设计模式20-观察者模式（Observer）
设计模式20-观察者模式（Observer）
概述
Define a one-to-many dependency between objects so that when one object changes state, all its dependents are notified and updated automatically.
在对象之间定义一个一对多的依赖，当一个对象状态改变的时候，所有依赖的对象都会得到通知并自动更新。也叫发布订阅模式，能够很好的解耦一个对象改变，自动改变另一个对象这种情况。
简单实现
JDK实现
- 定义被观察者
com.demo.design.pattern.behavioral.observer.Subject
public interface Subject {
/**
* 添加观察者（如果有必要，可以添加remove方法移除观察者接口并在抽象类中实现）
*
* @param observer 观察者
*/
void add(Observer observer);
/**
* 被观察者执行操作发生变化
*/
void operation();
}
com.demo.design.pattern.behavioral.observer.AbstractSubject
public abstract class AbstractSubject implements Subject {
/**
* 观察者对象，可以有多个不同的观察者执行不同的行为
*/
private final List observers = new ArrayList<>();
@Override
public void add(Observer observer) {
observers.add(observer);
}
void notifyObservers() {
// 唤醒所有的观察者并同步更新
for (Observer observer : observers) {
observer.update();
}
}
}
com.demo.design.pattern.behavioral.observer.SubjectImpl
public class SubjectImpl extends AbstractSubject {
@Override
public void operation() {
System.out.println("update self!");
this.notifyObservers();
}
}
- 定义被观察者
com.demo.design.pattern.behavioral.observer.Observer
interface Observer {
/**
* 更新操作
*/
void update();
}
com.demo.design.pattern.behavioral.observer.ObserverImpl
public class ObserverImpl implements Observer {
@Override
public void update() {
System.out.println("observer has received and update");
}
}
- 使用示例
@Test
public void observerPattern(){
Subject sub = new SubjectImpl();
sub.add(new ObserverImpl());
sub.operation();
}
源码解析
JDK
java.util.Observer
/**
* 通过实现 Observer 接口来接受被观察对象发生改变而发出的通知
*/
public interface Observer {
/**
* 当被观察对象发生改变时会调用此方法。当改变发生时，调用被观察对象的唤醒方法来通知所有观察者。
*
* @param o 被观察对象.
* @param arg 被观察对象唤醒方法参数
*/
void update(Observable o, Object arg);
}
java.util.Observable
/**
* This class represents an observable object, or "data"
* in the model-view paradigm. It can be subclassed to represent an
* object that the application wants to have observed.
*
* An observable object can have one or more observers. An observer
* may be any object that implements interface Observer. After an
* observable instance changes, an application calling the
* Observable's notifyObservers method
* causes all of its observers to be notified of the change by a call
* to their update method.
*
* The order in which notifications will be delivered is unspecified.
* The default implementation provided in the Observable class will
* notify Observers in the order in which they registered interest, but
* subclasses may change this order, use no guaranteed order, deliver
* notifications on separate threads, or may guarantee that their
* subclass follows this order, as they choose.
*
* Note that this notification mechanism has nothing to do with threads
* and is completely separate from the wait and notify
* mechanism of class Object.
*
* When an observable object is newly created, its set of observers is
* empty. Two observers are considered the same if and only if the
* equals method returns true for them.
*
* @author Chris Warth
* @see java.util.Observable#notifyObservers()
* @see java.util.Observable#notifyObservers(java.lang.Object)
* @see java.util.Observer
* @see java.util.Observer#update(java.util.Observable, java.lang.Object)
* @since JDK1.0
*/
public class Observable {
private boolean changed = false;
private Vector obs;
/** Construct an Observable with zero Observers. */
public Observable() {
obs = new Vector<>();
}
/**
* Adds an observer to the set of observers for this object, provided
* that it is not the same as some observer already in the set.
* The order in which notifications will be delivered to multiple
* observers is not specified. See the class comment.
*
* @param o an observer to be added.
* @throws NullPointerException if the parameter o is null.
*/
public synchronized void addObserver(Observer o) {
if (o == null)
throw new NullPointerException();
if (!obs.contains(o)) {
obs.addElement(o);
}
}
/**
* Deletes an observer from the set of observers of this object.
* Passing null to this method will have no effect.
* @param o the observer to be deleted.
*/
public synchronized void deleteObserver(Observer o) {
obs.removeElement(o);
}
/**
* If this object has changed, as indicated by the
* hasChanged method, then notify all of its observers
* and then call the clearChanged method to
* indicate that this object has no longer changed.
