java观察者模式实现和java观察者模式演化

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。

复制代码代码如下:

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

复制代码代码如下:

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

复制代码代码如下:

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

复制代码代码如下:

// 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

复制代码代码如下:

	
	// 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

复制代码代码如下:

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);
}