源码分析
Java内置观察者模式使用了一个类 Observable，一个接口 Observer
分析类 Observable
private boolean changed = false;
private Vector<Observer> obs;
/** Construct an Observable with zero Observers. */
public Observable() {
obs = new Vector<>();
}
notifyObservers()和notifyObservers(Object)依赖于 changed的状态，判断changed状态来决定观察者是否update，
在更新完成后clearChanged();
/**
* If this object has changed, as indicated by the
* <code>hasChanged</code> method, then notify all of its observers
* and then call the <code>clearChanged</code> method to
* indicate that this object has no longer changed.
* <p>
* Each observer has its <code>update</code> method called with two
* arguments: this observable object and <code>null</code>. In other
* words, this method is equivalent to:
* <blockquote><tt>
* notifyObservers(null)</tt></blockquote>
*
* @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
* <code>hasChanged</code> method, then notify all of its observers
* and then call the <code>clearChanged</code> method to indicate
* that this object has no longer changed.
* <p>
* Each observer has its <code>update</code> method called with two
* arguments: this observable object and the <code>arg</code> 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);
}特别注意notifyObserveres(Object)方法
1、arrLocal数组对象的作用－临时缓存，用于存储Observers的快照
2、sychronized(this)同步处理会导致两个问题，新加入的Observer对象不会通知，最近的unregistered对象仍会执行通知
实例说明
1、对象如何变成观察者
通过实现Observer接口，然后调用Observable对象的addObserver();
package org.watt.thinkinginjava.observer.java;
import java.util.Observable;
import java.util.Observer;
public class CurrentConditionDisplay implements Observer, DisplayElement {
private float temperature;
private float humidity;
private float pressure;
private Observable observable;
public CurrentConditionDisplay(Observable observable) {
this.observable = observable;
observable.addObserver(this);
}
public void display() {
System.out.println("CurrentConditionDisplay: " + temperature + " ," + humidity + ", " + pressure);
}
public void update(Observable o, Object arg) {
if (o instanceof WeatherData) {
WeatherData weatherData = (WeatherData) o;
this.temperature = weatherData.getTemperature();
this.humidity = weatherData.getHumidity();
display();
}
}
}
2、可观察者如何送出通知
可观察者继承Observable对象通过设置change状态
package org.watt.thinkinginjava.observer.java;
import java.util.Observable;
public class WeatherData extends Observable {
private float temperature;
private float humidity;
private float pressure;
public void measurementsChanged() {
//设置changed标识状态
setChanged();
notifyObservers();
}
public void setMeasurements(float temperature, float humidity, float pressure) {
this.setTemperature(temperature);
this.setHumidity(humidity);
this.setPressure(pressure);
measurementsChanged();
}
public float getTemperature() {
return temperature;
}
public void setTemperature(float temperature) {
this.temperature = temperature;
}
public float getHumidity() {
return humidity;
}
public void setHumidity(float humidity) {
this.humidity = humidity;
}
public float getPressure() {
return pressure;
}
public void setPressure(float pressure) {
this.pressure = pressure;
}
}
3、观察者如何接收通知
观察者实现update方法
public void update(Observable o, Object arg) {
//从WeatherData中获取数据
if (o instanceof WeatherData) {
WeatherData weatherData = (WeatherData) o;
this.temperature = weatherData.getTemperature();
this.humidity = weatherData.getHumidity();
display();
}
}
Observable用来定位是哪个主题通知你的，也可以把数据当作数据对象传给notifyObservers(args),在调用update时传入
public void update(Observable o, Object arg) {
//传入对象中获取
WeatherData weatherData = (WeatherData) arg; this.temperature = weatherData.getTemperature(); this.humidity = weatherData.getHumidity(); display();}
总结
1、内置观察者模式Obserable是个对象，一般情况下是面向接口编程，这个比较特别，而且setChanged等方法是protected，只有继承才能使用控制观察者对象；
2、Observer接口方法的使用，可以通过两种方式来传递数据，一种是继承Obserable生成的对象，另一种是使用update的第二个参数传入。