Java_观察者模式（Observable和Observer）

参考链接： http://blog.csdn.net/tianjf0514/article/details/7475164/

参考链接：https://my.oschina.net/bairrfhoinn/blog/177173?p={{currentPage+1}}

在Java中通过Observable类和Observer接口实现了观察者模式。一个Observer对象监视着一个Observable对象的变化，当Observable对象发生变化时，Observer得到通知，就可以进行相应的工作。

如果画面A是显示数据库里面的数据，而画面B修改了数据库里面的数据，那么这时候画面A就要重新Load。这时候就可以用到观察者模式。

SimpleObservable

package com.eastcom.first.spark.data.observer;import java.util.Observable;public class SimpleObservable extends Observable {private int data = 0;public int getData() {return data;}public void setData(int i) {if (this.data != i) {this.data = i;setChanged();// 只有在setChange()被调用后，notifyObservers()才会去调用update()，否则什么都不干。notifyObservers();}}}

SimpleObserver

package com.eastcom.first.spark.data.observer;import java.util.Observable;import java.util.Observer;public class SimpleObserver implements Observer {public SimpleObserver(SimpleObservable simpleObservable) {simpleObservable.addObserver(this);}@Overridepublic void update(Observable o, Object data) {// TODO Auto-generated method stubSystem.out.println("Data has changed to" + ((SimpleObservable) o).getData());}}

SimpleTest

package com.eastcom.first.spark.data.observer;public class SimpleTest {public static void main(String[] args) {SimpleObservable doc = new SimpleObservable();SimpleObserver view1 = new SimpleObserver(doc);SimpleObserver view2 = new SimpleObserver(doc);SimpleObserver view3 = new SimpleObserver(doc);doc.setData(1);doc.setData(2);doc.setData(2);doc.setData(3);doc.deleteObserver(view2);System.out.println("===================");doc.setData(1);doc.setData(2);doc.setData(2);doc.setData(3);}}

运行结果

Data has changed to1Data has changed to1Data has changed to1Data has changed to2Data has changed to2Data has changed to2Data has changed to3Data has changed to3Data has changed to3===================Data has changed to1Data has changed to1Data has changed to2Data has changed to2Data has changed to3Data has changed to3

对应一个类，既是观察者同时也是被观察者。具体实例如下：

ObserverA

package com.eastcom.first.spark.data.observer;import java.util.Observable;import java.util.Observer;public class ObserverA extends Observable implements Observer {@Overridepublic void update(Observable o, Object arg) {// TODO Auto-generated method stubObserverB observerB = (ObserverB) o;System.out.println("observerB changed, the new value of observerB.data is " + observerB.data);this.setChanged();this.notifyObservers();}}

ObserverB

package com.eastcom.first.spark.data.observer;import java.util.Observable;import java.util.Observer;public class ObserverB extends Observable implements Observer {int data = 0;@Overridepublic void update(Observable object, Object arg) {System.out.println("ObserverB found that ObserverA changed...");}public void setData(int data) {this.data = data;this.setChanged();this.notifyObservers();}}

ObserverClient

package com.eastcom.first.spark.data.observer;public class ObserverClient {public static void main(String[] args) {ObserverA a = new ObserverA();ObserverB b = new ObserverB();a.addObserver(b);b.addObserver(a);b.setData(2);}}

运行结果：

observerB changed, the new value of observerB.data is 2ObserverB found that ObserverA changed...