java实现观察者模式

javaapi中提供了两个类，可以帮助实现观察者模式
Observerable为发布者，Observer为订阅者
订阅者：发布者=1：1/1:n/n:m

import java.util.Observerable;
import java.util.Observer;
public class ObserverDemo {
public static void main(String[] args) {
    MyObservable myObservable = new MyObservable();
    myObservable.addObserver(new Observer() {
        @Override
        public void update(Observable o, Object value) {
            System.out.println(value);
        }
    });
    myObservable.setChanged();//这里是Observerable源码有个判断，为false直接返回
    myObservable.notifyObservers("hello world");
}


public static class MyObservable extends Observable {
    public void setChanged() {
        super.setChanged();
    }
}
}
JDK源码
    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)//这里
                    return;
                arrLocal = obs.toArray();
                clearChanged();
            }