它的源码实现，要看的只有一个Observable类和一个Observer接口。
JDK标准类实现观察者模式
import java.util.Observable; import java.util.Observer;
/**
* 使用java.util包中的标准类实现观察者模式
* @author will
*
*/
public class JDKObserverDemo {
public static void main(String[] args) {
JDKObserverDemo jod = new JDKObserverDemo();
// 被观察者
MyObservable myObservable = jod.new MyObservable("hello");
// 观察者
Observer myObserver = jod.new MyObserver();
// 注册
myObservable.addObserver(myObserver);
// 改变被观察者状态，触发观察者回调函数
myObservable.setValue("will");
}
class MyObservable extends Observable {
private String watchedValue; // 被观察的值
public MyObservable(String watchedValue) {
this.watchedValue = watchedValue;
}
public void setValue(String newValue) {
if(!watchedValue.equals(newValue)) {
watchedValue = newValue;
setChanged();
notifyObservers(newValue);
}
}
@Override
public String toString() {
return "MyObservable";
}
}
class MyObserver implements Observer {
@Override
public void update(Observable o, Object arg) {
System.out.println(o + "'s state changed, argument is: " + arg);
}
}
}
看了下JDK标准库中的Observer和Observable实现很简单，不想多说了。
下面是Quartz中的监听器实现。
QuartzScheduler被监听者
import java.util.ArrayList; import java.util.Date; import java.util.List;
/**
* Quartz核心类，相当于Observable（被观察者）
* @author will
*
*/
public class QuartzScheduler {
private ArrayList<SchedulerListener> internalSchedulerListeners = new ArrayList<SchedulerListener>(10);
// private ArrayList<JobListener> interanlJobListeners = new ArrayList<JobListener>(); // 一个Observable可以包含多组监听器
public Date scheduleJob(Trigger trigger) {
if(trigger == null) {
return null;
}
System.out.println("Schedule job, trigger: " + trigger);
notifySchedulerListenersScheduled(trigger);
return new Date();
}
public void unScheduleJob(Trigger trigger) {
if(trigger == null) {
return;
}
System.out.println("Unschedule job, trigger: " + trigger);
notifyShedulerListenerUnScheduled(trigger);
}
// 注册SchedulerListener
public void addInternalSchedulerListener(SchedulerListener schedulerListener) {
synchronized (internalSchedulerListeners) {
internalSchedulerListeners.add(schedulerListener);
}
}
// 移除SchedulerListener
public boolean removeInternalSchedulerListener(SchedulerListener schedulerListener) {
synchronized (internalSchedulerListeners) {
return internalSchedulerListeners.remove(schedulerListener);
}
}
public List<SchedulerListener> getInternalSchedulerListeners() {
synchronized (internalSchedulerListeners) {
return java.util.Collections.unmodifiableList(new ArrayList<SchedulerListener>(internalSchedulerListeners));
}
}
public void notifySchedulerListenersScheduled(Trigger trigger) {
for(SchedulerListener listener: getInternalSchedulerListeners()) {
listener.jobScheduled(trigger);
}
}
public void notifyShedulerListenerUnScheduled(Trigger trigger) {
for(SchedulerListener listener: getInternalSchedulerListeners()) {
listener.jobUnScheduled(trigger);
}
}
}
SchedulerListener
// 监听接口，回调函数，Client注册监听时需要提供回调函数实现 public interface SchedulerListener { void jobScheduled(Trigger trigger); void jobUnScheduled(Trigger trigger);
}
Trigger
// Trigger
public class Trigger {
private String triggerKey;
private String triggerName;
public Trigger(String triggerKey, String triggerName) {
this.triggerKey = triggerKey;
this.triggerName = triggerName;
}
public String getTriggerKey() {
return triggerKey;
}
public void setTriggerKey(String triggerKey) {
this.triggerKey = triggerKey;
}
public String getTriggerName() {
return triggerName;
}
public void setTriggerName(String triggerName) {
this.triggerName = triggerName;
}
public String toString() {
return String.format("{triggerKey: %s, triggerName: %s}", triggerKey, triggerName);
}
}
Test
public class Test { public static void main(String[] args) { QuartzScheduler qs = new QuartzScheduler(); SchedulerListener listenerA = new SchedulerListener() { @Override public void jobUnScheduled(Trigger trigger) { System.out.println("listenerA job unscheduled: " + trigger.getTriggerName()); } @Override public void jobScheduled(Trigger trigger) { System.out.println("listenerA job scheduled: " + trigger.getTriggerName()); } }; SchedulerListener listenerB = new SchedulerListener() { @Override public void jobUnScheduled(Trigger trigger) { System.out.println("listenerB job unscheduled: " + trigger.getTriggerName()); } @Override public void jobScheduled(Trigger trigger) { System.out.println("listenerB job scheduled: " + trigger.getTriggerName()); } }; // 注册Scheduler Listener qs.addInternalSchedulerListener(listenerA); qs.addInternalSchedulerListener(listenerB); Trigger triggerA = new Trigger("Key1", "triggerA"); Trigger triggerB = new Trigger("Key2", "triggerB"); qs.scheduleJob(triggerA); qs.scheduleJob(triggerB); qs.unScheduleJob(triggerA); }
}
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