*
* Each observer has its update method called with two
* arguments: this observable object and null. In other
* words, this method is equivalent to:
*
* notifyObservers(null)
*
* @see java.util.Observable#clearChanged()
* @see java.util.Observable#hasChanged()
* @see java.util.Observer#update(java.util.Observable, java.lang.Object)
*/
public void notifyObservers() {
notifyObservers(null);
}
/**
* If this object has changed, as indicated by the
* hasChanged method, then notify all of its observers
* and then call the clearChanged method to indicate
* that this object has no longer changed.
*
* Each observer has its update method called with two
* arguments: this observable object and the arg argument.
*
* @param arg any object.
* @see java.util.Observable#clearChanged()
* @see java.util.Observable#hasChanged()
* @see java.util.Observer#update(java.util.Observable, java.lang.Object)
*/
public void notifyObservers(Object arg) {
/*
* a temporary array buffer, used as a snapshot of the state of
* current Observers.
*/
Object[] arrLocal;
synchronized (this) {
/* We don't want the Observer doing callbacks into
* arbitrary code while holding its own Monitor.
* The code where we extract each Observable from
* the Vector and store the state of the Observer
* needs synchronization, but notifying observers
* does not (should not). The worst result of any
* potential race-condition here is that:
* 1) a newly-added Observer will miss a
* notification in progress
* 2) a recently unregistered Observer will be
* wrongly notified when it doesn't care
*/
if (!changed)
return;
arrLocal = obs.toArray();
clearChanged();
}
for (int i = arrLocal.length-1; i>=0; i--)
((Observer)arrLocal[i]).update(this, arg);
}
/**
* Clears the observer list so that this object no longer has any observers.
*/
public synchronized void deleteObservers() {
obs.removeAllElements();
}
/**
* Marks this Observable object as having been changed; the
* hasChanged method will now return true.
*/
protected synchronized void setChanged() {
changed = true;
}
/**
* Indicates that this object has no longer changed, or that it has
* already notified all of its observers of its most recent change,
* so that the hasChanged method will now return false.
* This method is called automatically by the
* notifyObservers methods.
*
* @see java.util.Observable#notifyObservers()
* @see java.util.Observable#notifyObservers(java.lang.Object)
*/
protected synchronized void clearChanged() {
changed = false;
}
/**
* Tests if this object has changed.
*
* @return true if and only if the setChanged
* method has been called more recently than the
* clearChanged method on this object;
* false otherwise.
* @see java.util.Observable#clearChanged()
* @see java.util.Observable#setChanged()
*/
public synchronized boolean hasChanged() {
return changed;
}
/**
* Returns the number of observers of this Observable object.
*
* @return the number of observers of this object.
*/
public synchronized int countObservers() {
return obs.size();
}
}
EventBus
事件总线，提供了实现观察者模式的骨架代码。Google Guava EventBus 就是一个比较著名的 EventBus 框架，它不仅仅支持异步非阻塞模式，同时也支持同步阻塞模式。
com.google.common.eventbus.EventBus
@Beta
public class EventBus {
private static final Logger logger = Logger.getLogger(EventBus.class.getName());
private final String identifier;
private final Executor executor;
private final SubscriberExceptionHandler exceptionHandler;
private final SubscriberRegistry subscribers = new SubscriberRegistry(this);
private final Dispatcher dispatcher;
// ...
}
利用 EventBus 框架实现的观察者模式，不需要定义 Observer 接口，任意类型的对象都可以注册到 EventBus 中，通过 @Subscribe 注解来标明类中哪个函数可以接收被观察者发送的消息。
优点
- 观察者和被观察者之间抽象耦合
不管是增加观察者还是被观察者都非常容易扩展，在系统扩展方面会得心应手。
- 建立一套触发机制
被观察者变化引起观察者自动变化。但是需要注意的是，一个被观察者，多个观察者，Java的消息通知默认是顺序执行的，如果一个观察者卡住，会导致整个流程卡住，这就是同步阻塞。
所以实际开发中没有先后顺序的考虑使用异步，异步非阻塞除了能够实现代码解耦，还能充分利用硬件资源，提高代码的执行效率。
另外还有不同进程间的观察者模式，通常基于消息队列来实现，用于实现不同进程间的观察者和被观察者之间的交互。
使用场景
-
关联行为场景。如用户注册或登陆成功后发送短信提醒，就可以通过观察注册或登陆是否成功来发送短信。
-
事件多级触发场景。
-
跨系统的消息交换场景， 如消息队列的处理机制。