源码中的设计模式-观察者模式

package java.util;

/**
 * A class can implement the <code>Observer</code> interface when it
 * wants to be informed of changes in observable objects.
 *
 * @author  Chris Warth
 * @see     java.util.Observable
 * @since   JDK1.0
 */
public interface Observer {
    /**
     * This method is called whenever the observed object is changed. An
     * application calls an <tt>Observable</tt> object's
     * <code>notifyObservers</code> method to have all the object's
     * observers notified of the change.
     *
     * @param   o     the observable object.
     * @param   arg   an argument passed to the <code>notifyObservers</code>
     *                 method.
     */
    void update(Observable o, Object arg);
}

　　

public class Observable {
    private boolean changed = false;
    // 存放观察者
    private Vector<Observer> obs;

    public Observable() {
        obs = new Vector<>();
    }

    // 注册观察者
    public synchronized void addObserver(Observer o) {
        if (o == null)
            throw new NullPointerException();
        if (!obs.contains(o)) {
            obs.addElement(o);
        }
    }    

   // 移除观察者 
    public synchronized void deleteObserver(Observer o) {
        obs.removeElement(o);
    }

    // 通知观察者
    public void notifyObservers() {
        notifyObservers(null);
     }

    // 实际通知观察者的方法
    public void notifyObservers(Object arg) {
        Object[] arrLocal;

        synchronized (this) {
        if (!changed)
                return;
            arrLocal = obs.toArray();
            clearChanged();
        }

        for (int i = arrLocal.length-1; i>=0; i--)
            ((Observer)arrLocal[i]).update(this, arg);
    }    

....
}

Observable中的方法都为线程安全的，继承该类结合Observer可实现观察者模式。